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Essential Revision Notes in

Paediatrics for the MRCPCH

Third edition

Edited by

Dr R M Beattie BSc MBBS MRCP FRCPCHConsultant Paediatric Gastroenterologist

Paediatric Medical UnitSouthampton General Hospital

Southampton

Dr Mike Champion BSc MBBS MRCP FRCPCHConsultant in Paediatric Inherited Metabolic Disease

Evelina Children’s HospitalGuy’s and St Thomas’ NHS Foundation Trust

London

Contents

Contributors vii

Preface to the Third edition xi

CHAPTERS

1. Cardiology 1Robert Tulloh

2. ChildDevelopment, ChildMental Health and Community Paediatrics 41Joanne Philpot and Ruth Charlton

3. Child Protection and Safeguarding 85Joanne Philpot and Ruth Charlton

4. Clinical Governance 95Robert Wheeler

5. Clinical Pharmacology andToxicology 105Steven Tomlin

6. Dermatology 125Helen M Goodyear

7. Emergency Paediatrics 147Serena Cottrell

8. Endocrinology andDiabetes 173Heather Mitchell and Vasanta Nanduri

9. Ethics and Law 209Vic Larcher and Robert Wheeler

10. Gastroenterology and Nutrition 229Mark Beattie and Hemant Bhavsar

11. Genetics 275Natalie Canham

12. Haematology andOncology 299Michael Capra

13. Hepatology 341Nancy Tan and Anil Dhawan

14. Immunology 373Pamela Lee and Bobby Gaspar

v

15. InfectiousDiseases 407Nigel Klein and Karyn Moshal

16. Metabolic Medicine 451Mike Champion

17. Neonatology 485Grenville F Fox

18. Nephrology 527Christopher J D Reid

19. Neurology 567Neil H Thomas

20. Ophthalmology 607Ken K Nischal

21. Orthopaedics 637Vel K Sakthivel

22. Respiratory 655Rebecca Thursfield and Jane C Davies

23. Rheumatology 689Nathan Hasson

24. Statistics 709Angie Wade

25. Surgery 727Merrill McHoney

Picture Permissions 761

Index 763

Contents

vi

Chapter 11GeneticsNatalie Canham

CONTENTS

1. Chromosomes1.1 Common sex chromosome

aneuploidies1.2 Common autosomal

chromosome aneuploidies1.3 CGH microarray1.4 MLPA1.5 Qf-PCR1.6 FISH testing1.7 Microdeletion syndromes1.8 Genetic counselling in

chromosomal disorders

2. Mendelian inheritance2.1 Autosomal dominant (AD)

conditions2.2 Autosomal recessive (AR)

conditions2.3 X-linked recessive (XLR)

conditions2.4 X-linked dominant (XLD)

conditions2.5 Constructing a pedigree

diagram (family tree)

3. Molecular genetics3.1 DNA (deoxyribonucleic acid)3.2 RNA (ribonucleic acid)3.3 Polymerase chain reaction

(PCR)

3.4 Reverse transcription PCR(rt-PCR)

3.5 Next generation sequencing3.6 Exome sequencing

4. Trinucleotide repeat disorders4.1 Fragile X syndrome

5. Mitochondrial disorders

6. Genomic imprinting

7. Genetic testing

8. Important genetic topics8.1 Ambiguous genitalia8.2 Cystic fibrosis8.3 Duchenne and Becker

muscular dystrophies8.4 Neurofibromatosis (NF)8.5 Tuberous sclerosis8.6 Marfan syndrome8.7 Homocystinuria8.8 Noonan syndrome8.9 Achondroplasia8.10 Alagille syndrome8.11 CHARGE syndrome8.12 VATER (VACTERL) association8.13 Goldenhar syndrome8.14 Pierre Robin sequence8.15 Potter sequence

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9. Fetal teratogens9.1 Maternal illness9.2 Infectious agents9.3 Other teratogens

10. Prenatal testing

11. Non-invasive prenatal testing

12. Preimplantation geneticdiagnosis (PGD)

13. Genetic counselling

14. Further reading

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Genetics

1. CHROMOSOMES

Background

Within the nucleus of somatic cells there are 22pairs of autosomes and one pair of sex chromo-somes. Normal male and female karyotypes are46,XY and 46,XX respectively. The normal chromo-some complement of 46 chromosomes is known asdiploid. Genomes with only a single copy of everychromosome or with three copies of each areknown respectively as haploid and triploid. A kar-yotype with too many or too few chromosomes,where the total is not a multiple of 23, is calledaneuploid. Three copies of a single chromosome ina cell are referred to as trisomy, whereas a singlecopy is monosomy.

Chromosomes are divided by the centromere into ashort ‘p’ arm (‘petit’) and a long ‘q’ arm. Acro-centric chromosomes (13, 14, 15, 21, 22) have thecentromere at one end and only a q arm.

Lyonization is the process in which, in a cell con-taining more than one X chromosome, only one isactive. Selection of the active X chromosome isusually random and each inactivated X chromo-some can be seen as a Barr body on microscopy.Genes are expressed only from the active Xchromosome.

Mitosis occurs in somatic cells and results in twodiploid daughter cells with nuclear chromosomeswhich are genetically identical both to each otherand the original parent cell.

Mitosis

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Meiosis occurs in the germ cells of the gonads andis also known as ‘reduction division’ because itresults in four haploid daughter cells, each contain-ing just one member (homologue) of each chromo-some pair, all genetically different. Meiosis involvestwo divisions (meiosis I and II). The reduction inchromosome number occurs during meiosis I and is

preceded by exchange of chromosome segmentsbetween homologous chromosomes called crossingover. In males the onset of meiosis and spermato-genesis is at puberty. In females, replication of thechromosomes and crossing over begins during fetallife but the oocytes remain suspended before thefirst cell division until just before ovulation.

Meiosis


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Translocations

• Reciprocal – exchange of genetic materialbetween non-homologous chromosomes

• Robertsonian – fusion of two acrocentricchromosomes at their centromeres, e.g. (14;21)

• Unbalanced – if chromosomal material hasbeen lost or gained overall

• Balanced – if no chromosomal material hasbeen lost or gained overall

Carriers of balanced translocations are usually phe-notypically normal but are at increased risk forhaving offspring with a chromosomal imbalance.There is also commonly an increased risk of mis-carriage and of reduced fertility.

Carriers of a robertsonian translocation involvingchromosome 21 are at increased risk of havingoffspring with translocation Down syndrome. Forfemale and male (14;21) translocation carriers theobserved offspring risks for Down syndrome areapproximately 15% and 5%, respectively. This maybe due to a selective disadvantage to spermatozoacarrying an extra chromosome. Remember, translo-cation carriers can also have offspring with normalchromosomes or offspring who are balanced trans-location carriers like themselves.

1.1 Common sex chromosomeaneuploidies

Turner syndrome (karyotype 45,X)

This affects 1 in 2500 live-born girls but it is afrequent finding among early miscarriages. Patientsare usually of normal intelligence. They havestreak ovaries that result in failure of menstruation,low oestrogen with high gonadotrophins and infer-tility. Normal secondary sexual characteristics maydevelop spontaneously or can be induced withoestrogens. If puberty is achieved, the uterus isusually normal and pregnancy is possible with theuse of donated ova. Short stature throughout child-hood with failure of the pubertal growth spurt istypical. Final height can be increased by earlytreatment with growth hormone. Other featuresmay include:

• Webbed or short neck• Low hairline• Shield chest with widely spaced nipples• Cubitus valgus (wide carrying angle)• Cardiovascular abnormalities (particularly aortic

coarctation in 10–15%)• Renal anomalies (e.g. horseshoe kidney,

duplicated ureters, renal aplasia) in a third• Non-pitting lymphoedema in a third

TripleX syndrome (karyotype 47,XXX)

This affects 1 in 1000 live-born girls. These patientsshow little phenotypic abnormality but tend to be oftall stature. Although intelligence is typically re-duced compared with siblings it usually falls withinnormal or low–normal limits. However, mild devel-opmental and behavioural difficulties are morecommon. Fertility is normal but the incidence ofearly menopause is increased.

Klinefelter syndrome (karyotype 47,XXY)

This affects 1 in 600 live-born boys. Phenotypicabnormalities are rare prepubertally other than atendency to tall stature. At puberty, spontaneousexpression of secondary sexual characteristics isvariable but poor growth of facial and body hair iscommon. The testes are small and associated withazoospermia, testosterone production is around50% of normal and gonadotrophins are raised.Gynaecomastia occurs in 30% and there is an in-creased risk of male breast cancer. Female distribu-tion of fat and hair and a high-pitched voice mayoccur but are not typical. Intelligence is generallyreduced compared with siblings but usually fallswithin normal or low–normal limits. Mild develop-mental and behavioural problems are more com-mon.

47,XYYmales

This affects 1 in 1000 live-born boys. These malesare phenotypically normal but tend to be tall. In-telligence is usually within normal limits but there isan increased incidence of behavioural abnormal-ities. Previous studies suggesting an increase incriminality have been disproved.

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1.2 Common autosomal chromosomeaneuploidies

Down syndrome (trisomy 21)

Down syndrome affects 1 in 700 live births overalland is usually secondary to meiotic non-disjunctionduring oogenesis, which is more common with in-creasing maternal age. Around 5% of patients havean underlying robertsonian translocation, most com-monly between chromosomes 14 and 21. Around3% have detectable mosaicism (a mixture of trisomy21 and karyotypically normal cells) usually resultingin a milder phenotype.

Phenotypic features include:

• Brachycephaly• Upslanting palpebral fissures, epicanthic folds,

Brushfield spots on the iris• Protruding tongue• Single palmar crease, fifth finger clinodactyly,

wide sandal gap between first and second toes• Hypotonia and moderate learning disability

The following are more common in patients withDown syndrome:

• Cardiovascular malformations in 40%,particularly atrioventricular septal defects

• Gastrointestinal abnormalities in 6%,particularly duodenal atresia and Hirschsprungdisease

• Haematological abnormalities, particularlyacute lymphoblastic, acute myeloid andtransient leukaemias

• Hypothyroidism• Cataracts in 3%• Alzheimer disease in the majority by 40 years of

age

Edward syndrome (trisomy18)

This typically causes intrauterine growth retardation,a characteristic facies, prominent occiput, overlap-ping fingers (second and fifth overlap third andfourth), rockerbottom feet (vertical talus) and shortdorsiflexed great toes. Malformations, particularlycongenital heart disease, diaphragmatic hernias,renal abnormalities and dislocated hips, are more

common. Survival beyond early infancy is rare butassociated with profound learning disability.

Patau syndrome (trisomy13)

Affected infants usually have multiple malforma-tions including holoprosencephaly and other centralnervous system abnormalities, scalp defects, micro-phthalmia, mid-line cleft lip and cleft palate, post-axial polydactyly, rockerbottom feet, renal anoma-lies and congenital heart disease. Survival beyondearly infancy is rare and associated with profoundlearning disability.

1.3 CGH microarray

CGH (comparative genomic hybridization) micro-array is a method of more detailed chromosomeanalysis than that provided by karyotyping. Patientgenomic DNA and control genomic DNA are differ-entially labelled with different fluorescent probesand then hybridized together. The ratio of fluores-cent intensity between patient and control DNA isthen compared which detects areas of copy numberdifference. This can detect microdeletions andmicroduplications as well as anomalies that wouldhave been visible on karyotype. The sensitivity ofthe test, and thus the size of the imbalances de-tected, are determined by the distances betweenand number of the fluorescent probes. High-resolution arrays can detect imbalances as small as200 base-pairs, but those in current diagnostic usetypically detect anomalies above 100 kilobases (kb).Arrays are not able to detect balanced rearrange-ments, so the karyotype is still appropriate in casessuch as recurrent miscarriage. Many small anoma-lies detected are inherited from a normal parent,and thus are probably not significant in the patho-genesis of developmental problems.

1.4 MLPA

MLPA (multiplex ligation-dependent probe amplifi-cation) is a multiplex PCR (polymerase chain reac-tion) method able to detect abnormal copy numbersof multiple genomic DNA sequences. This can beused at a gene level, detecting exon deletions or


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duplications, or at a chromosomal microdeletionlevel. Typically kits are generated with a set ofprobes such as all the telomeres, or the commonmicrodeletion syndromes.

1.5 Qf-PCR

Qf-PCR (quantitative fluorescence polymerase chainreaction) is a technique allowing fast assessment ofcopy numbers of whole chromosomes on smallsamples. Small sections of DNA from the sampleare amplified, labelled with fluorescent tags and theamounts measured by electrophoresis. This is mostcommonly used for identification of aneuploidy onprenatal samples. Typically only chromosomes 13,18 and 21, and perhaps the sex chromosomes, aretested because no other whole chromosome aneu-ploidy is survivable to term. Results are available in24–48 hours.

1.6 FISH testing

FISH (fluorescent in situ hybridization) is a tech-nique used to assess the copy number of specificDNA sequences in the genome. Fluorescentlylabelled probes are designed that are complemen-tary to the DNA sequences being assessed, and theyare allowed to hybridize to the chromosome spread.The number of copies can then be visualized asfluorescent spots using confocal microscopes. FISHcan be performed much more rapidly than formalkaryotyping. However, the use of MLPA, Qf-PCRand CGH microarray has largely superseded thisprocess, except in testing other members of a familyfor a known chromosomal anomaly.

1.7 Microdeletion syndromes

These are caused by chromosomal deletions thatare too small to see on standard microscopy butinvolve two or more adjacent (contiguous) genes.They can be detected using specific FISH testing,MLPA or CGH microarray.

Examples of microdeletion syndromes:

• 22q11 microdeletion (parathyroid glandhypoplasia with hypocalcaemia, thymus

hypoplasia with T-lymphocyte deficiency,congenital cardiac malformations, particularlyinterrupted aortic arch and truncus arteriosus,cleft palate, learning disability) also previouslycalled by many names including DiGeorgesyndrome. There appears to be an increasedincidence of psychiatric disorders, particularlywithin the schizophrenic spectrum

• Williams syndrome (supravalvular aorticstenosis, hypercalcaemia, stellate irides,characteristic facial appearance, learningdisability) due to microdeletions involving theelastin gene on chromosome 7

• 16p11.2 microdeletion syndrome (autism,seizures, learning disability) no real diagnosticphenotypic features meant that this was notpreviously identified, but with the widespreaduse of CGH microarray it is now apparent thatthis is the most common microdeletionsyndrome, found in 1 in every 100 on theautistic spectrum. Frequently also found in anormal parent, giving a high recurrence risk.

1.8 Genetic counselling inchromosomal disorders

As a general rule the following apply.

For parents of a child with trisomy 21

Recurrence risks will be around 1% above thematernal age-related risks for which there are tables.At age 36 years the background risk for Downsyndrome is 0.5%. Parents with a robertsoniantranslocation involving chromosome 21 have amuch higher recurrence risk.

For parents of a child with any other trisomy

Recurrence risks in future pregnancies for that spe-cific trisomy will be ,1%. However, couples aregenerally counselled that there is a 1% risk for anychromosome abnormality in future offspring, whichtakes into account the small risks that one parentmay be mosaic or may have an increased risk ofchromosome mis-segregation at meiosis.

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For parents of a child with amicrodeletion

Parental chromosomes should be checked. If theyare normal, recurrence risks will be ,1%. If oneparent carries the microdeletion then recurrencerisks will be 50%.

For parents of a child with any other

chromosome abnormality

Parental chromosomes should be checked. If theyare normal then recurrence risks are usually small(,1%). If one parent carries a predisposing trans-location then recurrence risks will be higher,depending on the nature of the translocation.

Prenatal karyotyping is available for any couplewho have had a previous child with a chromosomeabnormality.

2. MENDELIAN INHERITANCE

2.1 Autosomal dominant (AD)conditions

These result from mutation of one copy of a pair ofgenes carried on an autosome. All offspring of anaffected person have a 50% chance of inheritingthe mutation. Within a family the severity may vary(variable expression) and known mutation carriersmay appear clinically normal (reduced penetrance).Some conditions, such as achondroplasia andneurofibromatosis type 1, frequently start anewthrough new mutations arising in the egg or (morecommonly) sperm.

Examples of autosomal dominant

conditions

AchondroplasiaAlagille syndromeEhlers–Danlos syndrome (most)Facioscapulohumeral muscular dystrophyFamilial adenomatous polyposisFamilial hypercholesterolaemiaGilbert syndromeHuntington disease

Marfan syndromeMyotonic dystrophyNeurofibromatosis types 1 and 2Noonan syndromePorphyrias (except congenital erythropoieticwhich is AR)Tuberous sclerosisVon Willebrand disease

Conditions pre-fixed ‘hereditary’ or ‘familial’ are usually

autosomal dominant.

2.2 Autosomal recessive [AR]conditions

These result from mutations in both copies of anautosomal gene. Where both parents are carriers(with only one mutation and a normal copy), eachof their offspring has a 1 in 4 (25%) risk of beingaffected and a 2 in 4 (50%) chance of being acarrier. Carriers are usually indistinguishable fromnormal other than by DNA analysis.

Examples of autosomal recessive

conditions

AlkaptonuriaAtaxia telangiectasiaâ-ThalassaemiaCongenital adrenal hyperplasiasCrigler–Najjar syndrome (severe form)Cystic fibrosisDubin–Johnson syndromeFanconi anaemiaGalactosaemiaGlucose-6-phosphatase deficiency (von Gierkedisease)a

Glycogen storage diseasesHomocystinuriaHaemochromatosisMucopolysaccharidoses (all except Huntersyndrome)Oculocutaneous albinismPhenylketonuriaRotor (usually)Sickle cell anaemia


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Spinal muscular atrophiesWilson diseaseXeroderma pigmentosa

aDo not confuse with glucose-6-phosphate

dehydrogenase deficiency (favism) which is X-linked

recessive.

Most metabolic disorders are autosomal recessive –

remember the exceptions.

Risk calculations for AR disorders

Remember:

• People who have no family history of anautosomal recessive disorder have thebackground population carrier risk

• The parents of a child with an autosomalrecessive disorder are assumed to be carriers

• Where both parents are known to be carriers foran autosomal recessive disorder, any of theirchildren who are known to be unaffected areleft with a two-thirds carrier risk (because if thepossibility that they are affected is discounted,only three possibilities remain).

Autosomal recessive inheritance and

consanguinity

It is believed that everybody carries a few deleter-ious autosomal recessive genes. First cousins shareon average one-eighth of their genes because theyshare one set of grandparents. As a result, they aremore likely to be carrying the same autosomalrecessive disorders. For consanguineous couples ina family with a known AR disorder, specific risksshould be calculated and appropriate testing shouldbe arranged. For first-cousin parents who have noknown family history of any autosomal recessivedisorder, their offspring have around a 3% increasedrisk above the general background risk of anygenetic abnormality of 2% (i.e. a 5% overall risk).Screening should be offered for any autosomalrecessive disorder that is available and known to becommon in their ancestral ethnic group, e.g.:

• White people – cystic fibrosis• African/African–Caribbean people – sickle cell

anaemia• Mediterranean/Asian people – thalassaemia• Jewish people – Tay–Sachs disease and

multiple other recessive disorders

Although consanguinity is regarded as taboo inmany societies, around 20% of all marriages areconsanguineous (second cousin or closer). Thereare sound financial and societal reasons for consan-guineous marriages in societies where these rela-tionships are common, and the majority of offspringare healthy. Geneticists would never advise againstconsanguineous marriage (or indicate that a child’srecessive disorder is the fault of the marriage), butfamilies affected with recessive disorders have beenknown to employ carrier testing to assist in marriageplanning.

2.3 X-linked recessive (XLR)conditions

These result from a mutation in a gene carried onthe X chromosome and affect males because theyhave just one gene copy. Females are usually un-affected but may have mild manifestations as aresult of lyonization. New mutations are commonin many XLR disorders which means that the motherof an affected boy, with no preceding family history,is not necessarily a carrier. XLR inheritance is char-acterized by the following:

• No male-to-male transmission – an affectedfather passes his Y chromosome to all his sons

• All daughters of an affected male are carriers –an affected father passes his X chromosome toall his daughters

• Sons of a female carrier have a 50% chance ofbeing affected and daughters have a 50%chance of being carriers

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Examples of X-linked recessive

conditions

Alport syndrome (usually XLR; some AR forms)Becker muscular dystrophyDuchenne muscular dystrophyFabry diseaseFragile X syndromeGlucose-6-phosphate dehydrogenasedeficiency (favism)Haemophilias A and B (Christmas disease)Hunter syndrome (MPS II)Lesch–Nyhan diseaseOcular albinismRed–green colour blindnessTesticular feminization syndromeWiskott–Aldrich syndrome

2.4 X-linked dominant (XLD)conditions

These are caused by a mutation in one copy of agene on the X chromosome but both male andfemale mutation carriers are affected. As a result oflyonization, females are usually more mildly af-fected and these disorders are frequently lethal in

males. New mutations are common. For the reasonsoutlined above:

• There is no male-to-male transmission• All daughters of an affected male would be

affected• All offspring of an affected female have a 50%

chance of being affected

Examples of X-linked dominant

conditions include:

Goltz syndromeIncontinentia pigmentiRett syndromeHypophosphataemic (vitamin D-resistant)rickets

2.5 Constructing a pedigree diagram(family tree)

The basic symbols in common usage are shown inthe figure below. Occasionally symbols may be halfshaded or quarter shaded. This generally means thatthe individual manifests a specified phenotypic fea-ture denoted in an accompanying explanatory key,e.g. lens dislocation in a family with Marfan syn-drome.

Basic symbols used in pedigree diagrams


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3. MOLECULAR GENETICS

3.1 DNA (deoxyribonucleic acid)

DNA is a double-stranded molecule composed ofpurine (adenine + guanine) and pyrimidine (cyto-sine and thymine) bases linked by a backbone ofcovalently bonded deoxyribose sugar phosphateresidues. The two anti-parallel strands are heldtogether by hydrogen bonds which can be disruptedby heating and reform on cooling:

• Adenine (A) pairs with thymine (T) by twohydrogen bonds

• Guanine (G) pairs with cytosine (C) by threehydrogen bonds

3.2 RNA (ribonucleic acid)

DNA is transcribed in the nucleus into messengerRNA (mRNA) which is translated by ribosomes inthe cytoplasm into a polypeptide chain. RNA differsfrom DNA in that:

• It is single-stranded• Thymine is replaced by uracil (U)• The sugar backbone is ribose

3.3 Polymerase chain reaction (PCR)

This is a widely used method for generating largeamounts of the DNA of interest from very smallsamples. PCR can be adapted for use with RNAprovided that the RNA is first converted to DNA.

PCR is a method by which a small amount of targetDNA (the template) is selectively amplified to pro-duce enough to perform an analysis. This might bethe detection of a particular DNA sequence such asthat belonging to a pathogenic microorganism or anoncogene, or the detection of differences in genessuch as mutations causing inherited disease. There-fore the template DNA might consist of DNA

derived from peripheral blood lymphocytes, a tu-mour biopsy or a biological fluid from a patientwith an infection.

In order to perform PCR, the sequence flanking thetarget DNA must usually be known so that specificcomplementary oligonucleotide sequences, knownas primers, can be designed. The two unique pri-mers are then mixed together with the DNA tem-plate, deoxyribonucleotides (dATP, dCTP, dGTP,dTTP) and a thermostable DNA polymerase (Taqpolymerase, derived from an organism that inhabitsthermal springs):

• In the initial stage of the reaction the DNAtemplate is heated (typically for about 30seconds) to make it single stranded. As thereaction cools the primers will anneal to thetemplate if the appropriate sequence is present.

• The reaction is then heated to 728C (for about aminute) during which time the Taq DNApolymerase synthesises new DNA between thetwo primer sequences, doubling the copynumber of the target sequence.

• The reaction is heated again and the cycle isrepeated. After 30 or so cycles (each typicallylasting a few minutes) the target sequence willhave been amplified exponentially.

The crucial feature of PCR is that to detect a givensequence of DNA it only needs to be present in onecopy (i.e. one molecule of DNA): this makes itextremely powerful.

Clinical applications of PCR

• Mutation detection• Single cell PCR of in vitro fertilized embryo to

diagnose genetic disease before implantation• Detection of viral and bacterial sequences in

tissue (herpes simplex virus in CSF, hepatitis C,HIV in peripheral blood, meningococcal strains)

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Polymerase chain reaction

3.4 Reverse transcription PCT(rt-PCR)

This is a modification of conventional PCR used toamplify messenger RNA (mRNA) sequence in orderto look at the expression of particular genes withina tissue. mRNA is single stranded, unstable and nota substrate for Taq DNA polymerase. For that reasonit must be converted to complementary DNA(cDNA) using reverse transcriptase, a retroviral en-zyme, which results in a double-stranded DNAcopy of the original RNA sequence. PCR can thenbe performed in the normal way.

3.5 Next generation sequencing

DNA sequencing is used to identify point mutations,or small deletions/duplications, in a specific gene.Typically a small number of individuals’ DNA istested for mutations in one gene. This is expensivein terms of time and substrates. Next generationsequencing allows multiple parallel analyses to beperformed at the same time. This can be used to testa single individual’s DNA for mutations in multiplegenes, or to test large numbers of individuals at thesame time. Chips are being developed for specificrelated conditions caused by multiple genes, such


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as aortic dissection, Noonan syndrome, cardiomyo-pathies and cardiac arrhythmias. These will allowrapid genetic diagnosis of individuals with a clinicaldiagnosis. Next generation technology is also thebasis of exome sequencing.

3.6 Exome sequencing

Whole genome sequencing is expensive and time-consuming. The exome consists of only the codingsequences in the genome, i.e. the parts of thegenome that are translated into protein. This onlyrepresents around 5% of the total genome, but isestimated to contain 85% of all disease-causingmutations. Exome sequencing is a method of ana-lysing the entire exome for mutations. This is pri-marily a research tool used to identify unknowngenes responsible for mendelian disorders, but hasalso been used to identify functional variation asso-ciated with more common conditions such asAlzheimer disease.

4. TRINUCLEOTIDE REPEATDISORDERS

These conditions are associated with genes contain-ing stretches of repeating units of three nucleotidesand include:

• Fragile X syndrome – X-linked• Myotonic dystrophy – AD• Huntington disease – AD• Friedreich ataxia – AR• Spinocerebellar ataxias – AD

In normal individuals the number of repeats variesslightly but remains below a defined threshold.Affected patients have an increased number ofrepeats, called an expansion, above the disease-causing threshold. The expansions may be unstableand enlarge further in successive generations caus-ing increased disease severity (‘anticipation’) andearlier onset, e.g. myotonic dystrophy, particularlycongenital myotonic dystrophy after transmission byan affected mother. Between the normal range and

the affected range, there are two other expansionsizes. Premutation sizes are smaller than the lowestcopy number to cause disease and are not asso-ciated with a risk of the condition, but have a highrisk of increasing into the disease range duringgametogenesis, generating an affected child. Thisrisk can be gender dependent in some conditions.Intermediate alleles are smaller than the premuta-tion range, but larger than normal. They have a riskof increasing into the premutation range duringgametogenesis.

4.1 Fragile X syndrome

This causes learning disability, macro-orchidism,autism and seizures, and was historically associatedwith a cytogenetically visible constriction (‘fragilesite’) on the X chromosome. The inheritance is Xlinked but complex. Among controls there arebetween 6 and 45 stably inherited trinucleotiderepeats in the FMR1 gene. The intermediate allelesize is 50–58 repeats, and people with between 58and 230 repeats are premutation carriers but areunaffected. Only female gametogenesis carries arisk of expansion into the disease-causing range(230 to .1000 repeats) known as a full mutationwhich is methylated, effectively inactivating thegene. All males and around 50% of females withthe full mutation are affected, though females aretypically less severely affected. The premutationdoes not expand to a full mutation when passed onby a male. Male premutation carriers are known asnormal transmitting males and will pass the premu-tation to all their daughters (remember that theypass their Y chromosome to all their sons). Althoughpremutation carrier status is not associated withlearning disability, female carriers have a high risk(around 50%) of premature ovarian failure or earlymenopause. There is also a condition called fragileX-associated tremor and ataxia syndrome (FXTAS),which predominantly affects male premutation car-riers over the age of 50. Parkinsonism and cognitivedecline are also features. The lifetime male risk ofdeveloping FXTAS is 30–40% though 75% of menolder than 80 show signs.

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5. MITOCHONDRIAL DISORDERS

Mitochondria are exclusively maternally inherited,deriving from those present in the cytoplasm of theovum. They contain copies of their own circular16.5-kilobase chromosome carrying genes for sev-eral respiratory chain enzyme subunits and transferRNAs. Mitochondrial genes differ from nucleargenes in having no introns and using some differentamino acid codons. Within a tissue or even a cellthere may be a mixed population of normal andabnormal mitochondria known as heteroplasmy.Different proportions of abnormal mitochondriamay be required to cause disease in different tis-sues, known as a threshold effect. Disorders causedby mitochondrial gene mutations include:

• MELAS (mitochondrial encephalopathy, lacticacidosis, stroke-like episodes)

• MERRF (myoclonic epilepsy, ragged red fibres)• Mitochondrially inherited diabetes mellitus and

deafness (typically caused by the same mutationas seen in MELAS but at lower levels)

• Leber hereditary optic neuropathy (note thatother factors also contribute)

6. GENOMIC IMPRINTING

For most genes both copies are expressed but forsome genes, either the maternally or paternallyderived copy is preferentially used, a phenomenonknown as genomic imprinting. The unused copy isfrequently methylated, which inactivates the gene.These genes tend to aggregate together in imprintedregions on chromosomes. Abnormalities of inheri-tance or methylation of imprinted genes can there-fore cause disease even in the presence of twoapparently normal copies. The best examples arethe Prader–Willi and Angelman syndromes, bothcaused by cytogenetic deletions of the same regionof chromosome 15q, uniparental disomy ofchromosome 15 (where both copies of chromosome15 are derived from one parent with no copy ofchromosome 15 from the other parent), or abnorm-alities of methylation, which labels both chromo-somes as deriving from one parent. The diseasecondition is caused by the absence of one parent’scopy of genes in the region, rather than by exces-sive numbers of copies of the other.

Prader-Willi syndrome Angelman syndrome

Clinical

Neonatal hypotonia and poor feeding Unprovoked laughter/clappingModerate learning disability Microcephaly, severe learning disabilityHyperphagia + obesity in later childhood Ataxia, broad-based gaitSmall genitalia Seizures, characteristic EEG

Genetics

70% deletion on paternal chromosome 15 80% deletion on maternal chromosome 1530% maternal uniparental disomy 15 2–3% paternal uniparental disomy 15(i.e. no paternal contribution) (i.e. no maternal contribution);

remainder due to subtle mutations


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Other imprinting disorders

Silver–Russell syndromePrenatal onset growth retardation, relative macro-cephaly, triangular facies, asymmetry, fifth fingerclinodactyly and frequently normal IQ. Around 35%are caused by abnormal methylation of genes onchromosome 11p15, whereas 10% are associatedwith maternal uniparental disomy of chromosome7. The cause in the remainder is not yet known.

Beckwith–Wiedemann syndromePrenatal-onset macrosomia, facial naevus flammeus,macroglossia, ear lobe creases, pits on the ear helix,hemihypertrophy, nephromegaly, exomphalos (om-phalocele) and neonatal hypoglycaemia. There is anincreased risk of Wilms tumour, adrenocortical andhepatic tumours in childhood. Similar to Silver–Russell syndrome, the condition results fromabnormalities of inheritance or methylation ofchromosome 11p15 which contains severalimprinted genes, including the IGF-2 (insulin-likegrowth factor 2) gene. The results in BWS tend tobe directly opposite to those in Silver–Russellsyndrome.

7. GENETIC TESTING

Genetic tests can be thought of as diagnostic, pre-dictive or for carrier status. Informed verbal, andincreasingly written, consent (or assent) should beobtained before genetic testing.

Diagnostic tests

These are chromosomal investigations such as karyo-type and CGH microarray, or mutation analysis ofspecific genes. The latter is frequently used where thediagnosis is already suspected on clinical groundsbut genetic testing is useful for confirmation, or forcounselling or predictive testing in the wider family.

Predictive tests

When an individual is clinically normal but is atrisk for developing a familial disorder, such asHuntington disease, myotonic dystrophy or a famil-

ial cancer syndrome. Predictive testing is not usual-ly offered without a formal process of geneticcounselling over more than one consultation withtime built in for reflection. Where there are inter-vening relatives whose genetic status may be indir-ectly revealed, there are additional issues that mustbe taken into consideration. Written consent forpredictive testing is required by most laboratories.Nationally agreed guidance is that predictive testingin children for disorders that have no implicationsin childhood should not be undertaken until thechild is old enough to make an informed choice.

Carrier tests

These are usually undertaken in autosomal recessiveor X-linked recessive disorders where the result hasno direct implications for the health of the indivi-dual, but is helpful in determining the risks to theiroffspring. Carrier status may be generated as a by-product of diagnostic or prenatal testing. Nationalguidance is that specific testing for carrier statusshould be avoided in children until they are oldenough to make an informed choice.

Genetics in children

Diagnostic tests are obviously necessary and useful,as are predictive tests for disorders that may manifestin childhood, and have a screening programme ortreatment, such as the multiple endocrine neoplasias(MEN1, MEN2) and familial adenomatous polyposis.Predictive testing for adult onset disorders such asBRCA-1/-2 or Huntington disease are not appropriatein children, because they are unable to give informedconsent, and a diagnosis can never be removed onceit has been made. Many adults opt not to havepredictive tests for untreatable disorders such as Hun-tington disease, and an at-risk child should be al-lowed to make the same decision. Equally, carrierstatus for AR or X-linked disorders will impact only ona child’s reproductive decisions, not childhoodhealth, and thus is only tested when the child is ableto participate in the process and give proper informedconsent. Parents do occasionally request such testing,and a clinical geneticist would meet them in clinic todiscuss their reasons for testing and the reasons for areluctance to offer it.

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8. IMPORTANT GENETIC TOPICS

This section includes short notes on conditions thatform popular examination topics.

8.1 Ambiguous genitalia

Normal development of the reproductive

tract and external genitalia

A simplified outline is shown below.

Outline of the normal development of the reproductive tract and external genitalia

The 6-week embryo has undifferentiated gonads,mullerian ducts (capable of developing into theuterus, fallopian tubes and upper vagina), wolffianducts (capable of forming the epididymis, vas defe-rens and seminal vesicles) and undifferentiated ex-ternal genitalia.

In the presence of a Y chromosome the gonadsbecome testes that produce testosterone and muller-ian inhibiting factor (MIF). Testosterone causes thewolffian ducts to persist and differentiate and, afterconversion to dihydrotestosterone (by 5Æ-reductase),masculinization of the external genitalia. MIFcauses the mullerian ducts to regress.

In the absence of a Y chromosome the gonadsbecome ovaries which secrete neither testosteronenor MIF and, in the absence of testosterone, thewolffian ducts regress and the external genitaliafeminize. In the absence of MIF, the mullerian ductspersist and differentiate.

The causes of ambiguous genitalia divide broadlyinto those resulting in undermasculinization of amale fetus, those causing masculinization of afemale fetus, and those resulting from mosaicism fora cell line containing a Y chromosome and anotherthat does not. They are summarized in the diagramopposite.


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Ambiguous genitalia – outline of causes

8.2 Cystic ¢brosis

This results from mutations in the CFTR (cysticfibrosis transmembrane regulator) gene. The ˜F508mutation (deletion of three nucleotides coding for aphenylalanine residue at amino acid position 508)accounts for 75% of mutations in white people.Most laboratories now screen for 32 common muta-tions including ˜F508. Such testing identifies 90%of cystic fibrosis mutations in white people, but a

much smaller proportion in many other ethnicgroups. Therefore, negative molecular testing cannotexclude a diagnosis of cystic fibrosis.

8.3 Duchenne and Becker musculardystrophies

These result from different mutations within thedystrophin gene on chromosome Xp21.

Important distinguishing features of Duchenne and Becker muscular dystrophies

Duchenne muscular dystrophy Becker muscular dystrophy

Immunofluorescent Undetectable Reduced/abnormaldystrophin on musclebiopsyWheelchair dependence 95% at ,12 years 5% at ,12 yearsLearning disability 20% Rare

abnormal 46, XY

UNDERMASCULINIZED

MALE

Chromosomes

46,XX

MASCULINIZED

FEMALE

• e.g. 45, X/46,XY mosaic

NB. Complete testicular failure and complete androgen insensitivity ( testicular feminization

syndrome) cause apparently normal female external genitalia.

5

par tial testicular failure

par tial androgen insensitivity

5 -reductase deficiency

rare forms of congenital

adrenal hyperplasia

e.g. 3 -hydroxylase

17 -hydroxylase

rare syndromes

e.g. Smith-Lemli-Opitz (AR)

α

â

α

external androgens

e.g. OCP

endogenous androgens

e.g. common forms of

congenital adrenal

hyperplasia

- 21-hydroxylase

-11 -hydroxylaseâ

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In around a third of boys with Duchenne musculardystrophy, the condition has arisen as a new muta-tion, whereas a further third are the result of a newmutation in the mother. Mutation analysis in theaffected boy can often identify mothers who arecarriers, but a normal result does not exclude germ-line mosaicism, where mutated cells are present inthe ovaries but not the blood. A woman proven tobe a carrier has a 25% (1 in 4) recurrence risk, buta woman without the mutation in her blood still hasup to a 20% recurrence risk, and prenatal diagnosisis offered in all circumstances.

Given the high new mutation rate, both in theaffected child and in the mother, calculation of risksto other family members can be challenging. Therisk that the mother of an isolated case is a carrier istwo in three. The maternal grandmother’s risk is onein three, due to the chance of a new mutation inthe mother. Thus, the sister of the isolated affectedboy has a one in three risk of being a carrier, butthe maternal aunt has a one in six risk, and so on.

In practical terms, most families will have an identi-fiable mutation, and thus carrier identification willbe relatively easy. In the absence of a mutation, e.g.the affected individual has died with no DNAstored, or no mutation is identified (a small propor-tion), then the above risks can be modified usinglinkage to the X chromosome and Bayes theorem totake into account the number of unaffected malesin the family, and the creatine kinase (CK) levels inthe at-risk females. Carrier females can have ele-vated CK levels, although a normal result does notexclude carrier status because they follow a normaldistribution. A woman known to be at high risk, butwith no identifiable mutation, may only be able toopt to terminate male pregnancies if she wishes toavoid having an affected child.

8.4 Neuro¢bromatosis

There are two forms of neurofibromatosis (NF) thatare clinically and genetically distinct:

NF1 NF2Major features >6 Cafe-au-lait patches Bilateral acoustic neuromas

Axillary/inguinal freckling (vestibular schwannomas)Lisch nodules on the iris Other cranial and spinal tumoursPeripheral neurofibromas

Minor features Macrocephaly Cafe-au-lait patches (usually ,6)Short stature Peripheral schwannomas

Peripheral neurofibromasComplications Plexiform neuromas Deafness/tinnitus/vertigo

Optic glioma (2%) Lens opacities/cataractsOther cranial and spinal Spinal cord and nerve compressionstumours Malignant change/sarcomasPseudarthrosis (especiallytibial)Renal artery stenosisPhaeochromocytomaLearning difficultiesScoliosisSpinal cord and nervecompressionsMalignant change/sarcomas

Gene Chromosome 17 Chromosome 22Protein Neurofibromin Schwannomin


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8.5 Tuberous sclerosis

There are at least two separate genes that causetuberous sclerosis (TS), on chromosomes 9 (TSC1;hamartin) and 16 (TSC2; tuberin).

Clinical features of tuberous sclerosis

Skin/nails

• Ash-leaf macules• Shagreen patches (especially over the

lumbosacral area)• Adenoma sebaceum (facial area)• Subungual/periungual fibromas

Eyes

• Retinal hamartomas

Heart

• Cardiac rhabdomyomas, detectableantenatally, usually regressing duringchildhood

Kidneys

• Angiomyolipomas• Renal cysts

Neurological

• Seizures• Learning disability

Neuroimaging

• Intracranial calcification (periventricular)• Subependymal nodules• Neuronal migration defects

8.6 Marfan syndrome

This results from mutations in the fibrillin 1 (FBN1)gene on chromosome 15. Intelligence is usuallynormal. New diagnostic criteria do not include joint

laxity or hyperextensibility, and this alone in a tallindividual is not sufficient to suspect the diagnosisof Marfan.

Clinical features of Marfan syndrome

Musculoskeletal

• Tall stature with disproportionately longlimbs (dolichostenomelia)

• Characteristic facial appearance• Arachnodactyly• Pectus carinatum or excavatum• Scoliosis• High, narrow arched palate with dental

overcrowding• Pes planus

Heart

• Aortic root dilatation and dissection• Mitral valve prolapse

Eyes

• Lens dislocation (typically up)• Myopia

Skin

• Striae

Lungs

• Spontaneous pneumothorax• Apical bullae

8.7 Homocystinuria

(see also Chapter 16)

This is most commonly the result of cystathione-â-synthase deficiency and causes a Marfan syndrome-like body habitus, lens dislocation (usually down),learning disability, thrombotic tendency and osteo-porosis. Treatment includes a low methionine diet� pyridoxine.

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8.8 Noonan syndrome

This is an autosomal dominant condition. Around50% of individuals with Noonan syndrome havemutations in the PTPN11 (protein-tyrosine phospha-tase, non-receptor-type 11) gene on chromosome 12.A further 10–15% are caused by SOS1 (son of seven-less homologue 1 (Drosophila), on chromosome 2)and RAF1 (v-raf-1 murine leukaemia viral oncogenehomologue 1 on chromosome 3) causes another 5–10%. Multiple other genes on the RAS-MAPK path-way have also been implicated in small proportionsof cases. The karyotype is usually normal.

Clinical features of Noonan syndrome

Cardiac

• Pulmonary valve stenosis• Hypertrophic cardiomyopathy• Septal defects (atrial and ventricular septal

defects)• Branch pulmonary artery stenosis

Musculoskeletal

• Webbed or short neck• Pectus excavatum or carinatum• Wide-spaced nipples• Wide carrying angle (cubitus valgus)• Short stature in 80%

Other features

• Ptosis• Low-set and/or posteriorly rotated ears• Small genitalia and undescended testes in

boys• Coagulation defects in 30% (partial factor

XI:C, XIIC and VIIIC deficiencies, vonWillebrand disease, thrombocytopenia)

• Mild learning disability in 30%

8.9 Achondroplasia

A short-limb skeletal dysplasia resulting from speci-fic autosomal dominant mutations in the FGFR3(fibroblast growth factor receptor 3) gene on

chromosome 4. There is a high new mutation rate.Important complications are hydrocephalus, brain-stem or cervical cord compression resulting from asmall foramen magnum, spinal canal stenosis,kyphosis and sleep apnoea. Intelligence is usuallynormal.

8.10 Alagille syndrome

A variable autosomal dominant disorder resultingfrom deletions of or mutations in the JAG1 (jagged)gene on chromosome 20. Major features of thesyndrome include:

• Cardiac – peripheral pulmonary artery stenosis� complex malformations

• Eye – posterior embryotoxon, abnormalities ofthe anterior chamber

• Vertebral – butterfly vertebrae, hemivertebrae,rib anomalies

• Hepatic – cholestatic jaundice, paucity ofintrahepatic bile ducts

8.11 CHARGE syndrome

A malformation syndrome including:

• Colobomas• Heart malformations• Atresia of the choanae• Retardation of growth and development

(learning disability)• Genital hypoplasia (in males)• Ear abnormalities (abnormalities of the ear

pinna, deafness)• Cleft lip/palate and renal abnormalities are also

common

The majority of patients with CHARGE syndromehave new mutations or deletions of the CHD7(chromodomain helicase DNA-binding protein 7)gene on chromosome 8.

8.12 VATER (VACTERL) association

A sporadic malformation syndrome including:

• Vertebral abnormalities• Anal atresia � fistula


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• Cardiac malformations• Tracheo-oesophageal fistula• Renal anomalies, radial ray defects• Limb anomalies, especially radial ray defects

The cause is not yet known.

8.13 Goldenhar syndrome

Also known as oculo-auriculovertebral spectrum, orfirst and second and branchial arch syndrome. It ismainly sporadic and the cause is unknown. Majorfeatures include:

• Craniofacial – asymmetry, hemifacialmicrosomia, micrognathia

• Ears – malformed pinnas, deafness, preauriculartags

• Eyes – epibulbar (scleral) dermoid cysts,microphthalmia

• Oral – macrostomia, cleft lip/palate• Vertebral – hemivertebrae• Cardiac – cardiac malformations• Renal – renal malformations

8.14 Pierre Robin sequence

An association of micrognathia and cleft palatewhich may occur alone, but a proportion will have22q11 deletions or Stickler syndrome.

8.15 Potter sequence

Oligohydramnios as a result of renal abnormalities,urinary tract obstruction or amniotic fluid leakagemay lead to secondary fetal compression with jointcontractures (arthrogryposis), pulmonary hypoplasiaand squashed facies known as the Potter sequence.

9. FETAL TERATOGENS

9.1 Maternal illness

Maternal diabetes

Maternal diabetes is associated with fetal macro-somia, neonatal hypoglycaemia and increased risk

of a wide variety of malformations, particularlycardiac (transposition of the great arteries, aorticcoarctation, septal defects, cardiomyopathy), verte-bral (sacral abnormalities, hemivertebrae), renal(agenesis, duplex collecting systems), intestinal(imperforate anus, other atresias) and limb abnorm-alities (short femurs, radial ray abnormalities).

Maternal myasthenia gravis

This is associated with fetal arthrogryposis.

Maternal phenylketonuria

Although the fetus is unlikely to be affected byphenylketonuria (PKU: which is autosomal reces-sive), if an affected mother has relaxed her lowphenylalanine diet, the fetus is at risk of micro-cephaly, cardiac defects and learning disabilitysecondary to exposure to the raised maternalphenylalanine levels.

Maternal systemic lupus erythematosus

Maternal systemic lupus erythematosus (SLE) withanti-Ro and anti-La antibodies is associated with anincreased risk of fetal bradycardia and congenitalheart block for which pacing may be required. Aself-limiting neonatal cutaneous lupus may alsooccur.

9.2 Infectious agents

The following agents are associated with increasedfetal loss in the first trimester; hepatosplenomegaly,jaundice and thrombocytopenia in the neonate; andabnormalities particularly those affecting the centralnervous system, vision and hearing.

Fetal cytomegalovirus

Infection may be associated with microcephaly,intracranial calcification, chorioretinopathy, deaf-ness and learning disability.

Fetal toxoplasmosis

Infection with Toxoplasma species, a protozoan,may be associated with microcephaly, hydrocepha-

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lus, intracranial calcification, chorioretinopathy andlearning disability.

Fetal rubella

Infection with rubella virus is most often associatedwith deafness particularly in the first and earlysecond trimesters, but cardiac abnormalities (persis-tent ductus arteriosus, peripheral pulmonary steno-sis, septal defects), microcephaly, chorioretinopathy,cataract and learning disability are also associated.

Congenital syphilis, herpes and varicella

See Chapter 15, Section 11.1.

9.3 Other teratogens

Fetal alcohol syndrome

Pre- and postnatal growth retardation, neonatal irrit-ability, microcephaly, learning disability, hyperactiv-ity in childhood, cardiac defects (particularlyventricular and atrial septal defects), small nails onfifth fingers and toes, facial anomalies (short palpeb-ral fissures, ptosis, smooth philtrum, thin upper lip)and a variety of less common, often midline, mal-formations. It is likely that the effects on any onefetus are determined by the degree, timing andduration of exposure as well as the susceptibility ofthe fetus which is probably genetically determined.

Illicit drugs in pregnancy

Opiate drugs in pregnancy have a high risk ofdependency in the newborn, intrauterine growthretardation and still birth, but do not appear to beassociated with significant risk of structural anoma-lies. There are behavioural issues during childhood.Fetal cocaine has a higher risk of defects, apparentlyassociated with vascular disruption, such as limbreduction defects and porencephaly. Survivors ofthis do not appear to have long-term intellectualdeficit once their home circumstance has beentaken into account, though there is evidence ofsome attention and behavioural problems.

Fetal retinoic acid

Exposure to retinoic acid (which is used in thetreatment of acne) is associated with structural brainabnormalities, neuronal migration defects, microtiaand complex cardiac malformations.

Fetal valproate syndrome

Fetuses exposed to valproate have an increased riskof cleft lip and palate, neural tube defects, cardiacdefects, radial ray defects, learning disability andfacial anomalies (frontal narrowing including meto-pic craniosynostosis, thin eyebrows, infraorbital skingrooves, long philtrum, thin upper lip). These effectsappear to be dose dependent.

Fetal warfarin syndrome

Fetuses exposed to warfarin typically have nasalhypoplasia, stippled epiphyses and are at risk oflearning disability and brain, eye, cardiac and skele-tal malformations.

10. PRENATAL TESTING

• Chorionic villous sampling or biopsy (CVS orCVB) – a small piece of placenta is taken eithertransabdominally or transvaginally. CVS testingcan be safely performed from 11 weeks’gestation

• Amniocentesis – amniotic fluid is taken,containing cells derived from the surfaces of thefetus and amniotic membranes. Amniocentesisis usually performed from 15 weeks’ gestation

• Cordocentesis – a method of obtaining fetalblood that can be performed from 18 weeks’gestation

Chromosome and DNA testing can be performedon any of the above types of sample, and biochem-ical analyses can often also be performed if neces-sary. Each method carries a small risk ofmiscarriage. As a result, most couples opt for pre-natal testing only if they wish to terminate anaffected pregnancy. Although chromosome analysiscan be performed on any pregnancy, DNA analysis


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can be used only in families where known muta-tions have already been identified, and the family isat significant risk.

It is possible to identify the sex of an unborn fetusby prenatal testing and, in the case of X-linkedconditions where no specific mutation has beenidentified, this is often the only available prenataltest. However, it is illegal in the UK to terminate apregnancy on the basis of gender alone unless thechild is at risk of a genetic condition due to itsgender.

11. NON-INVASIVE PRENATALTESTING

Cell-free fetal DNA can be detected in the mother’scirculating blood from 4 weeks’ gestation. The vastmajority of the cell-free DNA is maternal, however,so testing is currently limited to the identification orexclusion of genetic material not present in themother, such as Y chromosome, or rhesus D inRhD-negative women. In those at risk of an X-linkeddisorder in sons, this process will remove the neces-sity for invasive testing in 50% of pregnancies.Currently it is not possible to test for trisomy 21 orother chromosomal anomalies by this method.

12. PREIMPLANTATION GENETICDIAGNOSIS

This technique is an in vitro fertilization (IVF)-basedprocess. At the 8- to 16-cell stage a single cell isremoved from each embryo for testing. Only em-bryos predicted to be unaffected are reimplantedinto the mother. Preimplantation genetic testing(PGD) is technically difficult and has a similar

viable pregnancy rate to IVF (25%). It is available inthe UK for a limited, although increasing, numberof conditions and virtually all inherited chromosomeanomalies. For funding purposes it is frequentlyregarded as fertility treatment, so families can find ithard to get NHS treatment. Overseas centres have awider range of conditions, but it is very expensive.

13. GENETIC COUNSELLING

This is the process of assisting families or individualsaffected by genetic disease to understand the causeof their condition, the risk of recurrence and theoptions available to them. It is entirely non-directiveand the aim is to deliver all available information toallow the family to make the appropriate decisions.Some families will opt for prenatal diagnosis andtermination, although this will not be acceptable forothers. Equally, with predictive testing, not everyoneat significant risk of a condition chooses to havetesting to clarify this risk further. Genetic counsel-ling will be offered to all, with no obligation topursue testing.

14. FURTHER READING

Firth HV, Hurst JA. Oxford Desk Reference –Clinical Genetics. Oxford University Press, 2005.

Harper PS. Practical Genetic Counselling, 7th edn.London: Hodder Education, 2010.

Jones KL. Smith’s Recognizable Patterns of HumanMalformation, 6th edn. Philadelphia: ElsevierSaunders, 2005.

Kingston HM. ABC of Clinical Genetics, 3rd edn.Oxford: Wiley-Blackwell, 2002

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