clinical manifestations of genetic mosaicism

CLINICAL MANIFESTATIONS OF GENETIC MOSAICISM

RCPA Short course in Medical Genetics and Genetic Pathology

Adjunct Prof Mac GardnerUniv Otago & Northern Regional Genetics, NZ

This word came from the ancient Greek mouseion meaning 'place holy to the Muses‘. This transmutated into the Latin musaeum meaning ‘decoration with small square stones‘, and the decorations may originally have been dedicated to the Muses.

In medieval Latin it was changed to musaicus/mosaicus and passed via Italian mosaicoand French mosaïque into English as mosaic.

DERIVATION OF THE WORD ‘MOSAICISM’

The starting position:

Mosaicism is common

Indeed, are we all mosaics?

Consider the mathematics:

A person ≈ 10,000,000,000,000 cells

This needed 9,999,999,999,999 mitoses

No QA system could get it right that many times!

MOSAICISM:

2 or more genetically different cell lineages in the constitution of one individual, derived from a single original zygote (one cell

line usually normal)

Mendelian: Some cells/tissues carry a classic gene mutation

Chromosomal: 2 (or more) karyotypically different cell lines*:

Epigenetic: Some cells/tissues have a different imprinting state (cf. David Amor re somatic UPD11)

Immunogenetic: All that rearranging of immunogenes

CNVs: May be rather frequent

Mitochondrial: The whole question of heteroplasmy

* k i l id

Excluding:

Cancer:

Many/practically all cancers are chromosomally ‘mosaic’, but these arise post‐constitutionally in 46,XY and 46,XX people (a.k.a. normal men and women)

Example of highly complex cytogenetic aberrations in a case of high‐grade myxofibrosarcoma

Willems SM, Debiec‐Rychter M, Szuhai K, Hogendoorn PC, Sciot R. Local recurrence of myxofibrosarcoma is associated with increase in tumour grade and cytogenetic aberrations, suggesting a multistep tumour progression model. Mod Pathol. 2006 Mar;19(3):407‐16.

Excluding:

Chromosomal loss with ageing:

• The quite common finding of loss of an X or a Y chromosome in an occasional cell in an older female or male population (and more notably in centenarians) may reflect “normal” age‐related anaphase lag.

Excluding:

X Chromosome inactivation in females:

• Every 46,XX female is mosaic with respect to X chromosome activation

• An epigenetic form of mosaicism

• Each cell makes its own autonomous decision about which X – maternal or paternal – remains the active one, at a very early stage of embryonic existence

Excluding:

Chimerism:

The existence of 2 cell lines of different origins:

(1) Fusion of 2 DZ twin embryos

(2) Iatrogenic engraftment

(3) Maternal residuum of fetal cells post partum, 46,XX//46,XY: A cause of maternal auto‐immune disease (systemic sclerosis/scleroderma)

Rak JM, et al. Male microchimerism and HLA compatibility in French women with sclerodema: a different profile in limited and diffuse subset. Rheumatology 2009 48(4):363‐6.

MOSAICISM CAN BE:

• SOMATIC,

• GONADAL, or

• SOMATIC‐GONADAL

and

• MENDELIAN, or

• CHROMOSOMAL

Distribution

Genetic type

MENDELIAN SOMATIC MOSAICISM:

The genetic error arises

• post‐zygotically, after initial differentiation of embryonic cell types

• in a cell destined to form a body lineage other than gonadal tissue.

Four examples:

• Neurofibromatosis type 1 (gene is neurofibromin)

• Neurofibromatosis type 2 (gene is NF2, a.k.a. merlin)

• Periventricular nodular heterotopia (one gene is FLNA)

• Atrial fibrillation (one gene may be GJA5)

NF1, a.k.a. VON RECKLINGHAUSEN’S DISEASE

Somatic mosaicism, also called ‘segmental’ mosaicism

Cutaneous signs confined to one region of the body

Tinschert S, Naumann I, Stegmann E, Buske A, Kaufmann D, Thiel G, Jenne DE. Segmental neurofibromatosis is caused by somatic mutation of the neurofibromatosis type 1 (NF1) gene. Eur J Hum Genet. 2000 8:455-9.

NF1, a.k.a. VON RECKLINGHAUSEN’S DISEASE

Somatic mosaicism, also called ‘segmental’ mosaicism

Cutaneous signs confined to one region of the body

Skin fibroblasts from café au lait macule deletion of the NF1 gene

Fibroblasts from skin elsewhere, and blood, normal

Tinschert S, Naumann I, Stegmann E, Buske A, Kaufmann D, Thiel G, Jenne DE. Segmental neurofibromatosis is caused by somatic mutation of the neurofibromatosis type 1 (NF1) gene. Eur J Hum Genet. 2000 8:455-9.

Mutation happened in this cell?

Cardiac tube

Mutation happened in this cell?

What he (and we) looked like at 3 weeks of age:

• 29 y/o female, quite sudden onset lower limb weakness and loss of bladder control

• Hearing grossly intact

• MRI showed spinal and intracranial tumours:

Neurofibromatosis type 2 (NF2)

a.k.a. Bilateral Vestibular Schwannoma

Tumour

Post-opPre-op

Schwannoma

Vestibular schwannoma

Question:

Will our children get this? What if we have another child?

NF2

NF2 and the neural crest ...

and Schwann cells

What she (and we) looked like at 3 weeks of age:

Cardiac tube

Neural tube

Neural crest cells

Neural

tube

At day 20 of life: genetically perfect

Neural crest cells

At day 21 of life: one (somatic) cell mutates

“First hit”

Orange cells = progeny cells from “first hit”


Cell can cross the midline


Purple cell = cell having “second hit”


Purple, blue cells = cells having “second hits”


Purple, blue, red = cells having “second hits”


Purple, blue, red = cells having “second hits”, whose progeny, being homozygous (or cpd het), can form aschwannoma

(Hypothetical) first & second hits at molecular level

gln320stop448 –1 g>a

methylation

How this knowledge, along with a basic understanding of embryology, can inform genetic counselling .....

Neural crest cells

Precursor yolk sac cells, which will ⇒blood, gametocytes

Neural crest cells

BILATERAL TUMOURS

Precursor yolk sac cells, which will ⇒gametocytes

Neural crest cells

BILATERAL TUMOURS


Blood cells

Neural crest cells

UNILATERAL TUMOURS


MINIMUM ESTIMATES OF MOSAICISM IN SPORADIC (-ve fam hx) CASES OF NF2

Bilateral vestib schwannoma: 33% +

Unilat “ “ : 60% +

But what fraction of the mosaicism includes the gonad?

Transmission risks to offspring for isolated cases of NF2, after negative mutation testing on peripheral blood

Age at dx Risk (cf. baseline risk = 50% in nonmosaic)____________________________________

VESTIBULAR SCHWANNOMA IS

____________BILAT________UNILAT____

< 20 30% 11% 20–29 16% 8%30–39 11% 6%40+ 9% 5%____________________________________

Mosaicism in neurofibromatosis type 2: an update of risk based on uni/bilaterality of vestibular schwannoma at presentation and sensitive mutation analysis including multiple ligation‐dependent probe amplification. D Gareth R Evans, R T Ramsden, A Shenton, C Gokhale, N L Bowers, S M Huson, G Pichert, A Wallace J. Med. Genet. 2007;44;424‐428.

Neurofibromatosis type 2 (NF2): a clinical and molecular review. Evans DGR. Orphanet J Rare Dis. 2009 19:16.

N.

Question:

Will our children get this? What if we have another child?

Answer:

Probably, no

NF2

• Periventricular nodular heterotopia ‐‐ a cause of epilepsy in females

Might some, mutation‐negative on blood, be due to localised (periventricular neurone) mutation in FLNA (filamin‐A)?

• Atrial fibrillation

Some may be due to localised (cardiac myocyte) mutation in ‘gap junction protein’ gene GJA5 (connexin‐40)

Gollob MH et al. Somatic mutations in the connexin 40 gene (GJA5) in atrial fibrillation. New Engl J Med 354: 2677-2688, 2006.

Immune dysfunction Adenosine deaminase deficiencyWiskott–Aldrich syndrome

Clotting disordersHaemophilia AHaemophilia B

Skeletal disorders AchondroplasiaOsteogenesis imperfectaMarfan syndromePseudoachondroplasia

Muscle disorders Duchenne muscular dystrophyCongenital myotonic dystrophy

Chromosomal instability Bloom syndromeFanconi anaemia

Tumour suppressor Neurofibromatosis type INeurofibromatosis type IITuberous sclerosis

Skin disorders Bullous ichthysiform erythrodermaIncontinentia pigmenti

Endocrine disorders Androgen insensitivity

Nervous-system Friedreich ataxia

Metabolic disordersTyrosinaemia type ILesch–Nyhan

Youssoufian & Pyeritz 2002

Recognition of somatic mosaicism: representative monogenic disorders

How much rare variation?

“The total somatic mutational load must be enormous. For example, theintestinal epithelium contains approximately 106 independent stem cells,each of which generates transient daughter cells every week or two. Thus,the intestinal epithelium of a 60-y-old is expected to harbor >109

independent mutations. This implies that, not far beyond the age of60y, nearly every genomic site is likely to have acquired a mutation inat least one cell in this single organ.”

SOMATIC ‘REVERTANT’ MOSAICISM OF LATER ORIGIN

The genetic ‘correction’ arises

• post‐zygotically

• well into embryonic/fetal development

• in a differentiated cell, destined to form just one body lineage

One example:

• Epidermolysis bullosa ‐‐ a blistering/dystrophic skin disorder

SOMATIC ‘MOSAICISM’ OF VERY EARLY ORIGIN:MZ TWIN DISCORDANCE


• post‐zygotically

• at or before the blastocyst/inner cell mass stage

• after splitting of embryo into 2

• in a cell destined to form all or almost all body lineages

Three examples:

• Dravet syndrome of severe epilepsy (gene is sodium channel SCN1A)

• NF1 (gene is neurofibromin)

• Trisomy 21 (may be ‘correction’, rather than an error)

Mendelian

Chromosomal

Twin with epilepsy →

(Dravet syndrome)

SCN1A mutation in all these tested tissues

← Unaffected twin

Normal SCN1A gene in all these tested tissues

Timing of de novo mutagenesis‐‐a twin study of sodium‐channel mutations. Vadlamudi L, Dibbens LM, Lawrence KM, Iona X, McMahon JM, Murrell W, Mackay‐Sim A, Scheffer IE, Berkovic SF. N Engl J Med 2010 363:1335‐40.

MZ

Normal allele mutates to an SCN1A allele

Zygote is normal at SCN1A locus

ONE TWIN WITH DRAVET SYNDROME

Monozygotic twins discordant for neurofibromatosis type 1 due to a postzygotic NF1 gene mutation.

Vogt J, Kohlhase J, Morlot S, Kluwe L, Mautner VF, Cooper DN, Kehrer-Sawatzki H. Institute of Human Genetics, University of Ulm, Ulm, Germany.

“Whereas DNA sequence analysis indicated somatic mosaicism for the NF1 nonsense mutation, c.4108C>T (p.Q1370X), in the affected twin, this lesion was apparently absent in his unaffected brother. . . . . .

We conclude that the twinning event, which would have taken place within three days post-fertilization, must have preceded the c.4108C>T mutation which is therefore predicted to have occurred during the blastocyst stage, leading to somatic mosaicism with normal cells lacking the mutation.”

MZ

Zygote is normal at NF1 locus

Twining occurs by day 3-4 morula/blastocyst

ONE TWIN WITH NF1

Jansen RPS, Bowman MC, de Boer KA, Leigh DA, Lieberman DB, McArthur SJ. What next for preimplantation genetic screening (PGS)? Experience with blastocyst biopsy and testing for aneuploidy. Hum Reprod 23: 1476-1478, 2008.

Hatching blastocyst →

Hatched blastocyst →

MZ

Normal allele mutates to an NF1 allele, in one cell of day 4 or 5 blastocyst

Zygote is normal at NF1 locus

Twining occurs by day 3-4 morula/blastocyst

ONE TWIN WITH NF1

MZ

‘Third’ chromo 21 lostZygote is

trisomy 21

ONE TWIN WITH TRISOMY 21

47,XX,+21 46,XX

SOMATIC MOSAICISM IN THE EARLY EMBRYO ~ CHROMOSOMAL


• post‐zygotically,

• in 1 or more cells in a preimplantation embryo

Four examples:

• Trisomy 21 mosaicism – a classic e.g. (As above, but didn’t twin)

• The concept of ‘chaotic mosaicism’ in the IVF embryo (but in vivo?)

• Fetal Trisomy 3

• ‘Hypomelanosis of Ito’ associated with mosaicism for a structural rearrangement

PGD -- Preimplantation Genetic Diagnosis

biopsiedblastomere

‘Chaotic mosiaicism’ in a day-3 embryo

Studied blastomeres

Hypothetical derivative

−3+5+13+14+18−19+20+22+X+Y-

+1+3−18+X+Y

3

1,18,X,Y

−1+3+18−X−Y

lysed blastomere

Courtesy: Lucille Voullaire

+3−3

+3−3

N

+3−3

N

To the placenta: ‘confined placental mosaicism’

†

+3−3

N

To the inner cell mass: true mosaicism of the embryo

†





Four examples:



• Fetal mosaic Trisomy 3 (majority of cells trisomic)


FETAL mosaic TRISOMY 3

FETAL TRISOMY 3





Four examples:



• Fetal mosaic Trisomy 3 (majority of cells trisomic)


Mosaicism for a duplication of terminal 3q: 3q26.3→3qter

HYPOMELANOSIS OF ITO

SOMATIC‐GONADAL MOSAICISM ~ CHROMOSOMAL



• in a cell destined to form somatic and gonadal tissue.

One example:

• Parental low‐level mosaicism for a deletion of chromosome 1q

GONADAL MOSAICISM



• in a mitotic (pre‐meiotic) gametocyte

• Consider the seminiferous tubule total length of 1 km . . . .

1,000/1,000,000,000 = 1 millionth of the tubule has this abnormality

Close to 1 km of tram track in view

A small ($2) coin comprises 1/50,000 of the distance.

GONADAL MOSAICISM ~ CHROMOSOMAL

One example:

• Parental low‐level mosaicism for a duplication ‐‐ dup(21)

mos dup(21)

• Couple, 3 previous miscarriages, 2 previous failed IVF cycles

• IVF/PGD using FISH, 13 embryos, 2 transfers, 0 pregnancies

• Of 13 embryos, 4 showed apparent dup(21)(q22)

• Of 1002 sperm, 66 showed the same apparent dup(21)

Somprasit et al., RBM Online 9, 225,

Cell from embryo Sperm cells

Reprod Biomed Online 9, 225, 2004

6% of the track between here and the tram

GONADAL MOSAICISM ~ MENDELIAN:

Two examples:

• Duchenne muscular dystrophy (gene is dystrophin)

• Achondroplasia (gene is FGFR3)

Gonadal mosaicism for Duchenne muscular dystrophy (gene is dystrophin)

Melis MA, Cau M, Congiu R, Puddu R, Muntoni F, Cao A. Germinal mosaicism in a Duchenne muscular dystrophy family: implications for genetic counselling. Clin Genet. 1993 43:247-9.

Natacci F, Baffico M, Cavallari U, Bedeschi MF, Mura I, Paffoni A, Setti PL, Baldi M, Lalatta F. Germline mosaicism in achondroplasia detected in sperm DNA of the father of three affected sibs. Am J Med Genet 2008 146A:784-6.

Gonadal mosaicism for achondroplasia (gene is FGFR3)

Peripheral blood:

Sperm:

PATHOGENIC MOSAICISM CAN BE:

• SOMATIC

e.g. Segmental NF1, Hypomelanosis of Ito

• overtly SOMATIC‐GONADAL

e.g. Father carries deleted no. 1 chromosome

• GONADAL

e.g. grandfather carries Duchenne, father carries abn chromosome 21

Arvid Lindau (1892 – 1958), a Swedish pathologist, who described the link between the retinal, cerebellar and visceral components of a disease, which he called “angiomatosis of the central nervous system”.

Eugen von Hippel (1867 ‐ 1939), a German ophthalmologist, first described “angiomatosis retinae” in 1911.

VHL ? Somatic mosaic

• 49 y/o man, onset giddiness, ataxia, vomiting loss of consciousness

• MRI cerebellar tumour

• Histo confirms haemangioblastoma

• Second small lesion in R cerebellar hemisphere

• Renal and pancreatic cysts

Cerebellar haemangioblastoma

• No retinal angiomas

• MRI r/v normal adrenal, inner ear anatomy

• Plasma metanephrine normal

VHL:

• Cerebellar/spinal hemangioblastoma

• Retinal angioma

• Pheochromocytoma (incl. extra‐adrenal)

• Renal cancer

• Renal cysts

• Pancreatic cysts

• Epididymal cysts

• Endolymphatic sac tumour

• +ve family hx

• But: on VHL gene sequencing no mutation; andon MLPA, no large deletion/duplication

Note: gene testing has high (97%) detection rate

Why no mutation found?

Possible explanations:

• A VHL‐like ‘phenocopy’ ?

• True VHL, mutation not able to be detected ?

• Somatic (or somatic‐gonadal) mosaicism ?

…… distinction important, in order to be able to advise son


a | Asymmetric fibrous dysplasia (non-neoplastic developmental disease of osseous tissue) of bones and irregular skin hyperpigmentation in a female with MCCUNE–ALBRIGHT SYNDROME101. (Reproduced with permission from REF. 131 © (1993) Oxford University Press).

*

b | Regenerating liver nodules (asterisk) that have a reversion of an inherited mutation in the fumarylacetoacetate hydrolase gene, which causes HEREDITARY TYROSINAEMIA TYPE I.(Reproduced with permission from REF. 22 © (1998) Elsevier Science (USA)


Cytogenetic abnormalities and fragile-x syndrome in Autism Spectrum DisorderKavita S Reddy Genzyme Genetics, Orange CA 92868, USAauthor email corresponding author email

BMC Medical Genetics 2005, 6:3J Med Genet. 1996 Apr;33(4):338-40.Mosaicism for the fragile X syndrome full mutation and deletions within the CGG repeat of the FMR1 gene.Milà M, Castellví-Bel S, Sánchez A, Lázaro C, Villa M, Estivill X.

clinical manifestations of genetic mosaicism

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