GENOMIC IMPRINTING

Mir Mehraj M.V.Sc Scholar

Animal Biotechnology

Introduction The differential expression of genetic material, at either

chromosomal or allelic level, depending on whether the genetic material has come from the male or female parent

(Hall et al, 1990)

An epigenetic form of gene regulation that results in only the copy inherited from father or mother to function.

(Jirtle and weidman et al,2007) It is specific for mammals (eutherians and marsupials) in vertebrates. ( Killian et al , O Niel et al 2000 ,Hore et al 2007)

The first description of the imprinting phenomenon was given by McGrath and Solter in 1984

Some genes are presumably modified during gametogenesis in such a way that only paternal or the maternal alleles are expressed after fertilization

Imprinting occurs by Epigenetic Mechanisms ( Delaval & Fiel et al 2004)

Imprinted genes represent less than 1% of the mammalian genome.

Igf2 (Paternally expressed) is the first imprinted gene that was identified in mouse.

( De Chaira et al , Robertson et al 1991)

A gene can behave as imprinted in one tissue and

be biallelically expressed in another (Solter D ., 1998)

Theories about Origin of GI

Parent-Offspring Conflict Theory / Kinship theory

Genetic interests of parent and offspring are different, therefore the offspring would manipulate the parents to ensure survival and vice versa. (Robert L. Trivers ., 1974)

Kinship theory is an gene centered extension to the Parental-Offspring conflict theory by David Haig (genetic conflicts during pregnancy)

Paternally derived genes try to extract greater resources from the mother. In turn, the mother tries to ensure equal distribution to all her offspring

Ligers v/s Tiglons

Ligers and Tiglons are progenies that come from matings between lions and tigers

Ligers: father is a lion and mother is a tiger

Tiglons: father is a tiger and mother is a lion

Different imprinted gene between the mother and father causes difference in size and appearance in size between ligers and tiglons

Female Liger Male Tiglon

Ovarian time bomb Theory (OTH)

Imprinting evolved in mammals to prevent

spontaneous development of unfertilized eggs and also trophoblastic disease of the ovaries (Varmuza and Mann et al, 1994)

Ovarian Teratomas (embryos without paternal genome) provide ground to this theory.

Mechanisms of Imprinting

DNA Methylation

Attachment of methyl (-CH3) groups to the bases of DNA.

Occurs at cytosine that follows guanine at CpG dinucleotides

DNA METHYL TRANSFERASE

There are 5 known methyl transferase enzymes

Dnmt1

Dnmt2

Dnmt3a

Dnmt3b

Dnmt3L

Dnmt3a, Dnmt3b de novo methyl transferases

Dnmt3b Methylation of centromeric satellite repeats

Dnmt1 & Dnmt2 Maintenance of

methylation

Dnmt3L Interact with 3a and 3b to stimulate methyl transferase activity

Igf2-H19 Insulator Model

Cluster containing maternally expressed H19 and paternally expressed Igf2

This cluster resides at 11 p 15.5 in humans

Regulated by an ICR designated imprinting center 1 (IC1) in humans and ICR or Differentially Methylated Domain (DMD) in mouse

Proper imprinting of H19 and Igf2 requires that the ICR/DMD is methylated on the paternal allele and unmethylated on the maternal allele

Non-coding RNAs A significant number of imprinted genes are

transcribed to give a non-coding RNA.

Non-coding RNAs include antisense transcript, small nucleolar RNAs (Sno RNAs), micro RNAs, pseudo genes and other RNA of unknown function

Random X-inactivation is associated coating of X chromosome with Xist (Plath K et al., 2003)

Imprinted X inactivation

In the postimplantation embryo, random XCI. (Bourmil & Lee et al., 2002)

Random and imprinted XCI are controlled by X-chromosomeinactivation center (XIC). ( Rougelle et al 2003) Components of the XIC are the Xist and Tsix genes,which

encode long nc RNAs , Xist & Tsix respectively.

Xist –Tisx & chromatin modification bring about X-inactivation

Histone modification & chromatin remodeling

Histone modifiations includes Acacetylation of lysines (HATs), Phosphorylation of serines (Kinases) and Methylation of lysines

Methylation of lysine-4 in H3 is associated with active genes and methylation of lysine-9 in H3 is associated with inactive genes

The allele-specific gene silencing in H19 is in part mediated by hypermethylation and histone deacetylation

(Pedone PV et al., 1999)

Genomic imprint cycles in Embryo

Erasure: Old imprint is totally erased at an early time-point in the PGCs of the developing foetus between 10.5 and 12.5 days post-coitus in mice (Hajkova et al.,2000)

Establishment : Male: Postnatally within diploid gonocytes prior to meiosis

Female: PGCs are arrested in diplotene stage (13.5 days of embryonic life) and not methylated until birth and

Methylation occurs during oocyte growth Maintenance: Imprinting of the embryonic cells is maintained

throughout life

Immediately after fertilization, the zygote faces a wave of global demethylation event, first in male pronucleus, followed by maternal pronucleus.

Imprint marks that were established in the gametes must resist this demethylation process.

Remethylation of the diploid genome occurs during gastrulation.

These imprints are then maintained throughout the life span of the Individual (Autran D et al.,2002)

Female:

In the oocytes, methyl transferases belonging to Dnmt3 family are required to set maternal specific methylation patterns for imprinted genes in mice

Dnmt3a, Dnmt3b and Dnmt3L seem to be operational here

Dnmt3L lacks a methyl transferase activity probably provides

sequence specificity for the other de novo methyl transferases, Dnmt3a and Dnmt3b, by directing them to the DNA region requiring normal methylation patterns. (Hata k et al., 2001)

Imprinting Control Region(ICR)

Regulates the allele-specific activity of imprinted genes in the cluster.

ICRs usually carry a germline derived methylation imprint.

Common feature they share is that they have a relatively high level of CpG dinucleotides and have simple sequence repeats in the vicinity

Also called ‘differentially methylated region’ (DMR) and ‘differentially methylated domain’ (DMD

Uniparental disomy (UPD)

Uniparental disomy (UPD) has been described for chromosomes 5, 6, 7, 9, 11, 13, 14, 15, 16, 21, 22 and the XY pair

(Petersen et al., 1992; Engel, 1993; Brzustowicz et al., 1994)

Reported cases of Cystic Fibrosis patients with maternal UPID for chromosome 7 (Spence et al., 1988; Voss et al., 1989)

Reported a child with SMA (Spinal muscular atrophy ) who inherited 2 copies of chromosome 5 from his father and none from his mother

(Allitto et al., 1993; Brzustowicz et al., 1994)

Imprinted Genes

SPECIES NUMBER OF GENES

Human 305

Mouse 179

Pig 81

Cattle 28

Sheep 20

Birds 18

Horse 3

Dog

Cat

Rabbit

Primates

Dog 1

Cat 1

Rabbit 1

Primates 2

Genomic Imprinting in Diseases

• Prader-Willi Syndrome First described by Prader et al ., 1956

1 in 14,000

Can be due to

• Deletion of the qll-13 region of the paternal chromosome 15 (Cassidy,

1992)

• Due to maternal UPD with a lack of paternal chromosome 15 (Nicholls et al., 1989)

Angelman syndrome

Deletion of the qll-13 region of the maternal chromosome 15 (Pembrey et al., 1989)

From paternal UPD . (Malcolm et al., 1991)

Silver Russel Syndrome

Maternal UPD of Igf2 & H19 genes on chromosome 7.

Beckwith-Wiedemann syndrome

Is a fetal overgrowth syndrome associated with Wilm’s tumor, rhabdomyosarcoma etc.

Paternal disomy of chromosome 11p15.5

Imprinted genes IGF2 and H19

Over expression of IGF2 and lack of H1

Cancer Igf2 loss of imprinting leads to wilm`s tumor. ( Jirtle et al ,1999)

Igf2 loss of imprinting a potential biomarker for colo rectal cancer predisposition .

(Cruz et al: M Cui et al , 2004 )

Hepatocellular carcinomas show Igf2R loss of imprinting.

(Angus t dsouza et al, 2004)

GI and Environment

You are what your mother ate.

Dietary supplementation of mice with extra folic acid, vitamin B12 ,choline alter the phenotype of their offspring via increased DNA methylation.

(Robert waterland et al,2007)

So a possible role of diet in genomic imprinting. ( Jirtle et al , 2008)

CONCLUSIONS

Genomic imprinting is an epigenetic modification.

It can be both of an advantage or disadvantage.

It can provide answers to some intractable questions surrounding gene regulation

Some questions still remain unanswered: Do we really need imprinting? Can we do away with imprinted genes?

•Thank You