GENOMIC IMPRINTING

1. GENOMIC IMPRINTING What is an imprinted gene? Using Prader-Willi and Angelman syndrome as an example describe the imprinting control centre and explain how the imprint is maintained or erased within the germ line. What are the clinical and diagnostic implications of errors in this process? ELAINE WHITFIELD

2. What Is an Imprinted Gene? • A gene which is expressed from only one allele in a parent-specific manner due to epigenetic modification • Mouse embryo manipulation in vitro provided first clear evidence for the phenomenom of gametic imprinting • Paternal genome more important for proliferation of the extraembryonic tissues • Maternal genome more important for post-implantation embryonic development

3. Selective breeding studies in mice and naturally occurring cases of UPD in humans have shown that only a minor proportion of the genome is imprinted • ~80 human genes have been determined to be imprinted to date • Imprinted genes are clustered within the genome • Imprinting centres - specific DNA sequences identify some of these clusters as targets of the imprinting machinery of the gamete

4. How Does Imprinting Occur? • Genes are ‘stamped’ so that the expression of the inherited information is modified according to whether it is passed on via the egg or sperm • Differential allele-specific DNA methylation is present in imprinted regions – localised to differentially methylated regions (DMRs) • The Prader-Willi / Angelman region on chromosome 15 is a well-characterised imprinted region

5. Prader-WilliSyndrome • PWS - ~1 in 20 000 births – failure to thrive during infancy, hyperphagia and obesity during early childhood, mental retardation & behavioural problems • Due to the loss of paternally expressed genes in ~2Mb imprinted domain at 15q11-q13 • No single gene has been shown to cause PWS when it’s expression is lost • PWS Imprinting Control Region (ICR) mapped to a segment of ~4kb that spans 1st exon and promoter of SNRPN/SNURF locus • 70% cases of PWS due to paternal deletions in 15q11-13 region • 20-30% cases due to maternal UPD • 5-10% cases due to paternally derived microdeletions of the ICR, loss of imprinting at the ICR (de novo methylation of the paternal allele) or balanced translocations that disrupt SNURF/SNRPN gene

6. Angelman Syndrome • AS - ~1 in 15 000 births – mental retardation, speech impairment & behavioural abnormalities • AS arises from the loss of a single gene – maternally expressed UBE3A – which is imprinted only in the brain • 65-70% cases maternally derived deletions of 15q11-q13 • 5% cases caused by paternal UPD • 10% cases maternal UBE3A mutations • 5% cases imprinting defects • 10% caused by an unknown defect

7. The PWAS region of chr. 15q11-q13

9. Erasure & re-establishment of the imprint in the germ line

10. Diagnosis of Prader-Willi / Angelman Syndrome • DNA methylation testing to detect abnormal parent specific imprinting will detect >99% of PWS and ~75% of AS affected individuals • If paternal expression is shown to be absent = PWS • If maternal expression is shown to be absent = AS • If normal methylation in ? AS direct mutation analysis of UBE3A may be considered

11. 174bp 100bp

12. If PWS or AS are detected it is important to determine the mechanism responsible De novo deletions and UPD have a very low recurrence risk Imprinting mutations have a recurrence risk of up to 50% UBE3A mutations have recurrence risk as high as 50% AS cases with no cause identified-recurrence risk is high and no testing available

13. In Conclusion: • Imprinting is a complex process which is critical for normal development • Few genes are imprinted & these are clustered • Defects in imprinting are implicated in abnormal fetal growth and disease. • Mechanism is important to discover as recurrence risk may be very high