FEATURE FEATURE portant information for patient management and for estimating recurrence risk. Molecular diagnostic testing for abnormal DNA methylation in the relevant imprinted domains can be done for a number of imprinting disorders and is already widely applied in PWS/ AS and BWS/SRS. The majority of molecular alterations within imprinting domains can be identified by alterations in DNA methylation in the respective imprinting control regions. A few retrospective studies have shown an increased incidence of epigenetic abnormalities causing both BWS and AS following the use of assisted reproductive technologies (ART). Although the increased relative risk is small for these DNA methylation errors these data highlight the importance of understanding the mechanisms behind genomic imprinting. The number of imprinted genes in human and mouse is currently just over 100 imprinted transcripts. Approximately 63 of these have been identified in humans. We designed experiments using uniparental tissues and DNA methylation at known imprinted genes to identify new imprinted genes5. We took advantage of two uniparental tissues; complete androgenetic hydatidiform moles (CHMs) and mature cystic ovarian teratomas. CHMs have two copies of each paternal chromosome and no maternal chromosomes. Mature cystic teratomas, on the other hand, have two copies of the maternal genome. Analyzing the genome-wide DNA methylation patterns in these tissues and comparing them to normal biparental tissue we identified a number of candidate imprinted genes and validated one of those genes both mouse and humans5. An expanded set of known imprinted genes could lead to the identification of the molecular cause of disorders of unknown etiology.
“Identifying the epigenetic marks and characterizing how they are read to regulate the expression of the primary genomic sequence is necessary for our understanding of human development and disease.�
The laboratory is also interested in the epigenetic contribution to intrauterine growth restriction (IUGR), a heterogeneous disorder in which babies are born with a birthweight less than the 10th centile for gestational age. IUGR has been associated not only with significant maternal and fetal/neonatal mortality and morbidity but also with adult-onset disorders such as hypertension, coronary artery disease, and type 2 diabetes. DNA methylation alterations have been shown to drive increased or decreased placental and fetal growth. By determining DNA methylation patterns in the placenta of children born small for gestational age, we identified that gain of methylation in WNT2 was significantly associated with reduced WNT2 expression in placenta and with low birthweight percentile in the neonate6. This gene has been demonstrated to have important function in mouse placental development. These data suggest that WNT2 expression can be epigenetically downregulated in the placenta by DNA methylation of its promoter and that high WNT2 promoter methylation is an epigenetic variant that is associated with reduced fetal growth potential6. We expect that future studies of the epigenome will elucidate other candidate genes that undergo epigenetic dysregulation and negatively impact placental and fetal health. The second focus in the Weksberg laboratory is the investigation of DNA methylation alterations in paediatric neurodevelopment and neuropsychiatric disorders. A number of neuropsychiatric disorders have been described with mutations or deletions in genes that are important for maintaining normal epigenetic regulation. Loss of function of these genes can disrupt normal establishment, maintenance, or reading of epigenetic marks, thereby resulting in altered chromatin structure and gene expression. In most disorders of this type, we still do not understand precisely how the mutation is related to the phenotype of the human disease. Many of these disorders are associated with intellectual disability (ID), as well as additional features including various congenital anomalies. The identification of alterations in DNA methylation associated with mutations in specific genes that function in epigenetic regulation will teach us more about what the responsibility of each epigenetic modifier is in the normal patterning of the epigenome. Other paediatric disorders we are investigat-
ing include autism spectrum (ASD), obsessive compulsive (OCD) and attention deficit hyperactivity (ADHD). For each of these disorders there have been genetic factors identified which explain a small proportion of such cases. We have proposed that epigenetic factors also contribute to the etiology of these disorders. These studies are all in the initial stages but we already have a number of interesting and encouraging results. We are currently investigating a number of candidate genes and pathways that may be relevant to these disorders. The field of epigenetics is generating exciting discoveries in parallel to genome sequencing initiatives. The NIH Roadmap Epigenomics Mapping Consortium was launched to produce a public resource of human epigenomic data to catalyze basic biology and disease oriented research7. Parallel initiatives include the NIH Epigenomics of Health and Disease Roadmap Program and CIHR Canadian Epigenomic Mapping Centres. These initiatives interface with the International Human Epigenomics Consortium, which was established to accelerate and coordinate epigenomics research worldwide8. This is an exciting time for all researchers involved in epigenetic research as we work towards deciphering the language of the epigenome at an exponential rate.
References 1. Berger, S.L., Kouzarides, T., Shiekhattar, R. & Shilatifard, A. An operational definition of epigenetics. Genes & development 23, 781-3 (2009). 2. Weksberg, R. Imprinted genes and human disease. American Journal of Medical Genetics Part C: Seminars in Medical Genetics 154C, 317-320. 3. Choufani, S., Shuman, C. & Weksberg, R. Beckwith– Wiedemann syndrome. American Journal of Medical Genetics Part C: Seminars in Medical Genetics 154C, 343-354. 4. Weksberg, R., Smith, A.C., Squire, J. & Sadowski, P. Beckwith-Wiedemann syndrome demonstrates a role for epigenetic control of normal development. Human molecular genetics 12 Spec No 1, R61-8. (2003). 5. Choufani, S. et al. A novel approach identifies new differentially methylated regions (DMRs) associated with imprinted genes. Genome research 21, 465-76. 6. Ferreira JC, C.S., Keating S, Chitayat D, Grafodatskaya D, Shuman C, Kingdom J, and Weksberg R. WNT2 promoter methylation in human placenta is associated with low birthweight percentile in the neonate. Epigenetics (2011). 7. Bernstein, B.E. et al. The NIH Roadmap Epigenomics Mapping Consortium. Nature biotechnology 28, 1045-8. 8. Eckhardt, F., Beck, S., Gut, I.G. & Berlin, K. Future potential of the Human Epigenome Project. Expert review of molecular diagnostics 4, 609-18 (2004).
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