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Epigenetics Underlying the Parent of Origin Effect in CDM

Inheritance of congenital myotonic dystrophy (CDM) is almost exclusively maternal and, while typically associated with large CTG expansions, is not always genetically differentiated from myotonic dystrophy type 1 (DM1) by repeat tract length. Correlations between CDM/DM1 genotype and phenotype can be improved through evaluation of somatic expansions. Yet it is clear that factors other than germ line repeat length underlie the bias toward maternal inheritance and the heterogeneity of CDM.

The laboratories of Drs. Karen Sermon (Vrije Universiteit Brussel) and Chris Pearson (Hospital for Sick Children) recently collaborated on an epigenetic analysis of the DM1 genetic locus in a cohort of DM1 and CDM patients. Prior reports showed that the DM1 locus resides in a 3.5 kB CpG island with putative CTCF sites, suggesting an epigenetic mechanism for DM1 regulation and disease phenotypes that diverge from CTG length assessments. Earlier reports also established variability in methylation status at that locus in both DM1 patients and DM1 transgenic mice. Drs. Sermon and Pearson hypothesized that CTG expansion might alter CpG methylation status and that a consequent regulatory dysfunction contributes to the severity of the CDM phenotype. 

Drs. Sermon and Pearson and team evaluated multiple generations of several families, including 20 individuals with CDM. Results showed nearly an absolute correlation between the methylation status upstream of the expanded CTG repeat and the occurrence of CDM (19/20 cases). By contrast, this pattern of methylation was rarely found among DM1 patients (2/59 cases). The authors suggest that CpG site methylation is an important contributing factor, with the development of CDM not being determined by CTG repeat length alone. 

Analysis of human embryonic stem cells (hESC) and chorionic villus samples from the study cohort identified upstream CpG site methylation only in maternally-derived samples; paternal samples never showed methylation upstream of expanded DMPK alleles.

Generational increases in both methylation and CTG expansion length were seen in each CDM family studied. Yet since CTG repeat lengths overlapped in DM1 and CDM, while upstream methylation was almost exclusive to CDM, the authors concluded that methylation status is a stronger indicator of CDM than absolute repeat length. Moreover, they speculate that the maternal inheritance bias of CDM may be a consequence of a failed survival of spermatogonia carrying the pathogenic methylation upstream of DMPK. Importantly, while their data suggests that it is rare, the authors do not exclude paternal inheritance for CDM.

Reference

CpG Methylation, a Parent-of-Origin Effect for Maternal-Biased Transmission of Congenital Myotonic Dystrophy.
Barbé L, Lanni S, López-Castel A, Franck S, Spits C, Keymolen K, Seneca S, Tomé S, Miron I, Letourneau J, Liang M, Choufani S, Weksberg R, Wilson MD, Sedlacek Z, Gagnon C, Musova Z, Chitayat D, Shannon P, Mathieu J, Sermon K, Pearson CE.
Am J Hum Genet. 2017 Mar 2;100(3):488-505. doi: 10.1016/j.ajhg.2017.01.033.

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