DMPK Hypermethylation in Sperm of Myotonic Dystrophy Type 1 Patients

Introduction: Myotonic dystrophy type 1 (DM1) is the most common autosomal dominant myopathy in adults (1 in 8,000 individuals). It results from of an unstable tri-nucleotide CTG repeat expansion (50 to 4,000 copies) located in the 3`-untranslated region of the dystrophia myotonica-protein kinase gene (DMPK). With rare exception, it is affected mothers who transmit the congenital and most severe form of the disease (CDM1, >1,000 repeats). Why CDM1 is hardly ever transmitted by fathers remains unresolved. Recently, researchers proposed a model of selective disadvantage to sperm cells with aberrant methylation in DMPK to explain why CDM1 is hardly ever transmitted by fathers.

Aim: The aim of this study was to challenge this model by determining whether DMPK is hypermethylated in sperm cells of DM1 male patients.

Methods: To determine the methylation state of DMPK when repeat expanded, we tested sperm from 5 patients undergoing preimplantation genetic diagnosis (PGD) procedures for DM1, with 300 to 1,000 CTG expansions. Methylation analysis was carried out by DNA bisulfite colony as well as locus-specific deep-sequencing to a restricted region located 650bp upstream to the CTGs.

Results: We found abnormal methylation levels of 26% and 60% by bisulfite DNA deep-sequencing in viable sperm cells of DM1 male patients. This was further validated by bisulfite allele-specific colony DNA sequencing (26 CpG sites).

Conclusions: The finding of this research rule out the proposition that DMPK hypermethylation leads to selective disadvantage of sperm cells in DM1 male patients. An alternative model should be suggested for the maternal inheritance of CDM1.