Project Three: Biological Determinants of Myotonic Dystrophy Variability

Project Co-Leaders: Johanna I. Hamel M.D. and Charles A. Thornton M.D., University of Rochester

Myotonic dystrophy types 1 (DM1) and 2 (DM2) are the first genetic diseases recognized to result from RNA toxicity, a complex process that reflects an interplay of synthesis and turnover of expanded RNA repeats, and phase separation of repeat RNA with RNA binding proteins into nuclear condensates (RNA foci). DM1 is one of the most variable genetic diseases known to medicine, where onset can occur at any time from mid-gestation to the 7th decade. The DM1 expanded repeat is genetically unstable, resulting in a wide range of different repeat lengths. Clinical variability of DM1 has been attributed to differences of repeat length, reflecting repeats that a person inherits (progenitor allele length) and additional length that arises through age-dependent somatic expansion. While initial studies indicated that clinical variability is mainly attributable to progenitor allele length, this has not been confirmed in recent studies. Further, the severity of involvement for individual muscles is not associated with repeat length in tissue since all adult DM1 patients show large expansions in muscle, regardless of whether the muscle is heavily affected or not. This project will re-assess the association of repeat length with disease severity, using a new platform for clinical assessment, remote study visits (RSVs), and a new molecular assay for repeat length. We posit that while somatic expansion is necessary for DM1 onset and progression, and occurs in all patients, it does not account for between-patient variability. Instead, we postulate that clinical variability reflects differences in how large expansions manifest as RNA toxicity. Discovering the genetic determinants of these differences, will require genome-wide unbiased analyses. However, it is first necessary to clarify the predictive value of repeat length, determine the residual variance that is not attributable to repeat length, and establish new methods to assemble large cohorts and analyze their repeat lengths. We also will investigate the mechanisms for RNA toxicity in DM2, which has added complexity that repeats are very large and located in an intron and thus the aims of this project to: 1) investigate the genetic determinants of DM1 severity by assessing DM1 severity in relation to repeat length in a large cohort and test association of candidate modifiers with DM1 severity; 2) test the RNA toxicity model for DM2 by assessment of splicing defects, CNBP intron 1 retention and nuclear RNA foci, while also evaluating the association of RNA peptides, in DM2 deltoid and tibialis anterior muscles that show weakness.