Modifying Factors in Striated Muscle Laminopathies

Mutations in the LMNA gene, which codes for lamins A and C, proteins of the nuclear lamina, are responsible for a wide spectrum of pathologies, including a group specifically affecting striated skeletal and cardiac muscles, with cardiac involvement being life-threatening. At the skeletal muscle level, a wide phenotypic spectrum has been described, ranging from severe forms of congenital muscular dystrophy to less severe forms of limb-girdle muscular dystrophy. The great clinical variability of striated muscle laminopathies, both inter- and intra-familial, can be observed in the age of onset, severity of signs and progression of muscle and heart involvement. To date, more than 400 LMNA mutations have been associated with striated muscle laminopathies (www.umd.be/LMNA/), highlighting strong clinical and genetic heterogeneity. A few recurrent mutations linked to a difference in severity have been identified. However, these genotype-phenotype relationships and the rare cases of digenism reported do not explain all the clinical variability of laminopathies. Therefore, there are probably other factors of severity than the causative mutation, called "modifier genes". Identification of such modifier genes has been initiated by studying a large family with significant clinical variability in the age of onset of muscle signs. A segregation analysis within this family identified 2 potential modifier loci. High-throughput sequencing restricted to these 2 regions according to phenotypic subgroups did not led to meaningful results so far. In addition, an international retrospective study of the natural history of early muscle laminopathies has allowed the investigators to highlight a strong inter-family clinical variability in patients carrying recurrent mutations. The investigators thus have strong preliminary data that could allow them to identify modifying genetic factors in a cohort of patients carrying a mutation in the LMNA gene. In order to identify these factors that mo