Autosomal Recessive Limb-Girdle Muscular Dystrophies

Abstract

Limb-girdle muscular dystrophy (LGMD) is a hereditary condition primarily affecting skeletal muscle, characterized by progressive, predominantly proximal weakness of the muscles of the pelvic and shoulder girdles, hyperCKemia, degenerative changes over the disease course on muscle imaging and dystrophic changes on muscle biopsy. There is phenotypic and genotypic variability among the disorders caused by different genes. LGMDs are classified by the patterns of inheritance into autosomal dominant LGMD (AD-LGMD) and autosomal recessive LGMD (AR-LGMD). The initial LGMD nomenclature system used “LGMD1” to indicate AD-LGMD and “LGMD2” to indicate AR-LGMD. An English alphabet assigned in order of discovery of the causative genes was placed as a suffix. The new nomenclature system uses the abbreviation LGMD-D for AD-LGMD and LGMD-R for AR-LGMD, followed by a number indicating the causative gene, in order of their discovery. This is because the rapidly growing list of LGMD-causative genes, especially for AR-LGMD, exhausted all the English alphabets. The prevalence of each LGMD-R subtype varies from country to country; however, LGMD-R1 (calpain 3-related LGMD) and LGMD-R2 (dysferlin-related LGMD) are considered the most common subtypes worldwide. Alpha-dystroglycanopathy is the largest group of LGMD-R, encompassing 12 genetically distinct subtypes due to mutations in genes encoding proteins involving in glycosylation of α-dystroglycan. This chapter will focus on the new nomenclature system and phenotypic variability of AR-LGMDs.