Chapter 28- Hereditary Inclusion-Body Myopathies

The term “hereditary inclusion-body myopathies” (HIBMs) was originally proposed by Askanas and Engel to identify different muscle disorders with autosomal-recessive or dominant inheritance and muscle pathology similar to that of inclusion-body myositis (s-IBM), except for the presence of lymphocytic inflammation. Although clinical presentation varies among different forms, they all have a progressive course leading to severe disability. Typical abnormalities of HIBMs muscle biopsy include the presence of i) myopathic changes with increased scatter of muscle fiber diameter and centralization of myonuclei, ii) muscle fibers containing various-sized and mainly rimmed vacuoles and iii) cytoplasmic and occasionally nuclear 15 to 21 nm diameter tubulofilaments. The most common form of HIBM was originally described in Persian-Jewish (PJ) families (MIM# 600737) and is due to mutations of the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene on chromosome 9, hence the name GNE myopathy. GNE codes for a bifunctional enzyme (GNE/MNK), with both epimerase and kinase activities, that stands centrally in the sialic acid biosynthetic pathway. A homozygous mutation converting methionine to threonine at codon 712 (p.M712T) has been found in all Middle Eastern patients of both PJ and non-PJ descent, thus suggesting a common founder effect GNE mutations are also responsible for distal myopathy with rimmed vacuoles or Nonaka myopathy (DMRV; MIM# 605820) therefore confirming that GNE myopathy and DMRV are the same disease. Similarly to Middle Eastern patients, strong linkage disequilibrium has been demonstrated in the majority of Japanese patients with the homozygous missense mutation p.V572L. Conversely, patients of other ethnic origin are usually compound heterozygous for mutations in different regions of the gene. More than 170 different GNE mutation have been characterized to date.