Inclusion Body Myopathy With Early-Onset Paget Disease With Or Without Frontotemporal Dementia 3

A number sign (#) is used with this entry because of evidence that inclusion body myopathy with Paget disease (IBMPFD3) is caused by heterozygous mutation in the HNRNPA1 gene (164017) on chromosome 12q13. One such family has been reported.

Heterozygous mutations in the HNRNPA1 gene also result in amyotrophic lateral sclerosis (ALS20; 615426).

For a general phenotypic description and a discussion of genetic heterogeneity of IBMPFD, see IBMPFD1 (167320).

Clinical Features

Kottlors et al. (2010) described a German family with 5 affected sibs with a limb girdle muscular dystrophy characterized by progressive predominantly proximal muscle weakness, mildly elevated serum creatine kinase levels, myopathic findings on muscle biopsy, and Paget disease of the bone. All affected individuals showed a pattern of muscle weakness beginning in the lower proximal extremities and later spreading to the foot dorsiflexors. Muscles of the abdominal wall and iliopsoas muscle were severely affected. Scapulae were only slightly winged, and upper extremities showed no weakness except in 1 severely affected patient. Although age of onset was between 35 and 43 years, affected members recalled having been slower and clumsier than peers as children. No patient had cognitive impairment. Histology showed myopathic changes with rimmed vacuoles and inclusion bodies on muscle biopsy. There was variability regarding Paget disease of bone; of the 3 severely affected and 2 more mildly affected members, only 2 had laboratory and radiologic evidence of Paget disease. The affected mother was deceased. The pedigree strongly suggested autosomal dominant inheritance.

Kim et al. (2013) confirmed that none of the affected members in the family reported by Kottlors et al. (2010) (family 2 of Kim et al. (2013)) had motor neuron dysfunction or cognitive impairment. Family member IV9 underwent a muscle biopsy, which showed atrophic fibers, central nuclei, and rimmed vacuoles characteristic of inclusion body myopathy.

Molecular Genetics

In a family with IBMPFD (family 2, originally reported by Kottlors et al. (2010)) in which mutations in VCP (601023) and other myopathy-related genes had been excluded, Kim et al. (2013) detected a missense mutation in the HNRNPA1 gene (164017.0001) that substituted a valine for a highly conserved aspartate that is centered in a motif, the prion-like domain (PrLD), conserved in multiple human paralogs of the HNRNP A/B family. The mutation was predicted to enhance prion-like behavior of the motif.

By sequencing coding exons of the HNRNPA1 gene, Le Ber et al. (2014) failed to identify pathogenic mutations in a cohort of 17 unrelated French patients with sporadic or familial occurrence of multiple system proteinopathy manifest as frontotemporal lobar degeneration (FTLD) and/or amyotrophic lateral sclerosis (ALS) that segregated with Paget disease of bone (PDB), and/or inclusion body myositis (IBM). No mutations were found in 60 probands with FTLD or FTLD/ALS. By sequencing the prion-like domain of the HNRNPA1 gene, Seelen et al. (2014) also failed to identify any nonsynonymous mutations in 135 patients with familial ALS, 1,084 patients with sporadic ALS, 68 patients with familial FTLD, 74 patients with sporadic FTLD, and 31 patients with sporadic IBM. All patients were from the Netherlands. The findings of both studies suggested that mutations in HNRNPA1 are a very rare cause of this spectrum of diseases.