Allelic heterogeneity of SMARD1 at the IGHMBP2 locus.

Spinal Muscular Atrophy with Respiratory Distress (SMARD) is an autosomal recessive disorder characterized by neurogenic muscular atrophy due to progressive anterior horn cell degeneration and early life-threatening respiratory failure ascribed to diaphragmatic dysfunction. SMARD is clinically and genetically heterogeneous. SMARD type 1 is characterized by onset of respiratory failure within the first weeks of life and has been ascribed to mutations in the immunoglobulin mu-binding protein 2 (IGHMBP2) gene on chromosome 11q13-q21. We report here the identification of nine novel IGHMBP2 mutations in five SMARD1 patients, including seven missense [ c.587A>G (p.Gln196Arg), c.647C>T (p.Pro216Leu), c.752T>C (p.Leu251Pro), c.1693G>A (p.Asp565Asn), c.1730T>C (p.Leu577Pro), c.1807C>T (p.Arg603Cys), c.1909C>T (p.Arg637Cys)] and two nonsense mutations [ c.1488C>A (p.Cys496X), c.2368C>T (p.Arg790X)]. Interestingly, 7 of 9 mutations occurred at highly conserved residues of the putative DNA helicase domain. The identification of novel IGHMBP2 variants will hopefully help diagnosing SMARD1 and contribute to a better functional characterization of IGHMBP2 gene product.