Exome sequencing identifies a DNAJB6 mutation in a family with dominantly-inherited limb-girdle muscular dystrophy

Abstract

Limb-girdle muscular dystrophy primarily affects the muscles of the hips and shoulders (the "limb-girdle" muscles), although it is a heterogeneous disorder that can present with varying symptoms. There is currently no cure. We sought to identify the genetic basis of limb-girdle muscular dystrophy type 1 in an American family of Northern European descent using exome sequencing. Exome sequencing was performed on DNA samples from two affected siblings and one unaffected sibling and resulted in the identification of eleven candidate mutations that co-segregated with the disease. Notably, this list included a previously reported mutation in DNAJB6, p.Phe89Ile, which was recently identified as a cause of limb-girdle muscular dystrophy type 1D. Additional family members were Sanger sequenced and the mutation in DNAJB6 was only found in affected individuals. Subsequent haplotype analysis indicated that this DNAJB6 p.Phe89Ile mutation likely arose independently of the previously reported mutation. Since other published mutations are located close by in the G/F domain of DNAJB6, this suggests that the area may represent a mutational hotspot. Exome sequencing provided an unbiased and effective method for identifying the genetic etiology of limb-girdle muscular dystrophy type 1 in a previously genetically uncharacterized family. This work further confirms the causative role of DNAJB6 mutations in limb-girdle muscular dystrophy type 1D.

Keywords: DNAJB6; Exome sequencing; Limb-girdle muscular dystrophy.

Figure 1. Mutations in DNAJB6 Cause LGMD1D

(A) An American family of Northern European descent with LGMD1. Exome sequencing was performed in individuals marked with a red asterisk, Sanger sequencing in those with a black asterisk. Proband is indicated by black arrowhead. Affected individuals are colored black and white to indicate dominant inheritance. (B) A c.265T>A mutation was discovered only in affected patients in the DNAJB6 gene, resulting in a p.Phe89Ile amino acid substitution.

Figure 2

Histopathological examination of LGMD1D muscle biopsy. (A) Low magnification photomicrograph of hematoxylin and eosin stained cryosection of biceps muscle biopsy, showing marked variation in fiber sizes, and a fiber with several rimmed vacuoles (arrow), highlighted in modified Gomori trichrome stain (inset, arrow), and scattered angular atrophic fibers (arrowheads). Hematoxylin and eosin, scale bar = 100 microns. Inset: High magnification view of region indicated by arrow, scale bar = 25 microns. (B) High magnification view of biceps muscle biopsy cryosection showing moderate variation in fiber size, a minimal increase in endomysial connective tissue, and a fiber containing numerous rimmed vacuoles (arrow). Modified Gomori trichrome, scale bar = 50 microns. (C) Higher power photomicrograph showing overly dark angular atrophic fibers typical of neurogenic atrophy (arrows). NADH enzyme histochemistry, scale bar = 100 microns.