In a cohort of 961 clinically suspected Duchenne muscular dystrophy patients, 105 were diagnosed to have other muscular dystrophies (OMDs), with LGMD2E (variant SGCB c.544A>C) being the most common

Abstract

Background: Targeted next generation sequence analyses in a cohort of 961 previously described patients with clinically suspected Duchene muscular dystrophy (DMD) revealed that 145/961 (15%) had variants in genes associated with other muscular dystrophies (OMDs).

Methods: NGS was carried out in DMD negative patients after deletion/duplication analysis followed by WES for No variant cases.

Results: The majority of patients with OMDs had autosomal recessive diseases that included Limb-Girdle Muscular Dystrophies (LGMDs), Bethlem, Ullrich congenital Myopathies and Emery-Driefuss muscular dystrophy. 3.5% of patients were identified with other disorders like Charcot-Marie Tooth and Nemaline myopathy. A small percentage of patients, 0.6% remain undiagnosed. Of a total of 78 genetic variants identified, 44 were found to be novel. Interestingly, a third of patients with OMDs were found to have LGMD2E/R4, a severe form of LGMD that afflicts young children with clinical symptoms similar to DMD. Almost one third of the unrelated LGMD2E/R4 patients had the same point mutation (c.544A>C) in the SGCB gene, suggestive of a founder effect, described here for the first time in India.

Conclusion: This study underscores the need for a complete genetic work up to precisely diagnose patients and to initiate appropriate counseling programs, disease management and prevention strategies.

Keywords: Duchenne muscular dystrophy; autosomal recessive disorder; limb girdle muscular dystrophy 2E‐R4; muscular dystrophy; next generation sequencing; overlapping symptoms.

FIGURE 1

Molecular diagnostic strategy used for 961 suspected DMD patients revealed 105 patients with other muscular dystrophies (OMD). An algorithm used to diagnose 961 patients with clinically suspected DMD. mPCR: Mutiplex PCR, MLPA: Multiplex ligation‐dependent probe amplification; NGS: Next‐generation sequencing; WES: Whole exome sequencing. The number of patients at each step are represented in red.

FIGURE 2

Mutational and functional profile of the SGCB gene in patients with LGMD2E. (a) Organization of the SGCB gene showing six exons with cDNA coordinates numbered below. Blue arrows indicate the positions of single mutations identified in patients. Red arrow indicates the most common c.544A>C variant; Pink arrow: shows the position of the c572delT variant. Exons 1and 2 encode the IC domain, exons 2 and 3 in code the trans‐membrane domain; exons 3,4,5 and 6 encode the EC domain and exons 1 and 6 encompass the 5′ and 3′ non‐coding sequences. (b) Comparison of disease severity of different mutations in the SGCB gene based on age of disease onset in years, in the LGMD2E patients. All but 544 a to C refers to all variants other than the c.544A>C. Calculated p value of 0.035 is considered statistically significant.

FIGURE 3

Age at onset and age at diagnosis of LGMDs compared to DMD. Comparison of the ages at onset of disease and ages of disease diagnosis (in years), in patients diagnosed with LGMDs, 2A, 2B, 2E, 2Q and DMD.

FIGURE 4

Patients with two or more variants in the genes associated with OMD, DMD or other diseases. (a) The percentage of patients in the cohort with one, two, three of four variants. (b) The number of patients with two or three variants with specific variants in bona fide genes for DMD, OMD (other muscular dystrophies) and or other disorders (Oth).

FIGURE 5

Comparison of symptoms observed in OMDs versus DMD demonstrates a definite overlap. A ratio of the number of patients with the nine indicated symptoms was compared with those seen in patients with DMD. (Ratio = OMD/DMD). Ratios >1 indicate greater occurrence of the symptom in the specific OMD. OMD: other muscular dystrophy: DMD Duchenne muscular dystrophy.