Molecular Genetics of Dystrophinopathies

Abstract

The term dystrophinopathies includes a spectrum of muscle diseases caused by mutations in the dystrophin ( DMD ) gene. The commonest mutations are intragenic deletions (65% of cases) and duplications (10%); the remaining 25% are small mutations (missense, nonsense, frameshifting and indels). Atypical mutations (deep intronic, or in 5′/3′ untranslated regions) account for no more than 1%. The functional consequences of mutations are related to the maintenance of the open reading frame, though exceptions to the rule are known. A precise molecular characterisation of dystrophin mutations is nowadays mandatory for the inclusion of patients in therapeutic trials, as the antisense‐mediated targeted exon skipping or the stop codon reversion uses gentamycin and ataluren drugs. Understanding the complexity of the dystrophin gene regulation has provided outstanding information about muscle functions, sarcolemma architecture and signalling circuits, muscle metabolism and other basic science mechanisms. Key Concepts: The dystrophin gene ( DMD ) is one of the largest disease genes in the human genome. Dystrophin deficiency accounts for different allelic conditions: Duchenne and Becker muscular dystrophies (DMD/BMD) are X‐linked recessive hereditary myopathies in which the skeletal muscle is affected with a nonreversible, age‐dependent degeneration. Allelic dystrophin mutations can also give rise to isolated cardiac involvement, or X‐linked dilated cardiomyopathy (XLDC). The commonest mutational event in the dystrophin gene is represented by intragenic deletions accounting for 65% of dystrophin mutations. Intragenic duplications account for 10% of all mutations and small mutations represent 25%. The functional consequences of all mutation types are mainly related to the maintenance of the open reading frame: ‘inframe’ mutations result in a shorter but partially functional dystrophin and are associated with BMD. Frame‐shift mutations are related to the absence of protein production and a DMD phenotype. The fine genetic characterisation of dystrophin mutations has become mandatory for making patients eligible for novel personalised trials.

Duchenne Muscular Dystrophy: From Diagnosis to Therapy

“…Among the quantitative methods available, multiplex ligation-dependent probe amplification (MLPA) is currently the most widely used. This method, testing simultaneously all 79 exons of the DMD gene, identifies the copy number variation (CNV) in a multiplex polymerase chain reaction based reaction [16].…”

“…In general, these approaches use an in vitro clonal amplification or PCR step to amplify the DNA molecules present in the sample, followed by the alternative approaches of massively parallel sequencing based on pyrosequencing (454 by Roche Applied Science, Inc. Indianapolis, IN, USA), reversible dye-termination (Illumina, Inc. San Diego, CA, USA), sequencing by ligation (SOLiD by by Life Technologies, Inc. Carlsbad, CA, USA) and Ion semiconductor sequencing (Ion Torrent System by Life Technologies), as main examples. These platforms are currently being used in clinical laboratories for molecular diagnosis by a target capturing the regions of genomic interest; indeed different studies applying NGS have proven its ability in detecting small mutations in the DMD gene [16,19].…”

A Comprehensive Review of Duchenne Muscular Dystrophy: Genetics, Clinical Presentation, Diagnosis, and Treatment

“…Distal rod deletions are usually present with BMD phenotypes. However, DMD usually results from deletions of the C terminus domain [45]. DMD patients' western blots display quantitative evidence of little or no dystrophin protein present in their cells.…”

Do NGS‐based techniques represent a first‐line testing in suspected Duchenne muscular dystrophy?

“…In the absence of dystrophin, the permeability of the cell membrane increases, allowing intracellular creatine kinase and intracellular calcium to enter the serum. Persistent inflammation is initially associated with necrosis and hypertrophy, followed by progressive loss of regeneration and muscle degeneration, which is characteristic of DMD 8 . Duchenne muscular dystrophy is a rapidly progressing disease, and all patients usually need to use a wheelchair by age 10; DMD patients develop a severe type of cardiomyopathy, which generally appears at age 10, and most of them die due to heart diseases and respiratory disorders 9,10 .…”

“…Persistent inflammation is initially associated with necrosis and hypertrophy, followed by progressive loss of regeneration and muscle degeneration, which is characteristic of DMD. 8 Duchenne muscular dystrophy is a rapidly progressing disease, and all patients usually need to use a wheelchair by age 10; DMD patients develop a severe type of cardiomyopathy, which generally appears at age 10, and most of them die due to heart diseases and respiratory disorders. 9,10 The average life expectancy in patients with Duchenne muscular dystrophy is between 20 and 30 years.…”