Dismantling Limb-Girdle Muscular Dystrophy: The Role of Whole-Exome Sequencing.

Muscular dystrophy encompasses a diverse group of genetically determined muscle disorders. The first clinical description of the disorder is attributed to Giovanni Semmola, who, in 1829, described 2 boys affected by a disorder with prominent muscular hypertrophy.1 Between1850and1868,Aran, Meryon, and Duchenne described a progressive atrophy of voluntarymuscles, ultimately termed pseudohypertrophic muscular paralysis of children by Duchenne.1,2 Other descriptions followed: familial atrophy of the pelvic girdle muscles (Leyden in 1876), scapulohumeral muscular atrophy (Erb in 1884), andmyopathy with facial weakness (Landouzy and Dejerine in 1884).1 The term limb-girdle muscular dystrophy (LGMD), suggested by Stevenson in 1953,3 and further detailed byWalton and Nattrass in a seminal article,2 refers to a group of muscular dystrophies with onset of weakness in the shoulder or pelvic girdles.4 The variable clinical course of this disorder was recognized even in these early descriptions.2,3 Traditionalneurologyemphasizestheroleofthehistoryand clinical examination in arriving at a diagnosis. The clinical approach to patients with suspected myopathy includes obtaining a history of symptom evolution, comprehensive pedigree analysis, evaluation of the distribution ofweakness, and identificationofextramuscularmanifestations.Neuromuscularneurologistsoftenusea“patternrecognition”approach,whichclassifiesmuscleweakness intospecific schema: limbgirdle,distal, humeroperoneal, etc. There is value to this clinical approach, which allows one to narrow the diagnosis down to a few conditions, or even a single condition, to inform subsequent confirmatory testing. A young boy with onset of weakness in the first decade, calf hypertrophy, Gowers maneuver, and a similarly affected maternal uncle is likely to have Duchenne muscular dystrophy. For a youngadultwithmyotonia, ptosis, temporalis atrophy, distal limb weakness, and a family history suggesting autosomal dominant inheritance, clinical suspicionofmyotonicdystrophy leads to confirmatorygenetic testing. However, genetic heterogeneity (ie, the existence of similar phenotypes due to different genetic mutations) limits the diagnostic capacity of phenotypic classifications. In1987, thediscoveryofthegeneticdefectandproteinproduct, dystrophin, in Duchennemuscular dystrophy revolutionized the approach tomuscular dystrophies.5 Aneraofmolecular diagnoses was born. As the search for genetic causes for neuromuscular disorders continued, it became apparent that LGMD was not a single disease but likely represented several disorders unified by their phenotype. After reports of autosomal recessive LGMD from the 1950s on, Fardeau and colleagues, in1989,describedanautosomal recessiveLGMDonthe French island of Réunion; in 1991, this disorder was found to link to chromosome 15q, and in 1995, it was identified as calpainopathy.6 In 1986,a reportofautosomaldominantLGMD established thegeneticheterogeneityof this syndrome.7Asubsequent family with autosomal dominant LGMD reported in 1988was found tohave a type of LGMD that linked to chromosome5q in 1992, and in2000, this familywas identifiedashaving amutation in themyotilin gene.8Agenetic classificationof LGMDs was proposed in 1995, based on the inheritance pattern: Type 1 LGMDs are autosomal dominant, and type 2LMGMDsare autosomal recessive.A letter defining theorder ofdiscovery of the chromosomal locus is appended to the numeric designation.9 The list of LGMDs with known genetic loci continuestogrowrapidly(8autosomaldominantLGMDs[LGMD1A1H) and 23 autosomal recessive LGMDs [LGMD2A-2W]).10 A definitive diagnosis of the type of LGMD is important; it avoids repeated testing or empirical, potentially toxic treatments for acquired causes such as inflammatory myopathy; provides patients with a sense of closure; assists genetic counseling; and aids the identification and treatment of complications.11 In the future, knowledgeof theunderlyinggeneticmutationwill benecessary forenrollment inclinical trials of targeted therapies. However, the genetic heterogeneity of the LGMDphenotypemakes establishing a definitive diagnosis challenging. The approach to diagnosis is complex and includes evaluation of the inheritance pattern and identification of specific clinical features that direct further testing.11 Conventional genetic testing formendeliandisorders such as LGMD involve genome-wide linkage to identifymutations that cosegregate within affected individuals, positional cloning, and, finally, targeted candidate gene sequencing.12 In autosomal recessive disorders, autozygosity mapping, which identifies regions of the genome that are homozygous in affected individualsbutnot inunaffected familymembers, is followed by gene sequencing to identify the causal mutation.13 These techniques assume the availability of several affected familymembersandareof limitedutilitywhenonlya fewcases are available or in sporadic cases owing to de novo mutations. In addition, genetic and phenotypic heterogeneity and incomplete penetrance limit these approaches.13 TheDNAsequencing techniquedescribedbySanger et al14 was used to sequence the human genome in 2001 (theHuman GenomeProject).15 In2004,next-generationsequencingmethods (whichusehigh-throughput,massivelyparallel sequencing platforms)were introduced,making itpossible tosequencesevRelated article page 1424 Opinion