DUCHENNE MUSCULAR DYSTROPHY

Duchenne Muscular Dystrophy — the most common severe childhood muscular dystrophy, where the specific DMD variant determines which of the emerging exon-skipping therapies a child is eligible for.

Whole genome sequencing reads the entire DMD gene — the largest gene in the human genome at 2.4 million base pairs — capturing all deletion breakpoints, duplications, and deep intronic variants that determine both diagnosis and treatment eligibility.

About this condition

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked recessive neuromuscular disorder caused by pathogenic variants in the DMD gene on chromosome Xp21.2, which encodes dystrophin — a structural protein essential for muscle fiber membrane integrity. Dystrophin deficiency leads to progressive muscle fiber degeneration, chronic inflammation, and replacement of muscle tissue with fibrotic and adipose tissue. DMD is the most common severe childhood muscular dystrophy, affecting approximately 1 in 3,500-5,000 male births worldwide. Onset typically occurs between ages 2-5 years with proximal muscle weakness, manifesting as difficulty running, climbing stairs, and rising from the floor (Gowers sign). Without treatment, loss of ambulation occurs by age 12-13, and death from respiratory or cardiac failure typically occurs in the late teens to mid-twenties.

Approximately 60-70% of DMD-causing variants are large deletions spanning one or more exons, concentrated in two hotspot regions (exons 2-20 and exons 44-55). Large duplications account for 5-15% of variants, and the remaining 20-30% are point mutations (nonsense, frameshift, splice site). The reading frame rule distinguishes DMD from the milder allelic condition Becker muscular dystrophy (BMD): out-of-frame variants that eliminate functional dystrophin production cause DMD, while in-frame variants that produce truncated but partially functional dystrophin cause BMD. This genotype-phenotype correlation guides prognosis and increasingly guides treatment selection.

The DMD treatment landscape has been transformed by genotype-specific precision therapies. Exon-skipping antisense oligonucleotides — eteplirsen (exon 51, ~13% of DMD patients), golodirsen/viltolarsen (exon 53, ~8%), and casimersen (exon 45, ~8%) — restore the reading frame to produce truncated but functional dystrophin, converting a DMD phenotype toward BMD. Ataluren targets nonsense (premature stop codon) variants, which account for approximately 10-15% of cases. Gene replacement therapy (delandistrogene moxeparvovec, approved 2023) uses micro-dystrophin delivered via AAV vector. Each of these therapies requires definitive DMD variant characterization to determine eligibility — making the genetic diagnosis not just diagnostic but directly therapeutic.

The reading frame rule determines whether a DMD variant produces Duchenne (severe, out-of-frame) or Becker (milder, in-frame) muscular dystrophy. Precise breakpoint characterization determines exon-skipping therapy eligibility — a decision that depends on knowing which exons are affected.

Gene locus: DMD (Xp21.2)

Standard deletion/duplication testing identifies the exons involved but not the precise intronic breakpoints. Deep intronic variants causing aberrant splicing are invisible to exon-focused panels. Whole genome sequencing provides the complete variant architecture.

Precise deletion breakpoints determine exon-skipping eligibility — exon-level testing alone is insufficient

MLPA (multiplex ligation-dependent probe amplification) — the standard first-tier DMD diagnostic test — identifies which exons are deleted or duplicated but cannot resolve intronic breakpoint locations with base-pair precision. For exon-skipping therapy eligibility, the exact breakpoint location determines whether the therapeutic exon skip can restore the reading frame. Additionally, complex rearrangements — inversions, non-contiguous deletions, and insertion-deletions — are not reliably detected by MLPA. Whole genome sequencing maps deletion breakpoints to the nucleotide level across all 79 DMD exons and the intervening 2.2 million base pairs of intronic sequence.

20-30% of DMD variants are point mutations that require full-gene sequencing to detect

One-third of DMD patients have point mutations — nonsense, frameshift, or splice site variants — rather than large deletions or duplications. MLPA does not detect point mutations; these patients require full DMD gene sequencing as a second diagnostic step. Among point mutation patients, those with premature stop codon (nonsense) variants may be eligible for ataluren therapy. Deep intronic variants that create cryptic splice sites represent an additional category detectable only by sequencing beyond the coding exons. Whole genome sequencing captures deletions, duplications, point mutations, and deep intronic variants in a single test — eliminating the sequential multi-test diagnostic pathway.

Duchenne Muscular Dystrophy — the most common severe childhood muscular dystrophy, where the specific DMD variant determines which of the emerging exon-skipping therapies a child is eligible for.