MYOTONIC DYSTROPHY

Myotonic Dystrophy — the most common adult muscular dystrophy, caused by unstable DNA repeat expansions that grow with each generation and produce a multi-system disorder affecting muscle, heart, brain, and endocrine function.

Whole genome sequencing characterizes CTG and CCTG repeat expansions in DMPK and CNBP, distinguishing myotonic dystrophy types 1 and 2, estimating expansion size, and detecting the premutation alleles at risk of expansion in future generations.

DMPK
CNBP

About this condition

Myotonic Dystrophy

Myotonic dystrophy (DM) is the most common adult-onset muscular dystrophy, affecting approximately 1 in 8,000 individuals worldwide. It is a multisystem disorder characterized by progressive muscle weakness and myotonia (delayed muscle relaxation), combined with involvement of the heart (conduction defects, cardiomyopathy), lens (posterior subcapsular cataracts), brain (cognitive changes, daytime sleepiness), endocrine system (insulin resistance, hypogonadism), and smooth muscle (gastrointestinal dysmotility). Two distinct genetic forms exist: DM type 1 (DM1) and DM type 2 (DM2), caused by different repeat expansions and with partially overlapping but clinically distinguishable phenotypes.

DM type 1 is caused by expansion of a CTG trinucleotide repeat in the 3' untranslated region of the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19q13.32. Normal alleles have 5-37 CTG repeats; premutation alleles have 38-49 repeats; full expansions causing DM1 range from 50 to thousands of repeats. DM1 demonstrates anticipation — repeat expansions typically grow with each successive generation, leading to earlier onset and more severe disease in offspring. The congenital form of DM1, almost always maternally transmitted, causes severe neonatal hypotonia, respiratory failure, and intellectual disability. DM type 2 is caused by CCTG tetranucleotide repeat expansion in intron 1 of the CNBP gene on chromosome 3q21.3, with normal alleles having fewer than 26 repeats and affected individuals having 75 to over 11,000 repeats.

Cardiac involvement in DM1 is a major determinant of mortality — progressive AV block and ventricular arrhythmias cause sudden cardiac death in 20-30% of DM1 patients. Regular cardiac monitoring (ECG, Holter, echocardiogram) and pacemaker/ICD implantation are critical components of management. Respiratory insufficiency develops in nearly all DM1 patients and is the leading cause of death. Non-invasive ventilation (BiPAP/CPAP) extends survival. Estimating the CTG repeat size from the initial genetic diagnosis guides prognosis: shorter expansions (50-150 repeats) correlate with milder, adult-onset disease; expansions above 1,000 repeats are associated with more severe or congenital presentation.

DM1 and DM2 require different genetic tests — CTG repeat expansion in DMPK (DM1) vs CCTG expansion in CNBP (DM2). Whole genome sequencing detects and measures both repeat types simultaneously.

Gene locus: DMPK (19q13.32) — DM type 1; CNBP (3q21.3) — DM type 2

Standard repeat expansion tests use Southern blot or TP-PCR and provide approximate size estimates. Whole genome sequencing resolves both DM1 and DM2 expansions with precise characterization — and identifies premutation carriers at risk of transmitting a larger expansion.

Distinguishing DM1 from DM2 requires testing different genes — and the phenotype alone cannot always differentiate them

DM1 and DM2 have overlapping clinical features — both produce myotonia, proximal-predominant weakness (in DM2), cataracts, and multisystem involvement. However, several critical differences distinguish them: DM1 causes congenital disease (DM2 does not); cardiac conduction disease is more severe and more penetrant in DM1; and DM2 tends to have milder, more proximally distributed weakness. The two conditions require different genetic tests targeting different chromosomal loci. A single ordered 'myotonic dystrophy test' typically evaluates only DMPK (DM1). Whole genome sequencing evaluates both DMPK and CNBP simultaneously, ensuring the correct molecular diagnosis regardless of which subtype is present.

Repeat expansion size informs cardiac surveillance intensity and prognosis

CTG repeat length in DM1 is an imperfect but meaningful predictor of disease course. Patients with larger expansions tend to have earlier onset, more severe multisystem involvement, and higher risk of cardiac conduction disease requiring intervention. This relationship guides surveillance intensity: DM1 patients with larger expansions undergo more frequent cardiac monitoring and have a lower threshold for pacemaker consideration. The repeat size also quantifies anticipation risk for family planning — a parent with a 200-repeat expansion may transmit a 400-repeat expansion, and a parent with a 500-repeat expansion is at substantial risk of transmitting a congenital-range expansion to offspring. Whole genome sequencing estimates serve as the baseline for this counseling.

Myotonic Dystrophy — the most common adult muscular dystrophy, caused by unstable DNA repeat expansions that grow with each generation and produce a multi-system disorder affecting muscle, heart, brain, and endocrine function.

Myotonic Dystrophy

Standard repeat expansion tests use Southern blot or TP-PCR and provide approximate size estimates. Whole genome sequencing resolves both DM1 and DM2 expansions with precise characterization — and identifies premutation carriers at risk of transmitting a larger expansion.

Distinguishing DM1 from DM2 requires testing different genes — and the phenotype alone cannot always differentiate them

Repeat expansion size informs cardiac surveillance intensity and prognosis

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