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.
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.
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.
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Common questions about whole genome sequencing.
What is the difference between whole genome sequencing and a targeted genetic test?
Targeted genetic tests — including standard hereditary cancer panels — read a pre-defined list of known variants in a specific set of genes. They are designed to find what they already know to look for. Whole genome sequencing reads your entire genome: all 6 billion base pairs, every gene, every region between genes. A Mayo Clinic study published in JAMA Oncology found that standard testing guidelines missed more than half of patients with inherited cancer mutations. Genome Test does not have a fixed list.
What will I receive when my results are ready?
Your Dante Genome delivers 200+ physician-ready reports organized by clinical category — hereditary cancer, cardiac conditions, rare diseases, pharmacogenomics, carrier status, and more. Reports are delivered to your secure Genome Manager and are formatted for direct clinical use. Your genome data is permanently retained and re-analyzed automatically as science advances.
What happens if a clinically significant variant is found?
If a pathogenic or likely-pathogenic variant is identified, it will be clearly flagged in your physician-ready report with clinical context, published evidence, and recommended next steps. We recommend sharing any clinically significant finding with your physician or a genetic counselor, who can guide decisions about surveillance, risk reduction, or cascade testing for family members.
How is this different from a consumer DNA test like 23andMe or AncestryDNA?
Consumer DNA tests use genotyping chips that read less than 0.1% of your genome — a tiny pre-selected set of common variants. They are optimized for ancestry and population-level traits, not clinical genetic findings. The Dante Genome Test sequences 100% of your genome at 30X coverage, the same standard used in clinical diagnostic settings. The two tests are not comparable in scope, methodology, or clinical utility.
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Your collection kit ships within 48 hours of ordering. Once your sample arrives at our CLIA-certified laboratory, sequencing and analysis takes 6–8 weeks. Results are delivered securely to your Genome Manager, where you can access your reports, share them with your physician, and receive automatic updates as new findings are validated against your genome.
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