Myotonic Dystrophy Type 1 (Curschmann-Steinert Disease)
Dr. rer. biol. hum. Soheyla Chahrokh-Zadeh, Dr. med. Imma Rost
Myotonic dystrophies, multisystemic diseases with an autosomal dominant inheritance pattern, represent the most frequently occurring muscular dystrophies in adults. The incidence of myotonic dystrophy type 1 (DM1) is estimated to be 1 in 8,000. The first symptoms, such as myotonia the delayed relaxation of a contracted muscle, usually occur in early adulthood. Additional symptoms are muscle weakness and fatigue as well as swallowing difficulties, a long face with little facial expression, cataract, diabetes mellitus and cardiac arrhythmia, and in men early development of frontal baldness and testicular atrophy. The clinical presentation of DM1 is categorized into four, partially overlapping phenotypes: asymptomatic, mild, classic and neonatal phenotype. The connatal form is characterized in particular by severe muscular hypotonia often with clubfoot and usually accompanied by respiratory insufficiency and poor drinking. Psychomotor development is frequently delayed. This form only occurs in DM1 and is predominantly seen when the mother passes on the disease.
DM1 is caused by a CTG repeat expansion in the 3’ untranslated region of the DMPK gene on chromosome 19, which codes a protein kinase. DM2 is important for differential diagnosis (proximal myotonic myopathy = PROMM OMIM Number: 602688); it is caused by expansion of a CCTG repeat in intron 1 of the ZNF9 gene on chromosome 3. The phenomenon of anticipation – i.e. decreasing age of onset and/or more increasing severity of symptoms from one generation to the next – is particularly strong in DM1. Healthy individuals have 5-36 CTG repeats in the DMPK gene, DM patients have up to approximately 1,000; children with the congenital form have over 1,000 repeats.
In predictive diagnostics, healthy, at-risk individuals, usually first-degree relatives of affected patients, are tested. According to the Genetic Diagnosis Act (GenDG), genetic counseling should be offered with any genetic diagnostic procedure. In the case of predictive genetic testing, the GenDG stipulates that genetic counseling must be carried out prior to testing as well as after having received the results, unless the at-risk person has provided a written waiver after having received written information on the content of the counseling.
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