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Myotonic Dystrophies (MDs)

MDs are a group of inherited disorders which characterized by prolonged muscle contractions (myotonia) and are not able to relax certain muscles after use. Other signs of MDs  include  muscle weakness, cataract, cardiac conduction defects, diabetes type2 and hormonal dysfunction.

There are two major types of myotonic dystrophy: type 1 (or Steinert myotonica)  and type 2 (or PROMM). Their signs and symptoms overlap, although type 2 tends to be milder than type 1 with onset in adulthood.

MD1

MD1 is characterized by progressive muscle wasting and weakness beginning in their 20's or 30's. MD1 affects at least 1 in 8,000 people worldwide. 

The diagnosis of the disease in LMG is established by identification of a triplet expansion CTG   in DMPK gene using Triple Primed-PCR (TP-PCR) analysis.

MD2

Myotonic dystrophy type 2 (DM2) or proximal myotonic myopathy (PROMM) is an autosomal-dominant disorder caused by an unstable [CCTG]n tetranucleotide repeat expansion in intron 1 of the CNBP/ZNF9 gene on chromosome 3q21.3. Disease onset occurs in late adulthood with a large spectrum of symptoms such as early onset cataract, myotonia, muscle stiffness and myalgia, proximal weakness, cardiac conduction defects, endocrinological abnormalities, and cognitive impairment.23

The diagnosis of the disease in LMG is established by identification of the tetraplet CCTG]n  expansion in CNBP/ZNF9 gene gene using Triple Primed-PCR (TP-PCR) analysis.

Reference

Papadopoulos C, Kekou K, Xirou S, Kitsiou-Tzeli S, Kararizou E, Papadimas GK. Early onset posterior subscapular cataract in a series of myotonic dystrophy type 2 patients. Eye (Lond). 2018 Mar;32(3):622-625. doi: 10.1038/eye.2017.280.