Laboratory Issues

There are many molecular pathology laboratories offering DNA testing for DM. The majority of laboratories are using a combination of PCR and Southern blot testing. The PCR allows accurate quantitation of the number of CTG repeats for normal alleles and those with small expansions. For the large CTG expansions, Southern blot analysis is required. All patients with DM have a CTG expansion in the 3' region of DMPK, and no other types of mutations have been identified in the gene. However, the gene for the rarer dominantly inherited myotonic dystrophy type 2 (DM2), which usually results in a more proximal myotonic myopathy and some of the multisystem manifestations of myotonic dystrophy, has been identified.25 Myotonic dystrophy type 2 is caused by a very large CCTG expansion in intron 1 of the zinc finger protein 9 gene (ZNF9). Gene testing for DM2 should be considered in patients with a DM-like phenotype who have a negative test for the DMPK CTG expansion. Proficiency testing for DM is available through the CAP, and proficiency specimens are sent to participants twice per year.

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