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A 45-year-old woman has proximal muscle weakness and stiffness, cataracts, and myotonia. Her DM1 test was negative, but she has a CCTG expansion.
AD; CCTG repeat expansion in CNBP (ZNF9)
- Milder than DM1
- Proximal weakness (vs distal in DM1)
- Myotonia, muscle pain/stiffness
- Cataracts, cardiac involvement
- NO congenital form
- Less anticipation
- Suspect in adults with proximal weakness, myotonia, muscle pain/stiffness, and cataracts and a negative DM1 test
- Confirm with molecular testing for the CCTG repeat expansion in CNBP (ZNF9); requires Southern blot or repeat-primed PCR because of the very large, somatically unstable expansion
- EMG shows myotonic discharges; molecular testing is definitive
- Annual ECG/cardiology surveillance for conduction defects and arrhythmia
- Mexiletine for symptomatic myotonia; manage muscle pain and stiffness
- Periodic cataract screening and diabetes/insulin-resistance monitoring
"DM2 is milder": DM2 has no congenital form, less anticipation, and predominantly proximal weakness (vs distal in DM1).
"DM2 = CCTG in CNBP": CCTG tetranucleotide repeat (not trinucleotide) expansion in CNBP. DM2 has a longer repeat unit (4 bases) than DM1 (3 bases, CTG).