[Cataract as an only clinical manifestation of myotonic dystrophy--a new example of somatic instability of CTG repeats expansion in myotonin protein kinase gene].

We reported a 69-year-old woman who developed a cataract as a single clinical expression of myotonic dystrophy (MD). There are many MD patients in her family including her 29-year-old daughter suffering from congenital MD. We compared CTG repeats expansion in the motonin protein kinase gene from the lens obtained at operation with that of her and her daughter's lymphocytes using two techniques, Southern blot and PCR amplification analyses. On the Southern analysis using Eco RI and BGII-digested DNA, lymphocytes from the patient and her daughter showed a band of abnormal size with about 80 and 2,500 repeats respectively in addition to the normal size allele. No band was visualized in the patient's lens because the amount of DNA was presumably too small. On PCR analysis the band of expanded allele was observed in the patient's lymphocytes only whereas the band of normal allele was present in all the specimens. The expanded CTG repeats were too large to be amplified by PCR as demonstrated on the Southern blot of her daughter's lymphocyte. Consequently the discrepancy in the size of CTG repeat expansion in lens and lymphocyte is a new example of somatic instability of the repeat which explains the cataract as an only clinical manifestation of MD in this patient.