1. ¹Division of Medical Genetics, Department of Obstetrics and Gynaecology, University Medical Centre Ljubljana, lajmerjeva 3, 1000 Ljubljana, Slovenia
2. ²Outpatient Paediatric Clinic Pula, Pula, Croatia
3. ³Department of Ophtalmology, University Medical Centre Ljubljana, Ljubljana, Slovenia
4. ⁴Department of Ophtalmology, General Hospital Pula, Pula, Croatia

Correspondence: Borut Peterlin, Division of Medical Genetics, Department of Obstetrics and Gynaecology, University Medical Centre Ljubljana, lajmerjeva 3, 1000 Ljubljana, Slovenia. E-mail: borut.peterlin@guest.arnes.si

Igor Medica and Natasa Teran contributed equally to this work.

Received 21 September 2006; Accepted 2 November 2006; Published online 5 December 2006.

Abstract

Myotonic dystrophy 1 (DM1) is known to diminish reproductive fitness in its severe form. Since no de novo mutations are known for this disease, it has the tendency to become extinct from a population. To explain the preservation of DM1 in a population, a hypothesis that a pool of subjects for the mutated gene exists in the apparently healthy (non-DM1) population was tested. In order to determine the (CTG) repeat number, PCR was performed in 274 patients found to have primary cataract of adult onset who showed no DM1 symptoms, and were not related to DM1 patients. In four cataract patients (1.46% 95% CI 0.5–3.7), a protomutation in the myotonin protein kinase gene was found which might lead to a complete mutation after transmission through the next generations. The number of (CTG) repeats in the remaining 270 cataract patients did not differ significantly from the control subjects in terms of the distribution of larger [(CTG)n 19] versus smaller [(CTG)n < 19] alleles. We consider the primary cataract patients to be the pool of DMPK protomutation from which DM1 mutation is maintained in the population.