POSTZYGOTIC INSTABILITY OF THE MYOTONIC-DYSTROPHY P[AGC]N REPEAT SUPPORTED BY LARGER EXPANSIONS IN MUSCLE AND REDUCED AMPLIFICATIONS IN SPERM

We have analysed the [AGC] expansion in leucocytes, muscle and sperm f rom 17 individuals affected by myotonic dystrophy (DM). Skeletal muscl e showed a larger repeat number than leucocytes in the same patient. A similar degree of expansion was detected in differently affected musc les of a single patient. The germline mutation (less than or equal to 350 repeats) was expanded in somatic cells of the progeny in all patie nts examined. Our results provide evidence of an early postzygotic ins tability of the [AGC] repeat in DM.