Introduction: Myotonic dystrophy is an autosomal dominant hereditary disorder with variable expression, associated with expansion of the CTG triplet on the gene which codifies myotonia situated on chromosome 19q. We present an unusual case of myotonic dystrophy in a pair of monozygotic twin sisters, determination of CTG expansion in lymphocytes of members of their family and in their father's spermatozoids.
Clinical case: The patients presented the phenomenon of anticipation of paternal transmission with an expansion of triplet CTG of lymphocyte DNA in a range of 300-1,400 identical repetitions in both. DNA of the paternal lymphocytes and spermatozoids showed a similar expansion of 75 repetitions.
Conclusions: CTG expansion is not due to previous expansion of DNA in the paternal gametes but to instability of DNA in the cellular mitoses following formation of the zygote.