MYOTONIC DYSTROPHY
is an inherited disorder in which the muscles contract
but have decreasing power to relax. With this condition,
the muscles also become weak and waste away. Myotonic
dystrophy can cause mental deficiency, hair loss and
cataracts. Onset of this rare disorder commonly occurs
during young adulthood. However, it can occur at any
age and is extremely variable in degree of severity.
The myotonic dystrophy gene, found on chromosome 19,
codes for a protein kinase that is found in skeletal
muscle, where it likely plays a regulatory role. An
unusual feature of this illness is that its symptoms
usually become more severe with each successive generation.
This is because mistakes in the faithful copying of
the gene from one generation to the next result in
the amplification of a genomic 'AGC/CTG triplet repeat',
similar to that found in Huntington disease. Unaffected
individuals have between 5 and 27 copies of AGC/CTG,
myotonic dystrophy patients who are minimally affected
have at least 50 repeats, while more severely affected
patients have an expansion of up to several kilobase
pairs.