Proximal myotonic myopathy

Hairpin Structure-forming Propensity of the (CCTG·CAGG) Tetranucleotide Repeats Contributes to the Genetic Instability Associated with Myotonic Dystrophy Type 2

The genetic instabilities of (CCTG.CAGG)(n) tetranucleotide repeats were investigated to evaluate the molecular mechanisms responsible for the massive expansions found in myotonic dystrophy type 2 (DM2) patients. DM2 is caused by an expansion of the repeat from the normal allele of 26 to as many as 11,000 repeats. Genetic expansions and deletions were monitored in an African green monkey kidney cell culture system (COS-7 cells) as a function of the length (30, 114, or 200 repeats), orientation, or proximity of the repeat tracts to the origin (SV40) of replication. As found for CTG.CAG repeats related to DM1, the instabilities were greater for the longer tetranucleotide repeat tracts. Also, the expansions and deletions predominated when cloned in orientation II (CAGG on the leading strand template) rather than I and when cloned proximal rather than distal to the replication origin. Biochemical studies on synthetic d(CAGG)(26) and d(CCTG)(26) as models of unpaired regions of the replication fork revealed that d(CAGG)(26) has a marked propensity to adopt a defined base paired hairpin structure, whereas the complementary d(CCTG)(26) lacks this capacity. The effect of orientation described above differs from all previous results with three triplet repeat sequences (including CTG.CAG), which are also involved in the etiologies of other hereditary neurological diseases. However, similar to the triplet repeat sequences, the ability of one of the two strands to form a more stable folded structure, in our case the CAGG strand, explains this unorthodox "reversed" behavior.

A family with PROMM not linked to the recently mapped PROMM locus DM2

Proximal myotonic myopathy is an autosomal dominantly inherited multisystem disorder, clinically similar to but genetically distinct from myotonic dystrophy (DM). A recently mapped second locus for myotonic dystrophy was thought to be an attractive candidate locus for PROMM, and this hypothesis was supported by reports of linkage to this locus in some PROMM families. We present a large German pedigree with PROMM in which linkage to this locus could be excluded, showing that PROMM is genetically heterogeneous.

Proximal myotonic myopathy: mini-review of a recently delineated clinical disorder

This mini-review describes proximal myotonic myopathy, a recently delineated, dominantly inherited disorder that is similar to but distinct from myotonic dystrophy. Proximal myotonic myopathy is not linked to the gene locus for myotonic dystrophy or to the loci of the genes of the muscle sodium and chloride channels associated with other myotonic disorders. Patients often present with myotonia and peculiar muscle pain in early adulthood and develop weakness of the thigh muscles later in life. Cataracts that are indistinguishable from those in myotonic dystrophy also occur commonly. The gene defect responsible for proximal myotonic myopathy awaits discovery. Because of the clinical similarities between proximal myotonic myopathy and myotonic dystrophy, clarification of the genetic differences will not only shed light on the pathomechanism of proximal myotonic myopathy, but may also increase our understanding of myotonic dystrophy.