The myotonic dystrophies, type 1 (DM1) and 2 (DM2) are progressive, autosomal, dominantly inherited disorders, mainly characterized by muscle weakness and atrophy but also by a variable impact on heart, eye, brain, and the endocrine and the gastrointestinal system (Meola 2000). The worldwide prevalence is approximately 1 in 8,000. They are considered to be most common in Western Europe and Japan, but less prevalent in Southeast Asia, and rare or absent in southern and central Africa (Emery 1991). A prevalence of 18 in 340,000 children has been reported (Darin and Tulinius 2000). The cause of myotonic dystrophies is an unstable inherited repeat DNA expansions. Expansions are elements occurring and repeated throughout the human genome, typically polymorphic in the general population. Repeats can become unstable during DNA replication and, depending on specific repeat motif and location, expanded repeats can become pathogenic. In disease states, the number of repeats exceeds the normal range, leading to various pathogenic mechanisms (Ranum and Cooper 2006). DM1 is associated with an expanded (CTG)n repeat (>50 to several thousands) within the noncoding 3′ untranslated region of the myotonic dystrophy protein kinase (DMPK) gene on chromosome 19q13.3. In DM2, another mutation exists, namely an expanded CCTG tetranucleotide repeat (from 75 to 11,000 repeats) in the first intron of the zinc finger protein 9 (ZNF9) gene on chromosome 3q21 (Day and Ranum 2005). This means that two unrelated genes are associated with similar phenotypes although there are differences, including the age of onset and severity of symptoms (Meola 2000). The first signs of a DM2 disease are typically shown in adulthood, and no study has as yet systematically described cognitive or behavioral abnormalities in a childhood DM2 phenotype. Consequently, the following chapter focuses on a description of DM1. In this disorder, the age of onset is variable, meaning that there are congenital cases, as well as children, adults and patients experiencing the first symptoms very late in life. DM1 is traditionally divided into categories, each presenting with specific clinical features and broadly associated with the age of onset and extent of genetic abnormality.
Cognitive and Behavioral Abnormalities of Pediatric Diseases
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