Triple repeat diseases

Myotonic dystrophy is a generalized adult-onset disorder with muscular spasms, weakness, and many other symptoms.11 It is one of the triple-repeat diseases (Table 26-4). The affected gene encodes a protein kinase of unknown function. The corresponding mRNA transcript has -2400 nucleotides. The gene has a CTG repeat (CTG) near the 3 -end with n< 30 normally. For persons with the mildest cases of myotonic dystrophy n may be over 50 while in severe cases it may be as high as 2000. As in other triple-repeat diseases the repeat number tends to increase in successive generations of people as does the severity of the diseased... 

The hereditary triple-repeat disease Huntington s chorea (Huntington disease), with an incidence of 5-10 per 100,000 persons, affects principally persons of age over 40 and is associated with a deficiency of GABA in basal ganglia.724 The cortex is also affected. Severe neurologic symptoms arise as a result of premature death of neurons in the basal ganglia. Convulsions may also arise because of a deficiency of GABA in the brain. 

Epilepsy may arise also from defects in a GABA transporter1145 or receptor.1146 One form of epilepsy is a triple-repeat disease of cystatin B (Table 26-4). Mutation in potassium channels,1147 glutamate receptors,1148 absence of neuropeptide Y,1149 and absence of L-isoaspartyl / D-aspartyl O-methyltransferase (Box 12-A)1150 have all been associated with epilepsy. 

TRIPLE REPEAT DISEASES OF THE NERVOUS SYSTEM Edited by Lubov T. Timchenko... 

Two studies have reported a positive association between MTHFR polymorphisms and pediatric ALL relapse risk (Table 1). The first study by Krajinovic et al. evaluated 201 children with ALL treated with methotrexate as per Dana Farber Cancer Institute (DFCl) protocols 87-01, 91-01, or 95-01. Thus, the study demonstrated that the MHFR T677A1298 haplotype was associated with a decreased disease-free survival in multivariate analysis. When children with either the MTHFR T677A1298 haplotype or the MTHFDl A195 8 variant also had a triple-repeat thymidylate synthase polymorphism, the correlation with decreased EFS was also significant (5). However, neither of the MTHFR polymorphisms alone was significantly associated with altered survival. These analyses did not include toxicity risks. 

There are many other neurodegenerative diseases, some with a high incidence, and others rare. They include Parkinson disease (p. 1790), Huntington disease (Table 26-4), spinal muscular atrophy (SMA a leading hereditary cause of infant mortality),1162a b amyotrophic lateral sclerosis (ALS), prion diseases (Box 29-E), ataxias, and other diseases caused by triple-repeat DNA sequences (Table 26-4) and X-linked adrenoleukodystrophy (ALD p. 945).1163 In the last, membrane function is disrupted. Although these diseases arise from a variety of causes many of them have in common amyloidosis, the deposition of insoluble proteins in or around neurons.11633... 

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