Anticipation and expanded trinucleotide repeat sequences

Anticipation implies that a disease occurs at a progressively earlier age of onset and with increased severity in successive generations. This may explain the non-Mendelian pattern of inheritance observed in some inherited diseases. This phenomenon has been observed in several neurological diseases including myotonic dystrophy, fragile X syndrome, Huntington's disease, and more recently in spinobulbar muscular atrophy, type 1 spinocerebellar ataxia, and spastic paraplegia. (55) Anticipation has been found to correlate with specific mutations in these syndromes: expanded trinucleotide repeat sequences. An expanded repeat sequence is unstable and may increase in size between family members, leading to increased disease severity of the disorder.

Anticipation has recently been described in bipolar affective disorder(12) and in unipolar affective disorder.(3) One recent study highlighted an association between cysteine-adenine-guanine (CAG) trinucleotide repeats and bipolar affective disorder in Swedish and Belgian patients. (56) CAG repeats have been detected by the repeat expansion detection method. This study, which was replicated in a different patient population, (5JJ demonstrated for the first time in a major psychiatric disorder that the mean length of CAG repeats was significantly higher in bipolar affective disorder patients compared to controls. More recently, an association between familial cases (but not sporadic cases) of bipolar affective disorder and CAG length has been observed. (58) This hypothesis has also been tested in a family sample of two-generation pairs with bipolar affective disorder. (59) A significant increase in CAG repeats between parents and offspring generations was observed, however, when the phenotype increased in severity, i.e. changed from major depression, single episode, or unipolar recurrent depression to bipolar affective disorder. A significant increase in CAG repeat length between generations was also found in female offspring with maternal inheritance, but not in male offspring. This is the first evidence of genetic anticipation in bipolar affective disorder families and should be followed by the identification of loci within the genome containing triplet repeats. CTG 18.1 on chromosome 18q21.1 and ERDA 1 on chromosome 17q21.3 are two repeat loci recently identified(60) which can be investigated in such study.

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