Trinucleotide repeats and neuropsychiatric phenotypes

Abstract

Genetic studies of affective disorders have yielded several chromosomal regions suggestive for linkage. Linkage analysis was performed in five families with unipolar affective disorder, collected from northern Sweden. Four candidate regions on chromosomes 16, 18, 21 and 4p were excluded.

Anticipation, increased disease severity and younger age-of-onset in successive family generations correlating with a longer repeat expansion, is a hallmark in disorders caused by trinucleotide repeat expansions (TREs). Mental impairment and psychiatric symptoms are seen in some of these disorders. This study aimed at investigating if TREs may be susceptibility factors for neuropsychiatric disorders displaying anticipation.

Fourteen Swedish families with affective disorders displaying anticipation were screened for CAG/CTG expansions as a possible explanation for the observed anticipation using the repeat expansion detection (RED) method. A significant difference in CAG/CTG repeat expansion distribution between affected offspring and healthy family members was seen. Two loci, ERDA1 on chromosome 17 and CTG18.1 on chromosome 18 were analyzed by PCR in the families. Eighty-nine percent of the RED expansions correlated in size with large alleles at these two loci. CTG18.1 expansions were more frequently seen in patients as compared to healthy individuals (odds ratio 2.6-2.8), and their size correlated inversely with age-of-onset. The allele distribution at ERDA1 was not significantly different between patients and healthy individuals.

Spinocerebellar ataxia type 8 (SCA8) is a neurodegenerative disorder associated with a CTG repeat expansion. The SCA8 repeat was analyzed using PCR in four sets of family and case-control materials of different origin. The frequency of individuals with expanded SCA8 repeats (>40 repeats) was higher in all the four patient groups as compared to healthy individuals. Expanded alleles within the SCA8 pathogenic size range (107-127 CTG repeats) were detected in 5 affected individuals and two healthy individuals. These findings suggest that repeat expansion alone may not be sufficient for the development of SCA8. It may however be a risk factor for psychiatric phenotypes.

In order to facilitate identification of disease genes containing an expanded trinucleotide repeat, a repeat detection and gene isolation system was established. The method is based on size separation of genomic fragments, followed by subcloning and library hybridization with an oligonucleotide probe. The expanded trinucleotide repeat is identified throughout the procedure using the RED method. Using this approach two disease genes were cloned, the Huntington's disease gene and the MJD/SCA3 gene. This cloning strategy should be applicable to isolation of any DNA fragment containing large trinucleotide repeat expansion in any species.

Familial spastic paraplegia type 4 (SPG4) is a neurodegenerative disorder characterized by spasticity and progressive weakness of the lower limbs, linked to chromosome 2p21-24 (SPG4). Anticipation as well as a CAG/CTG repeat expansion coding for a long polyglutamine tract have been reported in SPG4. Six families with SPG4 were screened for expanded CAG/CTG repeats as well as for proteins with an expanded polyglutamine tract. Neither repeat expansions cosegregating with the disease nor abnormal polyglutaminecontaining protein was detected. These findings were confirmed by the cloning of the SPG4 gene.

Both ERDA 1 and CTG 18. 1 repeat expansions exhibit repeat instability upon transmission. Fifty-three percent of parent-offspring pairs with ERDA1 expansions (n=51), and 67% of pairs with CTG18.1 expansions (n=12) were unstably transmitted. Contractions of repeats were more frequently seen upon paternal transmission at both loci, To understand mechanisms of repeat instability, these two loci were studied in monozygotic twin pairs. Thirty-three out of 101 MZ twin pairs carried repeat expansions at ERDA1 and/or CTG18.1 loci. No size difference was observed for any of these twin pairs, suggesting that the repeat instability is likely to be a prezygotic event. Somatic mosaicism, caused by repeat instability during somatic cell divisions, was observed in some individuals with an expanded CTG18.1 repeat. All MZ twin pairs and the majority of the parent-offspring pairs were in complete concordance for the presence or absence of mosaicisms, suggesting that genetic components acting in cis may be important for mitotic instability.