Amy Bauer

Long Repeat Tracts at SCA8 in Major Psychosis

Expansion at a recently identified unstable trinucleotide repeat on chromosome 13q21 has been reported as the molecular cause for spinocerebellar ataxia type 8 (SCA8). The trinucleotide repeat, which consists of a [CTA]n repeat and adjacent [CTG]n repeat, was reported to have a pathogenic range of 107-127 CTG repeats (or 110-130 combined CTA and CTG repeats) in a large ataxia kindred. This repeat region was also cloned by our group from a bipolar affective disorder (BPAD) patient, who has approximately 600 combined repeats, and large alleles (>100 repeats) were reported to be present in 0.7% of controls and 1.5% of major psychosis patients (n = 710 and n = 1,120, respectively). We have followed up these findings by screening three new samples of BPAD and schizophrenia (SCZ) patients and controls, including 272 individuals from 14 BPAD families from Sweden, 130 individuals from 32 SCZ and BPAD families/trios from the Azores Islands, and 206 SCZ individuals from the United Kingdom and Ireland, and 219 matched controls. We found large repeat alleles above the SCA8 pathogenic range in individuals from 3 of 32 Azorean pedigrees and in 1 of 206 SCZ individuals from the United Kingdom, and repeat alleles within the SCA8 pathogenic range in 1 of 14 Swedish families. Although the rarity of major psychosis patients carrying the SCA8 expansion mutation would require a much larger sample size to reach statistical significance, these results support the previously reported observation of increased occurrence of large repeats at SCA8 in major psychosis. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:873-876, 2000.

Detection of expansion regions in Portuguese bipolar families

We have studied 24 families with multiple affected members with bipolar disorder to test the hypothesis that in those families clinically showing genetic anticipation [Macedo et al., 1999] we would find large repeat expansions. The families meeting inclusion criteria had a minimum of two affected members over two generations and showed marked anticipation both in terms of age of onset and disease severity. We used the repeat expansion detection (RED) method to test patients (n = 24) and controls from these families and unrelated controls (n = 53). We also genotyped patients and family members from two families with large expansions at the known expansion loci on chromosomes 13, 17, and 18. The RED method revealed a higher number of large expansions in patients compared with controls (t-test; P < 0.0055: Mann-Whitney U; P = 0.02). The patients with the largest expansions were typed at the specific loci on chromosomes 13, 17, and 18 and the chromosome 18 expansion locus segregated with disease in one family, and a second family showed segregation with the expansion located at the SCA8 locus on chromosome 13. Genetic anticipation had been analyzed in this cohort of families, with correction for potential ascertainment bias, possible proband effects, cohort effects, regression to the mean, gender effects, and maternal vs. paternal transmission. None of these potential confounds appeared to account for the observed anticipation. We also identified that the presence of large expansions in affected family members derives primarily from two families from the genetically isolated Azores population. One family shows segregation with the chromosome 18 locus, whereas the other family segregates with expansions at the SCA8 locus. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:854-857, 2000.

Identification of a Highly Homogenous Population for Genetic Study of Psychiatric Disorders

Many psychiatric disorders are influenced by genetic factors, but the genetic components of complex diseases may not follow clear inheritance patterns. Although the patients may share a common clinical phenotype, the cause of the syndrome may consist of a heterogeneous collection of both genetic and/or environmental components. One method to minimize genetic heterogeneity in studies of complex disorders is to select a very homogenous study population. The average number of families with the same last name, when corrected for population size, is an excellent marker for the degree of homogeneity. We used surname analysis to evaluate the homogeneity of the Portuguese population of Madeira, comparing it with previous data on the homogeneity of populations of mainland Portugal, the Azores, and both rural and urban US populations. The average number of families with the same last name corrected for population size was 33.84 in Madeira, 30.88 in the Azores, and 21.42 in Coimbra (mainland Portugal) compared with 1.13 in rural and 0.38 in urban United States. This surname analysis supports the premise that the Portuguese population is a highly homogenous population, with the highest homogeneity in Madeira and the Azores, making it a good study population for molecular genetic analyses.

The Genetics of Attention-Deficit/Hyperactivity Disorder

Attention-deficit/hyperactivity disorder (ADHD) has an early childhood onset in the majority of cases. This has a considerable impact on the development of the affected individual, both directly (as a result of the symptoms) and indirectly (through the stresses imposed upon school, learning, socialization, and family life). Several lines of evidence point to a genetic component to ADHD. Family studies show a familial aggregation of ADHD, with a five- to sixfold increase in the incidence of ADHD among first-degree relatives. Twin studies reveal a higher concordance rate for ADHD among monozygotic twins compared with dizygotic twins. To date, molecular genetic research has focused on candidate genes in the dopaminergic system. Genes studied include the D2A1 allele of the dopamine D 2 receptor gene, the dopamine transporter gene, and the dopamine D 4 receptor gene. One of the major limitations to the study of the genetics of behavioral disorders in children has been the overlap among syndromes, including oppositional defiant disorder, conduct disorder, persistent (adult) ADHD, and bipolar disorder. Future research must address weaknesses in existing studies, including small samples sizes, restricted statistical power, and confounding factors such as comorbid illnesses, clinical heterogeneity with variable symptom severity, and unclear phenotypic boundaries .

Genetics of Schizophrenia: Current Findings and Issues

The genetics of schizophrenia are characterized by a set of complex questions, with few answers forthcoming. However, molecular genetic approaches remain the most promising avenue to the understanding of etiologic mechanisms. Powerful discoveries may arise in the near future from candidate gene studies that are benefiting from the extensive neurobiology research in schizophrenia over the past decades. Furthermore, there are promising new divisions of the phenotype into more manageable subtypes that may make both candidate gene and genome scan studies more revealing. The following review discusses selected highlights of the epidemiology, molecular genetic strategies, candidate genes, and genome scan investigations in schizophrenia.