Letters to the Editor

Mode of transmission of schizophrenia

OBJECTIVE

Although the evidences for the phenomenon of "anticipation" and parental "imprinting" have been shown in schizophrenia, they are inconclusive. The purpose of this study was to test these hypotheses by examining three successive generations.

METHOD

58 schizophrenic patients who had their maternal or paternal parent or grandparent, or both, affected with schizophrenia or related disorders were analyzed. Chi-square test was used to assess the association of the sex of the parent with more than one of the affected proband families. The differences in the age of onset of the illness between the successive three generations was calculated using the t-test.

RESULTS

In comparison to mothers' affected families, a large proportion of the father side affected families had more than one of their offspring affected with the illness. The age of onset in probands was lower in comparison to that of those on the parental side and the difference was more significant when the paternal side was affected. Interestingly, when the age of onset in the grandparents was compared with either of the parental sides of the probands no difference emerged, indicating lack of support from this study for the theory of anticipation. At any rate, the age of onset of probands was significantly lower in comparison to that of the paternal grandfather side. Further, skipping of a generation in the process of transmission was noted in some families.

CONCLUSIONS

It is hard to ignore our findings that suggest paternal side transmission.

Expanded (CAG)n, (CGG)n and (GAA)n trinucleotide repeat microsatellites, and mutant purine synthesis and pigmentation genes cause schizophrenia and autism

Unstable (CAG)n trinucleotide repeat microsatellites are hypothesized to cause schizophrenia. The (CAG)n microsatellite of dominant spinal cerebellar ataxia type 1 (SCA1) is a candidate schizophrenia gene. Autism results from expansions of (CGG)n and (GAA)n trinucleotide repeat stretches at fragile X syndrome (FRAXA), and the recessive Friedreich's ataxia (FA). Dominant ataxia genes may cause schizophrenia and recessive ataxia genes may cause autism. Syndromes with autism show purine synthesis defects (PSDs) and/or pigmentation defects (PDs). Autism is caused by very lengthy expansions of (CAG)n, (CGG)n and (GAA)n repeats, while schizophrenia results from much smaller (CAG)n and (CGG)n repeat expansions.

A critical review of genetic studies of schizophrenia. II. Molecular genetic studies

Recent molecular genetic studies of schizophrenia have, until now, been unable to demonstrate any specific major gene for schizophrenia. On the contrary, linkage and association studies have yielded almost exclusively negative or contradictory results. Such studies have involved certain candidate genes, such as the genes for dopamine receptors and other brain neurotransmitters. Some of these candidate genes have now actually been excluded as specific aetiological factors in schizophrenia. Similarly, studies searching for a major gene for susceptibility to schizophrenia involving the whole human genome or large parts of chromosomes have not yielded unambiguously positive results. However, the most recent empirical evidence suggests that many polygenes, acting together, could constitute a risk factor for schizophrenia. It is thus most probable that genetic susceptibility to schizophrenic psychoses is polygenic, and that their effects are dependent on interaction with physical and psychosocial environmental factors.

Association study of schizophrenia and the dopamine D3 receptor gene locus in two independent samples

Using a case-control design, an association of schizophrenia with the dopamine D3 receptor gene (D3RG) locus was investigated. Initial analysis of pooled results from published studies revealed a significant excess of individuals homozygous for either allele among the patients. The association was next tested in two cohorts ascertained independently at Pittsburgh, Pennsylvania and at Houston, Texas. The Pittsburgh sample was comprised of patients with schizophrenia (DSM-III-R) (n = 130). The controls belonged to two groups: adults screened for the absence of substance abuse or major psychiatric illness (n = 128), and neonates (n = 160). Multivariate analysis suggested an association with allele 1 of the biallelic D3RG polymorphism in comparison with the adult, but not the neonatal, controls. The association was most marked among Caucasian patients with a family history of schizophrenia (odds ratio 13.69, confidence intervals 1.80, 104.30). Survival analysis suggested an earlier age of onset among male patients homozygous for allele 2. The Houston cohort included Caucasian patients with schizophrenia or schizoaffective disorder (DSM-III-R criteria, n = 50), and normal controls matched for gender (n = 51). In this group, no significant associations were noted among all the patients or among subgroups of patients based on family history or age of onset. Possible reasons for the discordant results are discussed.

Relationship between homozygosity at the dopamine D3 receptor gene and schizophrenia

We have reported an association between schizophrenia and homozygosity of a Bal I polymorphism in the first exon of the dopamine D3 receptor gene (Crocq et al.: Journal of Medical Genetics 29:858-860, 1992). The present study consists of an attempt to replicate this finding in a further sample of 66 patients and 97 controls. Once again more patients than controls were homozygous, but the effect was not as strong as in our first study (chi 2 = 2.53, P = 0.05, one tailed). When pooled data from our two studies were analysed, excess homozygosity in patients remained highly significant (P = 0.002) with a particular excess of the 1:1 genotype (P = 0.01). This reflected a departure from Hardy-Weinberg equilibrium in the patients (P = 0.0005) but not the controls (P = 0.24). This led us to explore the possibility that there might be important differences between the patients in our two studies and that excess homozygosity might be a characteristic of particular subgroups of schizophrenics. Our findings suggest that the effect is consistently at its strongest in those patients who have a high familial loading and in those who have a good response to neuroleptic treatment, and that differences between our two samples might have contributed to the quantitatively different outcomes.