Biomedical subjects
Publications and source records attributed to Neil Risch.
A debate has arisen regarding the validity of racial/ethnic categories for biomedical and genetic research. An epidemiologic perspective on the issue of human categorization in biomedical and genetic research strongly supports the continued use of self-identified race and ethnicity.
Recent genetic investigations of autism have studied multiplex families, typically including families with multiple siblings who meet criteria for a diagnosis of autism. However, little is known about the specific behavioral characteristics of siblings with autism in these multiplex families. We investigated the behavioral phenotypic variability and similarity of 351 siblings with autism in 171 multiplex families using cluster analysis and correlations. The results of cluster analyses showed that the individuals with autism could be characterized on a severity gradient: a continuum based on severity of symptoms and impairment as measured by Autism Diagnostic Interview-Revised (ADI-R) scores, verbal-nonverbal status, and nonverbal IQ scores. Clusters based on scores from the ADI-R for the autism diagnostic criteria of the DSM-IV and nonverbal IQ scores still represented a severity gradient when the effects of verbal-nonverbal status were removed. The severity gradient was shown to be heritable, with a sib correlation of 30% or a heritability of 60%. In summary, in a sample of 171 autism multiplex families, there was no evidence of discrete behaviorally defined subgroups of affected individuals or families characterized by distinct patterns of behavioral symptoms. Rather, the clusters could be characterized along a single, heritable, continuous severity dimension.
Resting heart rate is significantly associated with cardiovascular morbidity and mortality. However, the extent to which resting heart rate is genetically determined is poorly understood, and no genes have been found that contribute to variation in resting heart rate. Because signaling through the beta1 adrenergic receptor is a key determinant of cardiac function, we tested whether polymorphisms in this receptor are associated with resting heart rate. A cohort of >1,000 individuals of Chinese and Japanese descent, from nuclear families, was genotyped for two polymorphisms, resulting in a serine/glycine substitution at amino acid 49 (Ser49Gly) and an arginine/glycine substitution at residue 389 (Arg389Gly), in the beta1 adrenergic receptor. For comparison, polymorphisms in the beta2 and beta3 adrenergic receptors were also evaluated. The Ser49Gly polymorphism was significantly associated (P=.0004) with resting heart rate, independent of other variables, such as body-mass index, age, sex, ethnicity, exercise, smoking, alcohol intake, hypertension status, and treatment with beta blockers. The data support an additive model in which individuals heterozygous for the Ser49Gly polymorphism had mean heart rates intermediate to those of either type of homozygote, with Ser homozygotes having the highest mean heart rate and with Gly homozygotes having the lowest. Neither the Arg389Gly polymorphism in the beta1 adrenergic receptor nor polymorphisms in the beta2 and beta3 adrenergic receptors were associated with resting heart rate. The heritability of heart rate was 39.7% +/- 7.1% (P<10-7).
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A recent report suggested that the HoxA1 and/or HoxB1 genes play a role in susceptibility to autism. To determine whether these findings could be confirmed, we screened these genes for DNA polymorphisms by sequencing all exons in 24 individuals with autism. We identified the same sequence variants in the genes that appeared in this report, which include one single-base substitution variant in HoxA1 and a common haplotype in HoxB1. We performed an association study by applying the transmission disequilibrium test to detect possible association of these variants to autism in 110 multiplex families. Our results demonstrated no deviation from the null hypothesis of no association. We have also separately examined transmissions within individual mating types, for paternal versus maternal alleles, to affected versus unaffected children, and for transmission to affected boys versus girls. None of these subsets revealed significant deviation from the null expectation. Our interpretation of these findings is that it is unlikely that HoxA1 and HoxB1 play a significant role in the genetic predisposition to autism.
We illustrate how homozygosity of haplotypes can be used to measure the level of disequilibrium between two or more markers. An excess of either homozygosity or heterozygosity signals a departure from the gametic phase equilibrium: We describe the specific form of dependence that is associated with high (low) homozygosity and derive various linkage disequilibrium measures. They feature a clear biological interpretation, can be used to construct tests, and are standardized to allow comparison across loci and populations. They are particularly advantageous to measure linkage disequilibrium between highly polymorphic markers.