Janet L. Sobell

Direct sequencing of the dopamine D2 receptor (DRD2) in schizophrenics reveals three polymorphisms but no structural change in the receptor

The dopamine D2 receptor gene (gene symbol DRD2) is a candidate gene for schizophrenia because the potency of certain neuroleptics correlates with their affinity for this receptor. Seven regions of likely functional significance including the coding sequences and the splice junctions were fully sequenced in the dopamine D2 receptor of 14 schizophrenics (and partially in several others) meeting DSM-III-R diagnostic criteria and in four unaffected non-Caucasians (97 kb of total sequence). No structural changes were found, suggesting that alteration in the structure of the dopamine D2 receptor is not commonly involved in the etiology of schizophrenia. However, two common and one uncommon intragenic polymorphisms were found. At least one of the polymorphisms was informative for linkage in 70% of Caucasians and 78% of Koreans.

Screening the monoamine oxidase B gene in 100 male patients with schizophrenia: A cluster of polymorphisms in African‐Americans but lack of functionally significant sequence changes

The monoamine oxidase B (MAO-B) gene was examined in 100 alleles derived from 80 Caucasian, 10 African-American, 5 Asian, and 5 Native American male patients with schizophrenia to identify sequence changes that might be associated with the disease. Approximately 235 kb of genomic sequence, primarily in coding regions, were screened by dideoxy fingerprinting, a modification of single-strand conformational polymorphism (SSCP) analysis that detects virtually 100% of sequence changes [Sarkar et al. (1992): Genomics 13:441-443; Liu and Sommer (1994): PCR Methods Appl 4:97-108]. No sequence changes of likely functional significance were identified, suggesting that mutations affecting the structure of the MAO-B protein are uncommon in the general population and are unlikely to contribute significantly to the genetic predisposition to schizophrenia. Eight polymorphisms were identified in African-Americans and Native Americans, but none were identified among Caucasians. Of the eight observed polymorphisms, a set of five transitions and one microdeletion was identified within approximately 17 kb of genomic sequence in the same 3 African-American individuals, while the remaining 7 African-Americans had a sequence identical to that in Caucasians. The presence of two such haplotypes, without intermediates, is compatible with the hypothesis that germline mutations can occur in clusters, as also suggested by other recent findings.

Application of DNA-Based Diagnosis to Patient Care: The Example of Hemophilia A

DNA-based carrier testing and prenatal diagnosis are rapidly expanding medical applications of recombinant-DNA technology. The ultimate goal of DNA-based diagnosis is the determination of the causative mutation, but, in general, this is possible only for large deletions, insertions, or certain nonsense mutation that, in most diseases, involve only a few percent of affected families. If direct diagnosis of the carrier state or fetal disease state is not feasible, indirect diagnosis can be performed by following the segregation of linked polymorphisms through the family pedigree. For such indirect diagnosis, DNA from multiple family members must be analyzed. Although this procedure is highly accurate in many families, errors can potentially occur because of meiotic recombination, genetic heterogeneity, new mutations, and nonpaternity. In this review, a general introduction to DNA-based diagnosis of mendelian diseases is presented and the methods and strategy are outlined. The use of these techniques for the diagnosis of hemophilia A is then described to illustrate the principles of diagnosis and to highlight some of the complexities encountered. DNA-based diagnosis is in its infancy and has the potential to revolutionize preventive medicine.

A common exonic polymorphism in the human D5 dopamine receptor gene

We report what is to our knowledge the first defined polymorphism in the human D5 dopamine receptor gene (D5DR).

Genotype-to-phenotype analysis : Search for clinical characteristics of a missense change in the GABAA-β1 receptor gene

Genotype-to-phenotype analysis reverses the classical approach to genetic disease in which an unknown genotype is sought for a known phenotype. This paper provides an example of genotype-to-phenotype analysis for the possible psychiatric effects of a missense mutation (H396Q) at a highly conserved residue of the beta 1 subunit gene of the gamma aminobutyric acid type A receptor. DNA samples from 1,507 Caucasians of Western European descent were screened, and 10 heterozygotes for H396Q were identified. These individuals were matched to homozygous normal individuals by age, gender, and length of available medical records. The complete medical records of these 20 individuals were reviewed blindly by two psychiatrists (D.C.S., L.L.H.) to assess psychiatric symptomatology, with an emphasis on anxiety and related disorders. However, no association was found between this missense change at a conserved amino acid and a dominant neuropsychiatric disease phenotype. Thus, this missense change may be neutral or only mildly deleterious, may only cause recessive disease in rare individuals, or may interact epistatically with some other gene(s).

APP mutations and schizophrenia

The amyloid precursor protein (APP) has been the subject of considerable scientific interest because of its putative role in Alzheimers disease and certain forms of cerebral amyloid angiopathy. Several mutations have been discovered in exon 17 including ones at codons 692, 693, 713, and 717, which may alter normal proteolysis and subsequently cause pathologic deposition of ~/A4, a 4 kDa breakdown product derived from exons 16 and 17 of the APP gene. An intriguing finding was reported by Jones and colleagues (Jones et al 1992) of an Ala 7~3 --* Va1713 mutation in a 62 year-old woman with chronic schizophrenia. It is difficult to evaluate the significance of the Ala 713 ~ Val ~i3 substitution since (1) neurohistopathological data are not available for this individual, (2) linkage data for family members are not available, and (3) screening by these authors of an additional 100 unrelated schizophrenics for the Maelli restriction site generated by the mutation did not reveal any positive findings. However, Ala ~13 is evolutionarily conserved in all of the I0 previously reported mammalian species (Johnstone et al 1991) and in M. fascicularis, suggesting that this residue is important for normal protein function.