Candidate Gene Study For Schizophrenia

Background: Several genome-wide linkage studies including our previous linkage study have suggested that chromosome 22ql2 and 6p may be harbor risk genes for schizophrenia. In the original candidate gene study, the NOTCH4 gene located in the 6p21.3, had been reported that there was an association with schizophrenia in a family-based association study in a British sample. However, all six subsequent replication studies failed to confirm the finding. On the other hand, with respect to 22q, significant linkage of intermediate phenotypes for schizophrenia in two independent studies was reported to markers within or near the beta-adrenergic receptor kinase 2 (ADRBK2) gene, which mediates homologus desensitization for a variety of neurotransmitters and hormones through phosphorylation of G protein-coupled receptors (GPCRs). We hypothesized that the ADRBK2 and NOTCH4 genes are susceptibility genes for schizophrenia.Methods: One hundred twenty three parent-offspring schizophrenic trios (16 Japanese trios and 107 Chinese trios) were recruited. For NOTCH4 gene, the single nucleotide polymorphisms (SNP) -- SNP1, and (CTG)n microsatalite repeat reported by original study were genotyped in all subjects through direct sequencing and PCR by capillary electrophoresis, respectively. For ADRBK2 gene, systematically mutation screening was performed in all 21 exonic and flanking intronic regions through direct sequencing in 48 schizophrenia probands (including 16 Japanese and 32 Chinese probands) randomly selected from our subjects, and the detected variants and eight SNPs reported in the JSNP database were genotyped in 64 trios. Results: For NOTCH4 gene, in addition to the original study'spolymorphisms, we detected four new SNPs - SNPs_A, B, C and D, which all are around SNP1 of the original study. We genotyped all samples for SNPs_A-D, besides SNP1 and (CTG)n of the original study. No significant associations were found between NOTCH4 and schizophrenia for all six polymorphisms. This finding remained negative when the Chinese sample was analyzed separately. However, when we took the clinical heterogeneity into account, our results showed that SNP_A may be associated with early-onset schizophrenia, and SNP1 may be associated with schizophrenia characterized by many negative symptoms. On the other hand, for ADRBK2 gene, although 15 novel single nucleotide variants were found in exonic and flanking intronic regions, the frequency of variants were very low, and only one rare missense variant and two silent variants were found in coding region. The missense variant Cys208Ser was only found in one family among 123 trios, and did not show any evidence for association with schizophrenia; Moreover, no association was obtained between the other 10 SNPs (two variants detected by mutation screening and 8 SNPs from JSNP) and schizophrenia.Conclusions: NOTCH4 is not a significant susceptibility gene for schizophrenia when clinical heterogeneity is ignored. However, NOTCH4 may associate with early-onset schizophrenia or schizophrenia with many negative symptoms, when clinical homogenous being taken into consideration. On the other hand, for ADRBK2 gene, our results suggest low nucleotide diversity in the ADRBK2 gene, and indicate that ADRJBK2 is unlikely to contribute strongly to schizophrenia susceptibility in this set of families.