15 September 2010. Neuregulin 3, the little-understood cousin of neuregulin 1, has been drawing attention as a schizophrenia candidate gene itself. Two recent studies back earlier hints that it plays some role in the disorder. In a study published June 15 in Molecular Psychiatry online, Assen Jablensky and colleagues find that single-nucleotide polymorphisms (SNPs) in NRG3 may contribute to a cognition-sparing form of schizophrenia, but not to schizophrenia with cognitive deficits. In the August 16 online version of the Proceedings of the National Academy of Sciences, Amanda Law and colleagues tie these same polymorphisms to schizophrenia and to the severity of delusions. They also reveal a plethora of new splice variants of the protein, some of which are altered in postmortem schizophrenia brain tissue, as well as evidence that the clinical risk SNPs can alter relative expression levels of certain splice variants.
NRG3 and its relatives in the neuregulin gene family make cell-cell signaling proteins (see Falls, 2003; Taveggia et al., 2005; Zhang et al., 1998). These neuregulin proteins serve as ligands for receptor tyrosine kinases of the ERbB family. Like NRG1 (see SRF related news story; SRF news story; SRF news story), NRG3 binds and activates ErbB4, which is encoded by another schizophrenia candidate gene (see SRF related news story).
In the only species that has been characterized—the mouse—NRG3 expression occurs mainly in the central nervous system, during both development and adulthood (Zhang et al., 1997; see Entrez Gene entry), and the gene inhabits chromosome region 10q22-q23, which has been implicated in schizophrenia (Fallin et al., 2003). Research also fingers this region in cognitive impairment and autism, among others (Balciuniene et al., 2007).
Studies have tied NRG3 single-nucleotide polymorphisms (SNPs) to schizophrenia in Chinese (Wang et al., 2008) and Scottish (Benzel et al., 2007) populations (see SRF related news story; also see the SZGene entry on NRG3). However, a study by Chen and colleagues in subjects of Ashkenazi Jewish descent found no clear relationship between NRG3 gene markers and schizophrenia itself, although it did connect three SNPs—rs10883866, rs6584400, and rs10748842—to a quantitative measure of delusions (Chen et al., 2009).
A mixed bag
The findings from Chen and colleagues sparked the interest of Jablensky, first author Bharti Morar, and others at the University of Western Australia in Perth. They sought to confirm and extend the findings in a new sample. To do so, they genotyped 411 patients with schizophrenia and 223 healthy control subjects for rs10883866 and rs6584400, the SNPs most strongly related to delusions in the Chen study. Case-control analyses of these mostly Anglo-Irish descendents tied rs6584400 to schizophrenia, in contrast to the Ashkenazi study.
Subjects also completed neurocognitive tests, and the researchers plugged the results into a latent structure analysis. This uncovered three groups of patients: 1) 180 with pervasive cognitive deficits, 2) 148 with relatively spared cognition, and 3) 83 patients, age 45 and up, who showed mild cognitive impairment, mainly slow information processing.
Morar and colleagues compared control subjects with the first two groups of patients, skipping the third group because their deficits might be age-related. As it turned out, the association of rs6584400 with schizophrenia came only from the group of patients with relatively intact cognition. Prior studies portray such patients as having Schneiderian first-rank symptoms, florid delusions, and positive thought disorder, in contrast to those with greater cognitive impairment. The latter tend to exhibit negative symptoms and impaired social functioning, but not complex delusions (see Jablensky, 2006). Merging these findings with their own and those from the Chen study, Morar and colleagues suggest that NRG3 contributes “to a subtype of schizophrenia with relatively preserved cognitive function but prone to florid and complex delusions.”
The researchers also checked the allelic association of the two SNPs to specific cognitive traits. For both rs10883866 and rs6584400, they found no connection to general intelligence or verbal memory, but these SNPs’ minor alleles did relate to performance on the Degraded Stimulus-Continuous Performance Task. This requires paying attention to a speedy presentation of a series of unclear digits.
The alleles seemed to affect patients and control subjects differently, with the minor alleles tied to better performance in subjects with schizophrenia and worse performance in healthy subjects. “This suggests that NRG3 may be modulating early attentional processes for perceptual sensitivity and vigilance, with opposing effects in affected individuals and healthy controls,” wrote Morar and colleagues.
Whether NRG3 promotes one form of schizophrenia or the disease overall, the mystery of how it might do so and what functions it serves prompted the study by Law and colleagues. Law, first author Wee-Tin Kao, and others at the National Institute of Mental Health, Bethesda, Maryland, started by checking for associations between NRG3 polymorphisms on the one hand, and schizophrenia or its symptoms on the other. They also explored possible explanations for how genetic variation in NRG3 might contribute to schizophrenia by altering the expression of different isoforms of the protein.
In family-based analyses, the team found associations between schizophrenia and 12 SNPs in a noncoding part of NRG3. That stretch includes the aforementioned rs6584400, rs10748842, and rs10883866. Deconstructing schizophrenia, Kao and colleagues verified earlier findings relating these three SNPs to delusions, measured in this study by the delusion severity subscale of the Positive and Negative Syndrome Scale. They also connected the three SNPs to the severity of positive symptoms and tied rs10748842 to negative symptom load.
However, the “replication” proved less than straightforward. In contrast to the Chen and Morar studies, which fingered the rarer alleles, the new study blamed the common ones. According to Kao and colleagues, ethnic differences in the populations studied might explain these contrasting results. Their own family-based analyses examined white Americans of Western European descent. "Flipping of disease alleles in different populations has been observed for a number of other diseases, such as autism (HTTLPR locus) and Alzheimer disease (apolipoprotein E ε4-related polymorphisms)," the authors write, adding that this "has been suggested to be a valid biological phenomenon as a result of heterogeneous effects of the same variant related to multi-locus interactions (i.e., effects of epistasis), environment, or LD, with causal variants that emerged on different genetic backgrounds."
They note, however, that the protective alleles in their study (at rs10883866 and rs6584400) are the ones associated with ”better” cognitive performance in patients with schizophrenia in the study by Morar and colleagues, "suggesting complex genetic association with the schizophrenia phenotype."
Many ways to splice it
To learn more about what the gene's products, Kao and colleagues sequenced NRG3 clones from adult human hippocampus and whole brain. They found that, through alternative splicing, NRG3 makes 15 different isoforms (see SRF related news story on similar findings for NRG1). In data that may tie some of these isoforms to disease, the researchers found greater expression of certain splice variants in dorsolateral prefrontal cortex tissue from subjects with schizophrenia versus healthy control subjects.
The researchers thought that rs10748842, a standout in the family-based analyses, might regulate NRG3 expression in the brain. Although it is a noncoding nucleotide, rs10748842 lies near a site that regulates transcription of a fetal form of NRG3, and mathematical modeling put it in the middle of a binding unit for a family of transcription factors. These clues led the researchers to try to connect allelic variation at rs10748842 to the expression of different NRG3 transcripts. They write that such variation "strongly predicts expression (P = 1.097E−12 to 1.445E−15) of specific, developmentally regulated NRG3 isoforms in the normal and developing human brain and in schizophrenia, whereby the ancestral (T) allele is associated with elevated expression." Again, the common allele seemed to bestow greater risk than the rare one.
This study adds flesh to the shadowy NRG3 and further builds the case for it as a schizophrenia candidate gene, and, with the work from the Morar team, supplies plenty of leads for interested researchers to untangle.—Victoria L. Wilcox.
Kao W-T, Wang Y, Kleinman JE, Lipska BK, Hyde TM, Weinberger DR, Law AJ. Common genetic variation in Neuregulin 3 (NRG3) influences risk for schizophrenia and impacts NRG3 expression in human brain. Proc Natl Acad Sci U S A. 2010 Aug 16. Abstract
Morar B, Dragović M, Waters FAV, Chandler D, Kalaydjieva L, Jablensky A. Neuregulin 3 (NRG3) as a susceptibility gene in a schizophrenia subtype with florid delusions and relatively spared cognition. Mol Psychiatry. 2010 Jun 15. Abstract