CHILD ABUSE AND ADULT DEPRESSION
To examine the interaction of the CRHR1 genotype and ELS, we initially analyzed data from our first sample group of 560 low-income, primarily (97.4%) African American adults living in an urban area. Of the 476 subjects with complete CTQ and BDI phenotypes, we first examined the effects of child abuse on adult depression. As expected based on prior literature, higher CTQ abuse scores predicted higher adult BDI scores (F3,472 = 37; P <.001) (). Additionally, when child abuse is divided into the none to mild and moderate to severe groups, we see a significant difference in BDI scores (none to mild: n=300, mean BDI score, 11.58; moderate to severe: n=176, mean BDI score, 19.74; P <.001).
Figure 2 Child abuse interacts with the CRHR1 genotype to enhance risk for depression in adults. A, Mean Beck Depression Inventory (BDI) scores (n = 476) are predicted by continuous scores on the Childhood Trauma Questionnaire (CTQ) physical, sexual, and emotional (more ...)
CRHR1 POLYMORPHISMS AND LD STRUCTURE
The gene encoding CRHR1 is located on chromosome 17q21.31 and contains 13 exons spanning 51 kb. Of the 502 participants in the study from whom we obtained DNA, 422 were genotyped for 15 SNPs located in a 57-kb region spanning the 5′ promoter region to the 3′ end of the CRHR1 gene, with an average intermarker distance of 4.1 kb (). We identified 2 blocks of LD: one within intron 1 spanning at least 8 kb (block 1), and the second from introns 2 through 9 spanning at least 18 kb (block 2). Ten of the SNPs showed a minor allele frequency greater than 5.0% and were thus included in the gene × environment regression analysis described later.
CRHR1 POLYMORPHISMS INTERACT WITH CHILD ABUSE TO PREDICT ADULT DEPRESSION
To test the primary hypothesis that genetic polymorphisms in the CRHR1 gene moderate the effects of child abuse on adult depression, we used linear regression–based methods. We first performed single-SNP analyses in which we regressed, using permutation analyses, continuous BDI scores on genotype (coded under an additive model), child abuse (none to mild vs moderate to severe), and the interaction between genotype and child abuse, adjusting for potential confounders such as age and sex. Statistical significance of main effects and interactions were evaluated using a permutation-based analysis (see the “Methods” section). Prior to adjusting for multiple comparisons, 7 of the 10 tested SNPs showed a significant interaction (lowest P=.008 for rs110402) with child abuse for the prediction of adult depression ().
INTERACTION OF INDIVIDUAL CRHR1 POLYMORPHISMS, CHILD ABUSE, AND DEPRESSION
To correct for multiple comparisons, we used spectral decomposition techniques to determine the effective number of independent SNPs in regions where LD exists. Using this method, we determined that α=.0094 provides an appropriate threshold to declare significance within the CRHR1 gene. This α level is derived from 5.3 independent tests accounting for the moderate extent of LD across the 10 tested SNPs.
The interactions between child abuse and 2 of the CRHR1 SNPs—rs110402 and rs7209436—remained significant after correction for multiple testing (), and 5 others showed significant interaction effects prior to correction ( and ). The rs110402 and rs7209436 SNPs both reside in the first LD block of CRHR1 that spans intron 1 of the gene and are in high LD (r2=0.93). For both SNPs, no significant genotype effect on depression was detected in the no to mild abuse group, with the mean BDI scores from these subjects ranging from 10.91 to 12.67 across genotypes (). Among individuals in the moderate to severe abuse groups, the rare allele of both SNPs had a protective effect on the severity of adult depressive symptoms. While individuals homozygous for the common alleles (CC for rs7209436 and GG for rs110402) had average BDI scores of 22.33 to 22.49 (reflective of moderate depressive symptoms), heterozygous individuals had significantly lower BDI scores (mean, 16.31–16.39, reflective of mild depressive symptoms) and individuals homozygous for the rare alleles (TT or AA, respectively) had a mean BDI score of 10.22 (reflective of low to absent levels of depressive symptoms) despite the presence of at least 1 type of moderate to severe child abuse. The data suggest an additive protective genetic effect of the rare allele of these 2 SNPs in those exposed to child abuse. We also note that similar interactive gene dosage patterns were seen across the other SNPs that were significant prior to correction, including rs242924 (), rs4792887, rs242940, rs173365, and rs242948 ().
Figure 3 Effect of rs242940 (A), rs173365 (B), rs4792887 (C), and rs242948 (D) genotypes and child abuse on adult depression. Error bars indicate SEM. The sample sizes for no to mild child abuse and moderate to severe child abuse are as follows: rs242940: AA, (more ...)
CRHR1 HAPLOTYPES SHOW SIMILAR GENE × ENVIRONMENT INTERACTION
A similar permutation-based linear regression approach was applied to investigate gene × environment interactions involving common CRHR1 haplotypes (haplotype frequency >10.0%) in haplotype blocks 1 and 2 () as well as a haplotype defined by the 3 SNPs carrying the strongest gene × environment interactions in the individual SNP analysis. In block 1, we observed 3 common haplotypes of approximately equal frequency (30.4%–34.1%) that summed to account for approximately 98.5% of the observed haplotypes (). Haplotype interaction analysis showed a significant interaction effect of child abuse and haplotypes within block 1. This association was accounted for by the haplotype formed of alleles T (rs7209436), C (rs4792887), and A (rs110402), which appears to be protective in a dose-dependent manner (P <.001) (). The other 2 common haplotypes within block 1 and those within block 2 did not show a significant interaction effect.
Figure 4 Effect of estimated CRHR1 haplotypes and child abuse on adult depressive symptoms. The frequencies of the estimated individual haplotypes within the first linkage disequilibrium block (A) and of the 3 most significant single-nucleotide polymorphisms (SNPs) (more ...)
We then investigated the haplotypes formed by the 3 most significant SNPs in the single-SNP analysis, of which 2 are located in block 1 (rs7209436 and rs110402) and 1 is between blocks 1 and 2 (rs242924). These 3 SNPs form 2 common “yin and yang” haplotypes in our sample accounting for more than 95.0% of the observed haplotypes (CGG=66.5% and TAT=28.8%) (). The haplotypes formed by these 3 SNPs showed a significant interaction with child abuse on adult BDI scores (P=.003). The presence of the rarer haplotype appears to decrease the risk of adult depressive symptoms in an additive manner in those with a history of child abuse.
GENETIC ADMIXTURE AND POPULATION STRATIFICATION
Varying degrees of genetic admixture of sub-Saharan African, European, and Native American ancestry have been reported in African American individuals31,32
and could lead to population stratification and thus spurious association results. To control for potential differences in genetic ancestry, we genotyped 134 AIMs.31,39
Differences in the extent of admixture in our sample do not account for the observed results because genetic ancestry was correlated with neither the extent of child abuse (Pearson correlations of genetic ancestry with CTQ total abuse scores were nonsignificant, ranging from −0.033 to 0.048 for the 4 different tested populations) nor adult BDI scores (Pearson correlations of genetic ancestry with BDI total scores were nonsignificant, ranging from −0.094 to 0.099). To confirm that admixture was not an important confounder in our analysis, we reinvestigated the interaction effect of rs110402, the SNP with the lowest P
value, by using linear regression and adding the percentages of sub-Saharan and European ancestry as independent variables. In this model, the interaction effect between the rs110402 genotype and child abuse on BDI scores was less than P
=.04 in a smaller sample of 210 individuals). Percentages of sub-Saharan and European ancestry were not significant predictors in this model, with P
values of.38 and.80, respectively. Because the β values for the interaction term were not significantly different with and without ancestry in the model (β [SE], 6.56 [2.70] and 5.59 [2.01], respectively), we also concluded that ancestry is not a confounder in our analysis. Overall, the interaction term remained at P
≤.05 for 5 of the 10 CRHR1
SNPs when adjusting for admixture.
SUPPORTIVE EVIDENCE FROM AN INDEPENDENT SAMPLE
These data suggest that certain alleles of the CRHR1 gene may provide a protective effect on the risk for depression in adults with a history of child abuse. To confirm this hypothesis, we genotyped rs7209436, rs110402, rs4792887, rs242924, and rs242940 in an independent sample that was ethnically (87.7% Caucasian) and socioeconomically (less impoverished) distinct from the original population described earlier. We analyzed the allele distribution of the previously identified protective SNP alleles and haplotypes TCA (rs7209436, rs110402, and rs4792887) () and TAT (rs7209436, rs110402, and rs242924) () in women with and without a lifetime diagnosis of MDD and with and without child abuse (n=199). For haplotype block 1, we found the following frequencies: TCA, 42.1%; CGG, 43.0%; and CTG, 13.1%. For the best SNP haplotype, we found frequencies of 41.6% for TAT and 56.0% for CGG.
We hypothesized that we would find an overrepresentation of 2 copies of the protective haplotypes in women with a history of moderate to severe child abuse but who had not developed MDD within their lifetimes. Indeed, we found a significant overrepresentation of the protective alleles of the tested SNPs (most significant in replication rs7209436 and rs242940) and of 2 protective copies of both the TCA and TAT haplotypes in this group (). In the group with child abuse but no MDD, 35.0% carried 2 copies of these haplotypes; however, in the group with no child abuse and no MDD, only 17.0% had 2 copies (). In contrast, in the group with a lifetime diagnosis of MDD, there were no significant differences in the distribution of haplotype copy numbers regardless of child abuse history.
The group with a history of child abuse but no MDD appears to be protected from depression despite the increased psychosocial risk. Notably, this group also has a relative increase in the copy numbers of protective CRHR1 alleles compared with those with a history of lifetime depression (), underscoring the potential protective effects of the identified genotypes in 2 independent and ethnically different samples.