Participants in the SAD group were 70% female, 95% Caucasian, 5% African American, and 1% Asian American, and middle aged (M = 40.1 years, SD = 8.3). They had an average GSS of 16.6 (SD = 3.3, min = 8, max = 24). Most participants (53%) had a diagnosis of unipolar Major Depressive Disorder, whereas 17% had a diagnosis of Biploar I Disorder, and 2% had Bipolar II Disorder (28% missing data). Among individuals with SAD, differences in GSS between the mood disorder diagnostic groups (i.e., unipolar, Bipolar I and II) were not statistically significant.
Participants in the control group were 58% female, 97% Caucasian, 3% African American, and middle aged (M = 39.8 years, SD = 10.7). Controls had a maximum GSS of 10 (M = 2.23, SD = 2.01). The SAD and control groups were not significantly different on the basis of age [F(1, N = 228) = 0.063, ns], gender [X2(1, N = 228) = 3.45, ns], or ethnicity [X2(2, N = 228) = 1.108, ns].
3.1 Allele and Genotype Association Tests
The distribution of all three genotypes (C/C, C/T, and T/T) and both alleles between the SAD and control groups at rs2675703 (P10L) did not differ [Genotype: Fisher’s Exact Test (2, N
= 228) = 5.09, p
= .09, ns
; Allele: Fisher’s Exact Test (1, N
= 456) = 2.29, ns
]. Upon observing that all seven individuals with the T/T genotype at P10L were in the SAD group, an autosomal recessive disorder interpretation was considered. If this were the case with P10L and SAD, the homozygous T/T genotype would be more common in SAD patients than in controls. A 2×2 test indicated SAD participants had a higher frequency of T/T than controls, when compared to the combined frequency of C/C and C/T together, Fisher's Exact test (1, N
= 220) = 4.38, p
< .05. Only 7 (5%) SAD participants had the T/T genotype compared to zero controls (). The effect size for this finding is medium, d
= 0.46, computed using the Arcsine test (Lipsey and Wilson, 2001
). The Odds Ratio, OR
= 5.63 [95% CI
1.22–26.01], indicated that individuals with the T/T genotype were 5.6 times more likely to be in the SAD group than in the control group. Demographic variables did not account for variance in this associated when tested using logistic regression; Genotype: OR
= 1.553 (95% CI
0.90 – 2.68), ns
; Allele: OR
= 1.67 (95% CI
0.94 – 2.97), ns
Frequency of genotype and alleles at the P10L & I394T loci.
The groups did not differ on frequency of genotypes and alleles at the I394T locus; Genotype: Fisher’s Exact Test (2, N = 217) = 1.50, ns, Allele: Fisher’s Exact Test (1, N = 434) = 0.668, ns. In addition, I394T genotype and allelic frequency did not predict group membership in logistic regression; Genotype: OR = 0.93 [95% CI 0.64 – 1.37], ns; Allele: OR = 0.805 (95% CI 0.53 – 1.22), ns.
The D444G locus in the melanopsin gene was found to be monomorphic in the present sample, with all participants having the homozygous genotype for the major allele (C). This is consistent with data reporting no polymorphisms at this site in Caucasian samples, and very low rates in non-Caucasian samples, accessed through online databases (dbSNP; Build 120/126).
The association between P10L genotype and GSS, with age, gender, race, and diagnosis (i.e., unipolar, Bipolar I, or Bipolar II) as covariates among SAD patients was nonsignificant, F(2, 94) = 1.33, ns. There was no association between I394T genotype and GSS among SAD patients, F(2, 86) = 1.75, ns.
3.2 Haplotype Analyses
Pairwise rates of linkage disequilibrium calculated with HaploView (Barrett et al., 2005) identified Logarithm of the Odds Ratio’s greater than 2 and D′ = 1 for each pair of markers, indicating the melanopsin gene exists as one haplotype block. Individuals with rare haplotypes (i.e., <2% frequency) were removed from further analyses as likely genotyping errors, yielding 190 individuals for the final haplotype analyses. Of the rare haplotypes deleted from the final analyses, the majority (70%) had neither the P10L or I394T coding variant, 8% had the P10L variant, 20% had the I394T variant, and 2% had both. Therefore, it is unlikely that potentially important haplotypes were removed.
There was no significant difference in haplotype distribution between the SAD and control groups, Fisher’s Exact Test (5, 378) = 3.75, ns. None of the covariates or the frequency of the 6 haplotypes predicted group membership (i.e., SAD or control) when tested with logistic regression controlling for demograpic variables. No association was found between haplotype frequencies and GSS among SAD patients.