Genetic Influences on Plasma Homocysteine Levels in African Americans and Yoruba Nigerians

Abstract

Plasma homocysteine, a metabolite involved in key cellular methylation processes seems to be implicated in cognitive functions and cardiovascular health with its high levels representing a potential modifiable risk factor for Alzheimer's disease (AD) and other dementias. A better understanding of the genetic factors regulating homocysteine levels, particularly in non-white populations, may help in risk stratification analyses of existing clinical trials and may point to novel targets for homocysteine-lowering therapy. To identify genetic influences on plasma homocysteine levels in individuals with African ancestry, we performed a targeted gene and pathway-based analysis using a priori biological information and then to identify new association performed a genome-wide association study. All analyses used combined data from the African American and Yoruba cohorts from the Indianapolis-Ibadan Dementia Project. Targeted analyses demonstrated significant associations of homocysteine and variants within the CBS (Cystathionine beta-Synthase) gene. We identified a novel genome-wide significant association of the AD risk gene CD2AP (CD2-associated protein) with plasma homocysteine levels in both cohorts. Minor allele (T) carriers of identified CD2AP variant (rs6940729) exhibited decreased homocysteine level. Pathway enrichment analysis identified several interesting pathways including the GABA receptor activation pathway. This is noteworthy given the known antagonistic effect of homocysteine on GABA receptors. These findings identify several new targets warranting further investigation in relation to the role of homocysteine in neurodegeneration.

Keywords: African Continental Ancestry Group; CD2-associated protein; cystathionine beta-synthase; genome-wide association study; homocysteine; metabolic networks and pathways; metabolomics.

Fig. 1

Schema of one-carbon metabolism pathways. DHF, dihydrofolate; THF, tetrahydrofolate; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; SLC19A1, solute carrier family 19 (folate transporter), member 1; MTHFR, methylenetetrahydrofolate reductase (NAD(P)H); TAT, tyrosine aminotransferase; MTRR, 5-methyltetrahydrofolate-homocysteine methyltransferase reductase; BHMT, betaine–homocysteine S-methyltransferase; MTR, 5-methyltetrahydrofolate-homocysteine methyltransferase; CBS, cystathionine-beta-synthase; CTH, cystathionase (cystathionine gamma-lyase); IL4I1, interleukin 4 induced 1; MAT2B, methionine adenosyltransferase II, beta; AHCYL1, adenosylhomocysteinase-like 1; DNMT1, DNA (cytosine-5-)-methyltransferase 1; LAP3, leucine aminopeptidase 3; ANPEP, alanyl (membrane) aminopeptidase; GGT6, gamma-glutamyltransferase 6.

Fig. 2

Manhattan plot of GWAS results. Red solid line indicates the genome-wide significant level (p = 5 × 10⁻⁸) and blue line shows p = 1 × 10⁻⁶.