Inferring mechanisms of copy number change from haplotype structures at the human DEFA1A3 locus

Runaway multi-allelic copy number variation at the α-defensin locus in African and Asian populations

Alpha defensins are anti-microbial peptides of the innate immune system. The defensin A1 and A3 genes are located in a repeat array of variable copy number (the DEFA1A3 locus) and encode the human neutrophil peptides 1, 2 and 3. The possibility that copy number variation (CNV) may be associated with infection susceptibility and autoimmune pathology motivated the study of DEFA1A3 CNV across populations. We enhanced two existing methods (one qPCR-based and one sequencing-based) to enable copy number estimation that discriminates between DEFA1 and DEFA3 genes. We used these methods to quantify A1/A3 copy number variation in 2504 samples from the 1000 Genomes high-coverage dataset as well as performing FiberFISH assays on selected samples to visualize the haplotypes. These methods produce accurate estimates and show that there are substantial differences between populations. The African population is a clear outlier with a high frequency of the ancestral pure DEFA1 haplotype, but also harbours exceptionally long haplotypes of 24 copies of both DEFA1 and DEFA3, whilst the East Asian population displays the highest mean level of DEFA3 copy number. Further, our findings demonstrate that qPCR can be an accurate method for CNV estimation and that defensins substantially extend the known range of copy number variation for a human protein-coding gene.

Low α-defensin gene copy number increases the risk for IgA nephropathy and renal dysfunction

Although a major source of genetic variation, copy number variations (CNVs) and their involvement in disease development have not been well studied. Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. We performed association analysis of the DEFA1A3 CNV locus in two independent IgAN cohorts of southern Chinese Han (total of 1189 cases and 1187 controls). We discovered three independent copy number associations within the locus: DEFA1A3 [P = 3.99 × 10−9; odds ratio (OR), 0.88], DEFA3 (P = 6.55 × 10−5; OR, 0.82), and a noncoding deletion variant (211bp) (P = 3.50 × 10−16; OR, 0.75) (OR per copy, fixed-effects meta-analysis). While showing strong association with an increased risk for IgAN (P = 9.56 × 10−20), low total copy numbers of the three variants also showed significant association with renal dysfunction in patients with IgAN (P = 0.03; hazards ratio, 3.69; after controlling for the effects of known prognostic factors) and also with increased serum IgA1 (P = 0.02) and galactose-deficient IgA1 (P = 0.03). For replication, we confirmed the associations of DEFA1A3 (P = 4.42 × 10−4; OR, 0.82) and DEFA3 copy numbers (P = 4.30 × 10−3; OR, 0.74) with IgAN in a Caucasian cohort (531 cases and 198 controls) and found the 211bp variant to be much rarer in Caucasians. We also observed an association of the 211bp copy number with membranous nephropathy (P = 1.11 × 10−7; OR, 0.74; in 493 Chinese cases and 500 matched controls), but not with diabetic kidney disease (in 806 Chinese cases and 786 matched controls). By explaining 4.96% of disease risk and influencing renal dysfunction in patients with IgAN, the DEFA1A3 CNV locus may be a potential therapeutic target for developing treatments for this disease.

Evolution of the rapidly mutating human salivary agglutinin gene (DMBT1) and population subsistence strategy

The dietary change resulting from the domestication of plant and animal species and development of agriculture at different locations across the world was one of the most significant changes in human evolution. An increase in dietary carbohydrates caused an increase in dental caries following the development of agriculture, mediated by the cariogenic oral bacterium Streptococcus mutans. Salivary agglutinin [SAG, encoded by the deleted in malignant brain tumors 1 (DMBT1) gene] is an innate immune receptor glycoprotein that binds a variety of bacteria and viruses, and mediates attachment of S. mutans to hydroxyapatite on the surface of the tooth. In this study we show that multiallelic copy number variation (CNV) within DMBT1 is extensive across all populations and is predicted to result in between 7-20 scavenger-receptor cysteine-rich (SRCR) domains within each SAG molecule. Direct observation of de novo mutation in multigeneration families suggests these CNVs have a very high mutation rate for a protein-coding locus, with a mutation rate of up to 5% per gamete. Given that the SRCR domains bind S. mutans and hydroxyapatite in the tooth, we investigated the association of sequence diversity at the SAG-binding gene of S. mutans, and DMBT1 CNV. Furthermore, we show that DMBT1 CNV is also associated with a history of agriculture across global populations, suggesting that dietary change as a result of agriculture has shaped the pattern of CNV at DMBT1, and that the DMBT1-S. mutans interaction is a promising model of host-pathogen-culture coevolution in humans.