Sequence analysis of the mannose-binding lectin (MBL2) gene reveals a high degree of heterozygosity with evidence of selection

Abstract

Human mannose-binding protein (MBL) is a component of innate immunity. To capture the common genetic variants of MBL2, we resequenced a 10.0 kb region that includes MBL2 in 102 individuals representing four major US ethnic groups. In all, 87 polymorphic sites were observed, indicating a high level of heterozygosity (total p ¼ 18.3 Â 10 À4 ). Estimates of linkage disequilibrium across MBL2 indicate that it is divided into two blocks, with a probable recombination hot spot in the 3 0 end. Three non-synonymous SNPs in exon 1 of the encoding MBL2 gene and three upstream SNPs form common 'secretor haplotypes' that can predict circulating levels. Common variants have been associated with increased susceptibility to infection and autoimmune diseases. The high frequencies of B, C and D alleles in certain populations suggest a possible selective advantage for heterozygosity. There is limited diversity of haplotype structure; the 'secretor haplotypes' lie on a restricted number of extended haplotypes, which could include additional linked SNPs, which might also have possible functional implications. There is evidence for gene conversion in the region between the two blocks, in the last exon. Our data should form the basis for conducting MBL2 candidate gene association studies using a locus-wide approach. Keywords: mannose-binding lectin; innate immunity; genetic variation; recombination hot spot; gene conversion IntroductionThe human mannose-binding protein (MBL) is an important component of the innate immune system and provides first-line protection against pathogens as an 'ante-antibody'. 1 MBL is a C-type collectin that functions as a pattern-recognition molecule in the immediate response to invading organisms. Carbohydrate structures on the surfaces of microorganisms (bacteria, fungi and parasites) are recognized by MBL, 2-5 which, in turn, lead to opsonization or activation of the lectin pathway of complement via associated mannosebinding lectin serine proteases (MASP2). [6][7][8] Decreased serum levels of MBL have been correlated with an increased risk for infection in both healthy children and immunocompromised individuals. Circulating levels of MBL and functional activity are partially regulated by genetic variation in the MBL2 gene, which encodes MBL. 9,10 The MBL2 gene is located on chromosome 10q11.2-21 and consists of four exons. 11 Each of the three structural gene polymorphisms in exon 1, known as the B, C and D alleles, interfere with the formation of higher MBL oligomers, leading to alterations in function and circulating levels. [12][13][14][15] Two strongly linked single-nucleotide polymorphisms (SNPs) lie in the proximal promoter (known as L/H and X/Y), as well as an SNP in the 5 0 UTR (known as P/Q); together, these upstream SNPs are linked to the three independent nonsynonymous SNPs (ie, B, C and D) to form the 'secretor haplotypes', which have previously been shown to partially account for alterations in complement activation and decreased circulating levels of MBL. 7,9,10,[16][17][18][19][20] Using ...

Association of a new mannose-binding lectin variant with severe malaria in Gabonese children

“…The g.388G4A, g.578G4A and g.797C4A variants were identified in another study, but without precise knowledge of the pattern of cooccurrence with another MBL2 variants (allelic haplotype). 30 The g.712A4T variant, found in our study with an overall frequency of 1.3%, was previously identified as g.712A4G (called -180A4G), with a 2% frequency in a sample of 24 Afro-Americans. 30 Allelic haplotypes under 'remarks' were thus defined in this study.…”

“…30 The g.712A4T variant, found in our study with an overall frequency of 1.3%, was previously identified as g.712A4G (called -180A4G), with a 2% frequency in a sample of 24 Afro-Americans. 30 Allelic haplotypes under 'remarks' were thus defined in this study. Amino-acid changes were deduced from sequence variations in bold.…”

Mannose-binding lectin and its genetic variants

“…Recently, sequencing of a 10 kb region that includes the whole MBL2 gene (with introns) was performed in individuals representing four major American ethnic groups as well as a follow-up in Dutch Caucasians. 46,47 In total, 87 polymorphic sites were detected. These studies confirmed the strong preservation of the classical seven MBL2 haplotypes.…”

“…Although it is not certain whether these putative selective and counter balancing forces are operating in present day populations, sequence analysis of the extended MBL2 haplotypes of different US ethnic groups indicates a higher degree of heterozygosity than expected across the MBL2 gene, which indeed supports the notion that such forces are working even today. 46 So far, the putative selective forces have not been determined and may at this stage be regarded as hypotheses. Two assumptions are prevailing and both would confer survival advantages in response to infections in the reproductive age particularly in small children when innate immune mechanisms are relatively more important than adaptive immune mechanisms.…”

Evolution of the mannose-binding lectin gene in primates

“…5,6 Thus, it has been suggested that this variability confers relative advantages for the hostgenerally associated with heterozygosity-and represents a balanced genetic system. [7][8][9] The MBL2 concentrations are influenced by the allele '0' 10 in the first exon and, independently, by a number of nucleotide substitutions in the promoter region of the MBL2 gene. Particularly, a base pair substitution in codon À221 (G-C substitution), changing the promoter polymorphism from Y to X, has a profound downregulating effect on the serum MBL2 level.…”