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

Mannose-binding lectin genetics: from A to Z

MBL (mannose-binding lectin) is primarily a liver-derived collagen-like serum protein. It binds sugar structures on micro-organisms and on dying host cells and is one of the four known mediators that initiate activation of the complement system via the lectin pathway. Common variant alleles situated both in promoter and structural regions of the human MBL gene (MBL2) influence the stability and the serum concentration of the protein. Epidemiological studies have suggested that genetically determined variations in MBL serum concentrations influence the susceptibility to and the course of different types of infectious, autoimmune, neoplastic, metabolic and cardiovascular diseases, but this is still a subject under discussion. The fact that these genetic variations are very frequent, indicates a dual role of MBL. This overview summarizes the current molecular understanding of human MBL2 genetics.

Mannose-binding lectin gene polymorphisms in relation to periodontitis

AIM

To investigate the correlation of six functional polymorphisms in the MBL gene with MBL plasma levels in relation to periodontitis.

MATERIAL AND METHODS

A total of 92 periodontitis patients and 70 controls, all of Caucasian origin, were included. Patients and controls were genotyped for the L/H, X/Y, P/Q, A/D, A/B and A/C polymorphisms. Distributions of genotypes, rate of allele carriage and allele frequencies were compared between patients and controls. Patients and controls were subdivided in groups of genotypes. Plasma MBL levels were compared between different genotype groups.

RESULTS

On the basis of genotyping, three phenotypes with regard to mannose-binding lectin (MBL) production were distinguished: high-producers, low-producers and deficient subjects. No differences in the genotype frequencies were observed between patients and controls. Within patients and controls, subjects with the high-producing genotypes had significantly higher MBL plasma levels than low-producers and deficient subjects (p<0.001). Plasma MBL was higher in low-producer patients compared with low-producer controls (p(adjusted)=0.021).

CONCLUSION

No association could be observed between MBL gene polymorphisms and susceptibility to periodontitis in Caucasians. However, now that genotyping could distinguish the low producing and deficient subjects from the high-producers, it was observed, for the first time, that MBL acts as a weak acute-phase protein in periodontitis.

Intriguing balancing selection on the intron 5 region of LMBR1 in human population

BACKGROUND

The intron 5 of gene LMBR1 is the cis-acting regulatory module for the sonic hedgehog (SHH) gene. Mutation in this non-coding region is associated with preaxial polydactyly, and may play crucial roles in the evolution of limb and skeletal system.

METHODOLOGY/PRINCIPAL FINDINGS

We sequenced a region of the LMBR1 gene intron 5 in East Asian human population, and found a significant deviation of Tajima's D statistics from neutrality taking human population growth into account. Data from HapMap also demonstrated extended linkage disequilibrium in the region in East Asian and European population, and significantly low degree of genetic differentiation among human populations.

CONCLUSION/SIGNIFICANCE

We proposed that the intron 5 of LMBR1 was presumably subject to balancing selection during the evolution of modern human.

Independent effects of genetic variations in mannose-binding lectin influence the course of HIV disease: the advantage of heterozygosity for coding mutations

BACKGROUND

The in vivo impact of mannose-binding lectin (MBL), a molecule involved in innate immunity, on the pathogenesis of human immunodeficiency virus (HIV)-1 infection and AIDS is unknown.

METHODS

A total of 1102 HIV-positive and 2213 HIV-negative adult subjects were screened for polymorphisms in the coding and promoter regions of MBL2, the gene that encodes MBL.

RESULTS

Variations in MBL2 did not influence the risk of acquiring HIV-1. Heterozygosity for coding mutations (O allele) and homozygosity for the -221 promoter polymorphism (X allele) in MBL2 were associated with a delay in and an accelerated rate of disease progression, respectively. MBL2 variations influenced the rate of progression to AIDS-defining illnesses. In a multivariate model, the effects of MBL2 variations were independent of several parameters known to influence disease progression, including steady-state viral load, baseline CD4(+) T cell counts, and delayed-type hypersensitivity skin test responses, an in vivo marker of cell-mediated immunity. The effects of MBL2 variations were most evident in those who possessed protective genotypes of CCR5 and a high copy number of CCL3L1, the most potent HIV-suppressive CCR5 ligand.

CONCLUSIONS

MBL2 genotypes are independent determinants of HIV disease progression and heterozygosity for MBL2 coding mutations confer disease-retarding effects. MBL-dependent immune responses may play a role in the pathogenesis of HIV infection.

MBL2 and hepatitis C virus infection among injection drug users

Background

Genetic variations in MBL2 that reduce circulating levels and alter functional properties of the mannose binding lectin (MBL) have been associated with many autoimmune and infectious diseases. We examined whether MBL2 variants influence the outcome of hepatitis C virus (HCV) infection.

Methods

Participants were enrolled in the Urban Health Study of San Francisco Bay area injection drug users (IDU) during 1998 through 2000. Study subjects who had a positive test for HCV antibody were eligible for the current study. Participants who were positive for HCV RNA were frequency matched to those who were negative for HCV RNA on the basis of ethnicity and duration of IDU. Genotyping was performed for 15 single nucleotide polymorphisms in MBL2. Statistical analyses of European American and African American participants were conducted separately.

Results

The analysis included 198 study subjects who were positive for HCV antibody, but negative for HCV RNA, and 654 IDUs who were positive for both antibody and virus. There was no significant association between any of the genetic variants that cause MBL deficiency and the presence of HCV RNA. Unexpectedly, the MBL2 -289X promoter genotype, which causes MBL deficiency, was over-represented among European Americans who were HCV RNA negative (OR = 1.65, 95% CI 1.05–2.58), although not among the African Americans.

Conclusion

This study found no association between genetic variants that cause MBL deficiency and the presence of HCV RNA. The observation that MBL2 -289X was associated with the absence of HCV RNA in European Americans requires validation.

Mannose-binding lectin variant associated with severe malaria in young African children

Mannose-binding lectin (MBL) is a serum protein which initiates innate immune responses to microbial pathogens by binding to non-self surface oligosaccharides. MBL deficiency is the most common congenital immunodeficiency of human and has been shown to predispose to infections, particularly in children and immune compromised. In a matched case-control study among 870 Ghanaian children, we examined the influence of six polymorphisms of the MBL2 gene on Plasmodium falciparum infection and severe malaria. A missense mutation resulting in low MBL activity (MBL2*C) was found in 35% of healthy controls, but in 42% of asymptomatically infected children (P=0.01), and in 46% of patients with severe malaria (P=0.007). Heterozygosity for MBL2*C was associated with increased odds of infection (odds ratio (OR), 1.6; 95% confidence interval (CI), 1.1-2.1), severe malaria (OR, 1.7; 95% CI, 1.2-2.4), and of severe anemia in particular (OR, 2.3; 95% CI, 1.4-3.8). The population attributable fraction of severe malaria cases attributable to MBL2*C heterozygosity was 17%. Our results suggest that the MBL pathway of the complement system is a critical determinant of both, susceptibility to P. falciparum infection and manifestation of severe malaria, particularly in young children in whom specific immune responses are weak or absent.

The MBL2 'LYQA secretor' haplotype is an independent predictor of postoperative myocardial infarction in whites undergoing coronary artery bypass graft surgery

BACKGROUND

Mannose-binding lectin (MBL) is an important component of innate immunity and activator of the lectin complement pathway. Within the MBL2 gene are seven 5' "secretor" haplotypes that code for altered serum MBL levels and complement activation. However, recent evidence suggests that 3' MBL2 haplotypes may also modify MBL function and circulating levels. Because MBL and the lectin complement pathway have been implicated in cardiovascular injury, we investigated whether MBL2 haplotypes are independently associated with an increased risk of postoperative myocardial infarction (PMI) in patients undergoing coronary artery bypass graft surgery.

METHODS AND RESULTS

Genotyping of 18 polymorphic sites within the MBL2 gene was performed in a prospective, longitudinal multi-institutional study of 978 patients undergoing primary coronary artery bypass graft-only surgery with cardiopulmonary bypass between August 2001 and May 2005. After adjustment for multiple comparisons by permutation testing, multivariate, stepwise logistic regression, including a score test, was performed controlling for patient demographics, preoperative risk factors, medications, and intraoperative variables to determine if MBL2 secretor haplotypes are independent predictors of PMI in whites undergoing primary coronary artery bypass graft surgery. Neither the 5' nor 3' MBL2 haplotypes alone were associated with an increased incidence of PMI. However, the incidence of PMI in whites (n=843) expressing the combined MBL2 5' LYQA secretor haplotype (CGTCGG) and 3' haplotype (CGGGT) was significantly higher than in whites not expressing the haplotype (38% versus 10%; P<0.007). Moreover, the combined MBL2 LYQA secretor haplotype was an independent predictor of PMI in whites after primary coronary artery bypass graft surgery after adjustment for other covariates (P<0.02; adjusted OR: 3.97; 95% CI: 1.30 to 12.07). The combined MBL2 LYQA secretor haplotype in whites was also an independent predictor of postoperative CKMB levels exceeding 60 ng/mL (P<0.02; adjusted OR: 4.48; 95% CI: 1.95 to 16.80). Inclusion of the combined MBL2 LYQA secretor haplotype improved prediction models for PMI based on traditional risk factors alone (C-statistic 0.715 versus 0.705).

CONCLUSIONS

The combined MBL2 LYQA secretor haplotype is a novel independent predictor of PMI and may aid in preoperative risk stratification of whites undergoing primary coronary artery bypass graft surgery.

Gene polymorphisms associated with reduced hepatic expression of porcine mannan-binding lectin C

Previous studies showed that low expression of mannan-binding lectin C (MBL-C) in pigs was not due to single-nucleotide polymorphisms (SNPs) in the coding region of pig MBL2. In these studies, we compared the 5' flanking regions of porcine MBL1 (1907 bp) and MBL2 (1880 bp) in normal and diseased pigs with low or high hepatic expression of MBL2. Hepatic expression of MBL-C was very low in all pigs submitted for postmortem diagnosis. In various European pig breeds, a G(-1081)A substitution was linked to very low hepatic MBL-C expression, and was more frequent in diseased pigs. A C(-251)T substitution with less influence on MBL-C expression was more common in various breeds but was not associated with disease. MBL2 polymorphisms were associated with some disease groups and with the presence of some etiologic agents. These findings indicate that some promoter polymorphisms impair MBL-C expression in pigs and may increase their susceptibility to disease.

Mannose-binding lectin contributes to the severity of Guillain-Barré syndrome

In Guillain-Barré syndrome (GBS), complement activation plays a crucial role in the induction and extent of the postinfectious immune-mediated peripheral nerve damage. Mannose-binding lectin (MBL) activates the complement system via the lectin pathway after recognition of repetitive sugar groups on pathogens. We investigated whether the MBL2 genotype, serum MBL level, and MBL complex activity are associated with the development and severity of GBS. Single nucleotide polymorphisms in the promoter region (-550 H/L and -221 X/Y) and exon 1 (A/O) of the MBL2 gene were determined in 271 GBS patients and 212 healthy controls. The frequencies of the H allele, HY promoter haplotype, and HYA haplotype, which are related to high MBL activity, were all increased in GBS patients compared with healthy controls (p < or = 0.03), particularly in severely affected GBS patients (MRC-sum score < 40) (p < or = 0.02). Severe weakness was also associated with high MBL concentrations and MBL complex activity in sera from GBS patients (p < 0.01). The MBL2 B allele was associated with functional deficiency and relatively mild weakness. These results support the hypothesis that complement activation mediated by MBL contributes to the extent of nerve damage in GBS, which is codetermined by the MBL2 haplotype.

Mannose-binding lectin-2 genetic variation and stomach cancer risk

Deficiency of the mannose-binding lectin (MBL) protein, an antigen-recognition molecule involved in systemic and mucosal innate immunity, is determined by variant alleles in MBL2 gene promoter and exon-1 regions. We conducted a population-based study on 305 stomach cancer cases and 427 controls in Warsaw, Poland to determine whether MBL2 gene variants predispose to stomach cancer. Single nucleotide polymorphisms (SNPs) in MBL2 were determined by TaqMan. The 5 tested MBL2 variants are in complete linkage disequilibrium and comprise 6 different haplotypes. The risk of stomach cancer was increased in subjects carrying the H/H promoter genotype (OR = 1.8, 95%CI 1.1-2.9; p = 0.020) relative to L/L carriers, after adjustment for age, gender, education and smoking. Carrying at least one D exon-1 allele was associated with nonsignificant excess risk (OR = 1.5, 95% CI 0.9-2.4; p = 0.081). In haplotype analysis, the HYD haplotype was associated with increased risk of stomach cancer when compared with HYA, the most common haplotype (OR = 1.9, 95% CI 1.1-3.2; p = 0.021). In diplotype analysis, subjects carrying the YA/D haplotype combination showed the highest risk (OR = 3.0, 95% CI 1.2-7.1; p = 0.015), compared with YA/YA. Further analyses to examine the joint effect of MBL2 and IL-1B polymorphisms, previously shown to predispose to stomach cancer, indicated that the combination of at-risk IL-1B genotypes (CT or TT at location -511) and HYD MBL2 haplotype was associated with a 3.5-fold risk (OR = 3.5, 95% CI 1.6-7.6; p = 0.001). Our findings suggest that the codon 52 D MBL2 variant causing a cysteine > arginine replacement, but not B and C variants producing glycine substitutions, is specifically associated with gastric cancer risk.

Evolutionary insights into the high worldwide prevalence of MBL2 deficiency alleles

Human mannose-binding lectin (MBL) is a member of the collectin protein family that binds a broad range of microorganisms and activates the lectin-complement pathway of innate immunity. Common alleles of MBL2 disrupt the MBL protein or modulate the amount of protein produced, resulting in MBL deficiency. The clinical manifestations of MBL deficiency have been extensively studied but the actual role of this lectin in immunity to infection remains a matter of strong debate. MBL is commonly thought to play a key role in protective immunity, because MBL deficiency has been associated with an increase in susceptibility to infectious diseases. However, the high worldwide prevalence of multiple MBL2 deficiency or low-producing alleles suggests the converse that MBL deficiency confers protection. To explore the underlying forces accounting for the high worldwide prevalence of MBL2 deficiency alleles, we characterized genetic diversity in and around the MBL2 genomic region in 1166 chromosomes from 24 worldwide populations. Our results clearly demonstrate that the patterns of MBL2 variation are compatible with neutral evolution, as opposed to negative, positive or balanced natural selection. The high worldwide frequencies of MBL2 alleles associated with the production of little or no protein therefore result exclusively from human migration and genetic drift. The evolutionary neutrality of MBL2 strongly supports the notion that MBL2 variation does not have strong effects on population fitness, suggesting, therefore, that this lectin is largely redundant in host human defences.

Evidence for an association between mannose-binding lectin 2 (MBL2) gene polymorphisms and pre-term birth

PURPOSE

Human mannose-binding lectin, encoded by the MBL2 gene, is an important component of innate immunity and an important regulator of inflammatory processes. MBL2 gene polymorphisms are associated with an increased risk of neonatal infections and some data suggest a relation between the maternal MBL2 genotype and the risk of premature delivery. In this study, we evaluated whether there is an association between the fetal MBL2 genotype and prematurity.

METHODS

A microarray-based on-chip PCR method was used to simultaneously detect five common MBL2 polymorphisms (codon 52, 54, 57; promoter -550, -221) in 204 DNA samples isolated from archival blood cards. MBL2 genotypes of infants born before the 36th week of pregnancy (N = 102) were compared to a control group of infants born at term after the 37th week (N = 102).

RESULTS

The frequency of the codon 52 polymorphism was significantly higher in the pre-term group compared to the term group (10.8% versus 4.9%, P = 0.04), while the frequency of the codon 54 polymorphism was equal in both groups (11.3% versus 11.8%). Interestingly, carriers of genotypes (O/O) likely conferring deficient MBL plasma levels were more common in the group of premature birth (9.8% versus 2.9%, P = 0.05), while the promoter -550 C/C genotype was underrepresented in the pre-term birth group (24.5% versus 39.2%, P = 0.03).

CONCLUSION

Our data add to the knowledge about genetic predisposition to prematurity and suggest that the fetal MBL2 genotype might be an additional genetic factor contributing to the risk of premature delivery.

Diversity of the MBL2 gene in various Brazilian populations and the case of selection at the mannose-binding lectin locus

The mannose binding lectin (MBL2) polymorphism is responsible for a common immunodeficiency in the human species. There were suggestions that the MBL2 polymorphism has been under balancing selection, based on the high global frequency of alleles generating MBL deficiency and on the worldwide distribution of diseases negatively associated with them. To describe the distribution of MBL2 allelic haplotypes in Brazilian populations and to discuss the evolution of this polymorphism, we analyzed six South Brazilian populations (152 Guarani Amerindian, 239 Kaingang Amerindian, 107 admixed, Brazilian 32 Afro-Brazilian, 202 Euro-Brazilian and 16 Oriental-Brazilian). Eight haplotypes were observed: MBL2*HYPA, LYQA, LYPA, LXPA, LYPB, LYQC, HYPD, and LYPD. In addition, through sequencing of the promoter and exon 1 from Amerindian and Oriental individuals, three new single-nucleotide polymorphisms (SNPs) were found in the MBL2 promoter region in the Kaingang. Analysis of the sequencing data by neutrality tests (Tajima's D and Fu and Li's D* and F*) revealed no deviation from selective neutrality equilibrium in the Guarani and Kaingang. Significant Fay and Wu's H results are explained by the recent gene flow in these populations. Contrarily to previous thoughts, stochastic evolutionary factors seem therefore to have had a predominant role in shaping the MBL2 polymorphism, at least in the Amerindians.

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

Mannose-binding lectin (MBL2) variants that decrease the plasma level of the protein or encode dysfunctional proteins are frequently associated with the severity of a number of infections and autoimmune disorders. The high frequencies of these variants in most populations of the world are probably maintained by some selective advantage against widespread diseases. We found 14 new MBL2 allelic haplotypes, two of them with non-synonymous variants, by screening 136 children with uncomplicated malaria, 131 children with severe malaria and 39 older healthy schoolchildren. We also found a significant association of a novel variant with susceptibility to severe malaria (P=0.010). Increased MBL plasma levels and corresponding MBL2 genotypes were associated with lower concentration of several cytokines and chemokines in plasma of malaria patients. We suggest that malaria could have been one of the evolutionary driving forces shaping the MBL2 polymorphism in the African population.

Mannose-binding lectin and its genetic variants

Mannose-binding lectin (MBL) is a collagen-like serum protein that mediates activation of the complement system and is of importance for host defence. Common variant alleles situated both in the promoter and structural region of the human MBL gene (MBL2) influence the stability and the serum concentration of the protein. Epidemiological studies have suggested that genetically determined variation in MBL serum concentration influences the susceptibility to and the course of different types of infections, autoimmune, metabolic and cardiovascular diseases, but this is still a subject of debate. The fact that these genetic variations are very frequent indicates a dual role for MBL in host defence. In this survey, we summarize the current molecular understanding of human MBL genetics.

SNP500Cancer: a public resource for sequence validation, assay development, and frequency analysis for genetic variation in candidate genes

The SNP500Cancer database provides sequence and genotype assay information for candidate SNPs useful in mapping complex diseases, such as cancer. The database is an integral component of the NCI Cancer Genome Anatomy Project (http://cgap.nci.nih.gov). SNP500Cancer reports sequence analysis of anonymized control DNA samples (n = 102 Coriell samples representing four self-described ethnic groups: African/African-American, Caucasian, Hispanic and Pacific Rim). The website is searchable by gene, chromosome, gene ontology pathway, dbSNP ID and SNP500Cancer SNP ID. As of October 2005, the database contains >13 400 SNPs, 9124 of which have been sequenced in the SNP500Cancer population. For each analysed SNP, gene location and >200 bp of surrounding annotated sequence (including nearby SNPs) are provided, with frequency information in total and per subpopulation as well as calculation of Hardy-Weinberg equilibrium for each subpopulation. The website provides the conditions for validated sequencing and genotyping assays, as well as genotype results for the 102 samples, in both viewable and downloadable formats. A subset of sequence validated SNPs with minor allele frequency >5% are entered into a high-throughput pipeline for genotyping analysis to determine concordance for the same 102 samples. In addition, the results of genotype analysis for select validated SNP assays (defined as 100% concordance between sequence analysis and genotype results) are posted for an additional 280 samples drawn from the Human Diversity Panel (HDP). SNP500Cancer provides an invaluable resource for investigators to select SNPs for analysis, design genotyping assays using validated sequence data, choose selected assays already validated on one or more genotyping platforms, and select reference standards for genotyping assays. The SNP500Cancer database is freely accessible via the web page at http://snp500cancer.nci.nih.gov.

Detection of three single nucleotide polymorphisms in the gene encoding mannose-binding lectin in a single pyrosequencing reaction

Mannose-binding lectin (MBL) is a key molecule of innate immunity. Binding of MBL to carbohydrates present on pathogens activates the lectin pathway of complement activation, resulting into opsonization and anti-microbial protection. Three frequently occurring single nucleotide polymorphisms (SNPs) are described in the coding region of the MBL2 gene that are associated with abnormal polymerization of the MBL molecule, decreased serum concentrations of high molecular weight MBL, and strongly impaired function. Clinical studies have shown that these MBL SNPs are associated with increased susceptibility to infections, especially in immune-compromised persons, as well as with accelerated progression of chronic diseases. The present study describes a novel method to detect the three major MBL SNPs by pyrosequencing. The close proximity of these SNPs allows their detection in one single pyrosequencing reaction, resulting in clearly distinguishable patterns for each allele combination described until now. This method can be used for the easy and reliable detection of MBL SNPs to identify the basis of functional MBL deficiency in clinical diagnostics and research.

Association of mannose-binding lectin gene (MBL2) polymorphisms with rheumatoid arthritis in an Indian cohort of case-control samples

Single nucleotide polymorphisms in the mannose-binding lectin (MBL2) gene, as well as the serum MBL2 level, have been associated with various autoimmune diseases. We investigated whether such polymorphisms and/or the serum MBL2 level were associated with rheumatoid arthritis (RA) in an Indian population. The frequency of the B variant (codon 54) of the MBL2 gene was quite frequent in the healthy Indian population and was significantly (P=6.35x10(-6)) lower in RA patients. We replicated this association (P=1.78x10(-5)) in an independent cohort of control individuals. Promoter polymorphism at -550 nt showed a significant overrepresentation (P=0.003) of the minor allele G in severe RA patients compared with the less severe group. Haplotype LYA frequency was significantly (P=0.03) high in the less severe group, while the frequency of the HYA haplotype was significantly (P=0.04) increased in the severe RA patients. No statistically significant difference in serum MBL2 was observed as a whole, but the individuals homozygous for the LYA haplotype had significantly lower (P=0.017) serum MBL2 levels compared with individuals homozygous for the HYA haplotype. Therefore, the B variant of the MBL2 gene may be associated with protection from RA in our study population, and the promoter polymorphism (-550 nt) seems to have some role in disease progression.

An analysis of genetic variation across the MBL2 locus in Dutch Caucasians indicates that 3′ haplotypes could modify circulating levels of mannose-binding lectin

Mannose-binding protein (MBL) is a critical component of innate immunity and provides first-line protection against pathogens. Both circulating MBL serum levels and functional activity have been correlated with common genetic variants in the MBL2 gene. Associations between MBL deficiency and severe infections have been reported in immuno-incompetent patients and for autoimmune disorders; however, measured MBL serum levels do not fully correlate with the 'secretor haplotypes'. Previously, the MBL2 locus was resequenced and determined that a recombination hotspot divides MBL2 into two haplotype blocks. It was sought to investigate whether additional variants, in either block structure could associate with MBL serum levels. Therefore, 31 common variants were analysed across the locus in 212 DNA samples of healthy Caucasian individuals with known MBL serum concentrations. The additional 5' variants were in strong linkage to the elements of the 'secretor haplotypes'; functional alleles B, C and D also lie on restricted haplotypes. Four variants in the 3' block (Ex4-1483T>C, Ex4-1067G>A, Ex4-901G>A and Ex4-710G>A) are components of a distinct haplotype block. The results of this study suggest that additional 5' variants as well as markers of distinct 3' haplotype blocks in MBL2 may contribute to circulating protein levels, but further studies are required to confirm these observations. Last, there could be a selective advantage for diversification of the 3' region of the gene.

The ‘involution’ of mannose-binding lectin

Mannose-binding lectin (MBL) acts as a serum opsonin in innate immune defense and induces complement activation by the lectin pathway. In humans, low levels of functional serum MBL are caused by the dominant action of three single nucleotide substitutions in exon 1 that disrupt the glycine-rich backbone structure of the protein. The presence of common MBL variant alleles is associated with both infectious and autoimmune diseases. Conversely, it has also been suggested that MBL variants are maintained because of selective advantages for the host. In man, the MBL genetic system comprises one functional gene (MBL2) and one expressed pseudogene (MBL1P1), whereas the lower primate, the rhesus monkey resembles rodents with two functional MBL genes. We have investigated the molecular mechanisms behind the evolutionary loss of MBL expression from lower primates to man, including silencing of the MBL1P1 gene and the generation of MBL2 variant structural alleles and promoter polymorphisms leading to the present human MBL2 haplotypes. We present data showing that the MBL1P1 gene has been repeatedly hit throughout evolution and silenced eventually by mutations in the glycine residues of the collagen-like region. Our results indicate that the MBL1P1 gene has been selectively turned off during evolution through the same molecular mechanisms causing the MBL2 variant alleles in man, suggesting an evolutionary selection for low-producing MBL genes.

Effects of natural selection on interpopulation divergence at polymorphic sites in human protein-coding Loci

To develop new strategies for searching for genetic associations with complex human diseases, we analyzed 2784 single-nucleotide polymorphisms (SNPs) in 396 protein-coding genes involved in biological processes relevant to cancer and other complex diseases, with respect to gene diversity within samples of individuals representing the three major historic human populations (African, European, and Asian) and with respect to interpopulation genetic distance. Reduced levels of both intrapopulation gene diversity and interpopulation genetic distance were seen in the case of SNPs located within the 5'-UTR and at nonsynonymous SNPs, causing radical changes to protein structure. Reduction of gene diversity at SNP loci in these categories was evidence of purifying selection acting at these sites, which in turn causes a reduction in interpopulation divergence. By contrast, a small number of SNP sites in these categories revealed unusually high genetic distances between the two most diverged populations (African and Asian); these loci may have historically been subject to divergent selection pressures.

Evolution of the mannose-binding lectin gene in primates

The mannose-binding lectin MBL2 plays an important role in the innate immune system. It binds carbohydrates surface, acts as an opsonin and activates the complement system. With the aim of studying the evolution of the MBL2 gene in primates, we sequenced its coding region in 12 non-human primate species and compared them with the human sequence. We demonstrated that nucleotide and amino-acidic sequences of the MBL2 among primates are highly homologous, underlining the importance of this molecule in the defense system against pathogen invasions. In particular, in the collagen-like domain that confers the characteristic structure to MBL2 protein, the identity among primates is really high. In the carbohydrate recognition domain, we evidenced some primates' group-specific amino-acidic mutations not resulting in changes of the structure or function of this MBL2 domain. Phylogenetic analysis did not evidence any positive selective pressure in MBL2 gene among non-human primates. Our findings indicate that MBL2 is well conserved in agreement with its important role in the immune system: in non-human primates, we did not observe the same 'plasticity' of the MBL2 human gene, where a frequency of more than 1% of nucleotide variations was described in the coding and promoter regions.