The Association Between Mannose-Binding Lectin Gene Polymorphism and Rheumatic Heart Disease

Abstract

Mannan-binding lectin (MBL) is an innate pattern recognition molecule known to play a key role in pathogen clearance. As MBL2 gene polymorphism is associated to an increased susceptibility to infection, we aimed to determine genetic variations in the MBL2 gene in rheumatic heart disease (RHD). Genetic variations in the promoter and exon 1 region of the MBL2 gene were analyzed in 107 patients with RHD and 105 controls by real-time polymerase chain reaction. The frequency of MBL2* A/A genotype was significantly higher in RHD patients (71/107, 66.36% vs 52/105, 49.52%, p ≤ 0.02, OR = 1.99, 95% CI, 1.15–3.50). A/A genotypes were associated with higher levels of MBL in RHD compared with controls with the same genotype (p ≤ 0.004). The frequency of HYPA/HYPA, HYPA/LYQA, and LYQA/LYQA haplotypes was also increased in RHD (p ≤ 0.03, OR = 1.98, 95% CI, 1.05–3.73). However, the frequency of MBL2 variant alleles (termed “O”) was lower among patients (39/214, 18.2% vs 63/210, 30.0%, p ≤ 0.006, OR = 0.52, 95% CI, 0.33–0.82), which was also seen for O/O genotypes (3/107, 2.8% vs 10/105, 9.5%, p ≤ 0.05, OR = 0.27, 95% CI, 0.07–1.03). This data suggests a role for MBL genotypes in the susceptibility to RHD. However, it still remains unclear whether A/A homozygosity is a risk factor for RHD or rheumatic fever itself.

Introduction

Rheumatic fever (RF) is a consequence of upper-respiratory tract infections caused by group A streptococci (GAS) that continues to affect between 3% and 5% of children and adolescents in Brazil [1]. Rheumatic heart disease (RHD) represents the most severe manifestation of RF, affecting 30% to 50% of RF patients who develop chronic and progressive valvular lesions due to immune-mediated damage [2]. The most affected valve is the mitral, followed by the aortic and tricuspid. RHD is the main cause of chronic cardiopathy in young adults, resulting in high levels of morbidity and mortality, and as such, remains a major concern to public health in developing countries [1, 2].

Although there is much evidence for the role of GAS in the etiology of RF, the pathogenesis of RF and RHD remains an enigma. It is now widely accepted that RF and RHD are related to an autoimmune reaction triggered by GAS in a susceptible host [3, 4]. The pathogenic mechanisms include antigenic mimicry between streptococcal antigens and heart constituents that lead to reactive inflammation and to valvular lesions [5, 6]. It is intriguing why only a small proportion of GAS infected individuals actually develops RF and consequently RHD. The host response to streptococcal antigens is under genetic influence, and it is therefore pertinent to search for genetic markers that influence the development of RF and RHD [3, 4].

Mannan-binding lectin (MBL) is a plasma protein involved in the primary defense against microorganisms. MBL binds to carbohydrates, exposing terminal mannose, glucose, N-acetylmannosamine, N-acetylglucosamine, or fucose, presented by a wide range of pathogens, including GAS [7, 8]. Such structures are often referred to as pathogen-associated molecular patterns. MBL binding to pathogen-associated molecular patterns promotes complement activation and direct complement-mediated killing or killing by phagocytosis. MBL therefore constitutes part of the innate immune defenses, conferring host protection before adaptive immune responses have been activated. MBL deficiency, with a frequency of 10% to 20%, is the most common inherited immunodeficiency and is associated with increased susceptibility to infection and some autoimmune diseases [9, 10, 11, 12]. The high frequency of MBL deficiency naturally leads to speculations as to situations where this might be beneficial, such as cases associated with undesirable complement activation and tissue injury.

The human MBL2 gene is located in the chromosome region 10q11.1-q21 and contains four exons. The intact protein is formed by oligomers of subunits each consisting of three identical polypeptide chains of 32 kDa. Three different missense point mutations in exon 1 result in changes of the primary amino acid sequence. Substitutions in codon 52 (CGT to TGT) exchanges arginine with a cysteine (allele D); in codon 54 the changing of GGC to GAC causes the substitution of glycine with aspartic acid (allele B), and in codon 57 the change of GGA to GAA causes the substitution of glycine with glutamic acid (allele C). The level of MBL is also influenced by mutations in the promoter region, most strongly by the X/Y variants resulting from substitution in base pair −221, less so by substitution in bp −550 (H/L), and even less by P/Q at bp +4 located in the 5′-untranslated portion of MBL2 gene [7, 13, 14]. All together, seven common haplotypes: HYPA, LYPA, LYQA, LXPA, HYPD, LYPB, LYQC and one rare LYPD are known [13, 15, 16]. Many investigations on associations between MBL and diseases have been made by genotyping for the exon variations only, without taking into account that the LXPA haplotype is the most common cause of MBL deficiency in Europeans. In addition to the decrease in MBL levels, the missense mutant variants also abolish the capacity of MBL to bind the MBL-associated serine proteases, MASPs, eliminating the ability of activating complement [13, 17].

While MBL is supposed to protect against infectious diseases, high MBL levels could have a proinflammatory role in chronic diseases [18, 19, 20, 21]. Previously we found significantly increased MBL levels in patients with RHD, suggesting that MBL deficiency could exert a beneficial role against the development of the disease [18]. Now we wish to extend this finding by searching for possible associations between MBL alleles and chronic RHD in the same patient cohort.