Mannose binding lectin polymorphisms in systemic lupus erythematosus in Spain

Abstract

Low mannose binding lectin (MBL) producer genotypes have been considered as a systemic lupus erythematosus (SLE) risk factor. The aim of this study was to explore whether polymorphisms in the MBL gene are associated with susceptibility to SLE and disease-specific clinical manifestations or with disease severity in SLE patients in Son Llàtzer Hospital. MBL2 exon 1 and promoter polymorphisms were genotyped and MBL plasma levels were quantified by ELISA in 39 SLE cases and in 96 healthy controls. High MBL producer genotypes HYPA and LXPA were the most frequent haplotypes (65 and 62 participants, respectively). LYQC/HYPD, LXPA/LYQC and LYPB/HYPD were only found in SLE, and all of them were related to severe MBL deficiency. SLE patients showed a trend towards more severe MBL deficiency (MBL <100 ng/mL) compared to controls (10 [25.6%] and 11 [11.46%] respectively, P = 0.07). The wild-type genotype was more frequent in controls compared to SLE. The trend towards more severe MBL deficiency in SLE and the fact that some of the low-MBL producer genotypes were only found in SLE patients, suggest that low MBL levels or MBL2 variant could be a risk factor for the development of SLE.

Keywords

Mannose binding lectin (MBL)polymorphisms systemic lupus erythematosus (SLE)

Introduction

Systemic lupus erythematosus (SLE) is a multifactorial disease and susceptibility is related to genetic, hormonal, immunological and environmental factors. A strong genetic link has been identified through the use of genome-wide association and family studies, and more than 30 SLE related loci have already been identified.¹ Mannose binding lectin (MBL) is a pattern-recognition receptor protein molecule encoded by the MBL2 gene and its deficiency is linked to susceptibility to secondary infections² and presumed to be a contributory factor in the development of SLE.³ Defects in MBL could entail a deficient clearance of apoptotic material, thus increasing exposure to autoantigens that drive the production of autoantibodies and further tissue inflammation.^3–5 MBL production is controlled by the MBL2 gene that lies in chromosome 10, and polymorphisms of the structural regions of the gene or its promoter have been associated with relative or absolute serum MBL deficiencies.³ Three mutations in exon 1 of the MBL2 gene are currently known: codon 52 (rs5030737; C>T; Arg>Cys), called D variant, codon 54 (rs1800450; G>A; Gly>Asp), called B variant and codon 57 (rs1800451; G>A; Gly>Glu), called C variant. B, C and D variants are referred to collectively as O, while A is the wild-type. Individuals with the wild-type genotype (A/A) have generally high MBL levels; heterozygotes A/O have about 10% of wild-type serum concentrations of MBL and homozygotes O/O have very low or absent MBL levels.⁶ Other polymorphisms in the promoter 1 region modulating MBL levels have been identified: at position −550 (rs11003125; G>C; polymorphism H/L) and −221 (rs7096206; C>G; polymorphism X/Y),⁷ and a P/Q variant in a 5’untranslated region at position +4, (C>T, rs7095891).⁸ Due to a strong linkage disequilibrium between the polymorphisms present in the promoter and the structural variants in exon 1 of the MBL2 gene, only seven common haplotypes have been described (HYPA, LYPA, LXPA, LYQA, HYPD, LYPB and LYQC) and there are therefore 28 possible diplotypes, the frequency of which varies among populations.⁷ The combination of structural gene and promoter polymorphisms results in up to 1000-fold variations in MBL concentrations in different individuals. Among haplotypes carrying the wild-type A allele, HYPA and LYQA are related to high MBL levels, whereas LYPA is related to medium to low levels.^6,8 Moreover, ethnic differences have also been studied and it has been observed that low MBL level related genotypes are present in 10% of Caucasians and up to 40% of Africans.^8–10 This has been related to the possibility that low MBL levels could protect against intracellular infections like tuberculosis.¹¹ The role of the MBL pathway in complement activation and in the clearance of apoptotic cells suggests that genetic variability in MBL may be involved in the pathogenesis of SLE. On the one hand, previous studies have shown an association between MBL deficiency and SLE, SLE severity, renal disease,^3,12–15 infections,¹⁶ thrombosis and antiphospholipid syndrome,¹² while others have found an association restricted to European-derived patients,^17,18 and a meta-analysis showed that MBL variant alleles were a significant SLE risk factor.^14,15 On the other hand, a study in Spain did not find a significant association between SLE and MBL deficiency.¹⁹ There have been few studies on MBL and SLE in the Spanish population.^12,19,20

The aim of this study was to explore whether polymorphisms in the MBL gene are associated with susceptibility to SLE and disease-specific clinical manifestations, or with disease severity in a cohort of SLE patients.

Material and methods

Patients seen between 2010 and 2012 at Hospital Son Llàtzer in Palma de Mallorca (Balearic Islands) with a diagnosis of SLE that fulfilled American Rheumatology SLE criteria²¹ and a control group of healthy volunteer blood donors were included in the study. Two populations were included in the control group: first, healthcare workers who had their medical history collected and a blood test performed to exclude any disease. The blood test included a general test and determination of ANA, antiDNA antibodies, rheumatoid factor and complement factors. All the results had to be within normal ranges to be considered as healthy. Second, the control group included healthy house-hold contacts of tuberculosis patients who were examined at the Tuberculosis Unit of our hospital. As in the other group, their medical history was collected and a blood test was performed to exclude any medical condition. These patients have participated in another published study.²² All patients and controls provided written informed consent. A code was assigned to each participant, and demographic, clinical and analytical data (full blood count, complement, antiDNA antibodies and urinary sediment) as well as Systemic Lupus Erythematosus Disease Activity Index (SLEDAI)²³ at inclusion were entered into an anonymous database. The study was approved by the Ethics Committee of the Balearic Islands on 22 June 2011.

Methods

Blood sampling protocol

The blood was aseptically collected into plain and ethylenediaminetetraacetic acid tubes. For all samples, serum was separated immediately and transferred into cryovials and preserved at −80°C for further testing.

Quantification of MBL

MBL serum concentrations were determined by enzyme-linked immunosorbent assay (ELISA) performed in microwells coated with a monoclonal antibody against the MBL carbohydrate-binding domain in a commercial kit (oligomerized mannan-binding lectin; AntibodyShop®, Gentofte, Denmark). MBL serum concentrations were expressed as ng/mL.

SNP genotyping

EDTA blood samples were used for genomic DNA isolation. Genomic DNA was extracted using the Maxwell® 16 Blood DNA Purification Kit (Promega, USA).

Six SNP in the MBL2 gene (−550 G/C [rs11003125], −221 C/G [rs7096206], +4 C/T [rs7095891], codon 52 CGT/TGT [rs5030737], codon 54 GGC/GAC [rs1800450] and codon 57 GGA/GAA [rs1800451]) (GenBank accession numbers AF360991) were analysed. Genotyping was performed by polymerase chain reaction with sequence-specific primers (PCR–SSP).²⁴ Polymorphisms within the promoter and exon 1 of the MBL2 gene were analysed, using primer pairs spanning from the promoter region through exon 1. The primers used were as follows: MBL2-1F -GGGAATTCCTGCCAGAAAGTAGAG, MBL2-1RGGA-TCCTAAGGAGGGGTTCATCT, MBL2-2F-ACTAGTCACGCAGTGTCACAAGGA, MBL2-2R-CAGGCAGTTTCCTCTGGAAGGTA. In short, 50 ng of genomic DNA were amplified in a 50 µL volume of reaction mixture containing 1× PCR buffer, 2.5mM of MgCl2, 0.2 mM of dNTPs, 0.5 mM of each primer and 1 U GoTaq® Hot Start Polymerase (Promega, USA) on a T1 Thermocycler (Biometra, Germany). Thermal cycling parameters for amplification of both regions were 95°C for 10 min (initial denaturation); 40 cycles of 30 s at 95°C (for denaturation), 30 s at 55°C (for annealing), 45 s at 72°C (for extension) and 10 min at 72°C (for final extension). PCR products were cleaned up by use of Excela Pure 96-Well UF Plate (EdgeBio, USA) and 1 μL of the purified product was directly used as a template for sequencing, using the BigDye terminator v. 3.1 cycle sequencing kit (Applied Biosystems, USA) on an ABI 3730XL DNA sequencer, following the manufacturers’ instructions. Polymorphisms were identified by assembling the sequences with respective reference sequences (NG_008196.1), using Sequencher, version 5.0 (available at: http://genecodes.com/), and were reconfirmed visually from their respective electropherograms.

Statistical analysis

First, a descriptive analysis of the different populations of the study was performed. MBL levels were expressed as ng/mL. Qualitative data were presented as percentages and quantitative data were presented as mean (+/− SD) or median and range. Diplotype frequencies were compared between cases and controls by Chi-squared test or Fisher’s exact test when necessary. MBL levels were compared between cases and controls by Mann–Whitney U test. P values with Yates correction and odds ratio (OR) with 95% confidence interval (CI) were calculated using EPIDAT 3.1 software. A power of 0.8 was established and a two-tailed P value less than 0.05 was considered statistically significant unless otherwise specified. The Bonferroni correction was used to adjust P value for multiple comparisons. The significance cutoff was established at P <0.05/n, n being the number of comparisons made.

The analysis was carried out with SPSS 13.0 and EPIDAT 3.1 software. A subgroup analysis of Caucasian participants was performed because of the potential genetic differences described in literature.

Results

Thirty-nine SLE cases and 96 healthy controls were included. There were 37 (94.9%) women in the SLE group and 89 (92.7%) in the control group. There were no statistical significant differences in gender, age or ethnicity in SLE patients or controls (Table 1).

Table 1.

Demographic characteristics of SLE patients and controls.

	SLE	Control
Age (mean +/− SD)	47.67 (+/−12.51)	43.94 (+/−15.39)
Female (n (%))	37 (94.9%)	89 (92.7%)
Ethnicity (n (%))	Caucasians: 35 (89.7%) -Spain: 28 (71.8%) -Germany: 1 (2.6%) -Argentina: 3 (7.7%) -Ireland: 1 (2.6%) -Poland: 1 (2.6%) -Brazil: 1 (2.6%) Non-Caucasians: -Nigeria: 3 (7.7%) -Bolivia: 1 (2.6%)	Caucasians: 96 (100%) -Spain: 93 (96.9%) -Germany: 1 (1%) -Argentina: 1 (1%) -Moldavia: 1 (1%)

A descriptive analysis of SLE disease activity of the SLE patients was carried out (Table 2). Due to technical problems, MBL level could not be determined in one of the cases.

Table 2.

Activity index and clinical manifestations in SLE patients.

	n (%)
SLEDAI
0–2 (inactive)	27 (69.2)
3–4 (mild)	6 (15.4)
5–7 (moderate)	2 (5.1)
>8 (severe)	4 (10.3)
Clinical manifestations
Arthritis	20 (51.3)
Nephritis	11 (28.2)
Serositis	14 (35.9)
Neuropsychiatric	7 (17.9)
Haematologic	13 (33.3)

Frequencies of MBL2 genotypes in exon 1 and allele B

All the structural genetic variants of the MBL gene were within the range of Hardy–Weinberg equilibrium.

There were no statistical significant differences between MBL2 genotypes in SLE patients compared to controls; A/A: 21 (53.8%) vs. 69 (71.9%), A/O: 14 (35.9%) vs. 26 (27.1%) and O/O: 4 (10.3) vs. 1 (1%), respectively.

Allele B was analysed and a trend towards a higher frequency in SLE patients compared to controls was observed (13 [33.3%] and 20 [20.8%] respectively), but did not reach a statistically significant difference (P = 0.19).

Considering Caucasians exclusively, the wild-type allele was also more frequent in the control group compared to SLE patients (69 [71.9%] vs. 18 [51.4%], respectively, although no statistical significance was achieved (P = 0.04, OR 0.41, CI 0.18–0.91), because adjusting with Bonferroni correction, statistical threshold was determined in P = 0.017. Allele B was more frequent in the SLE group compared to the controls (13 [37.1%] and 20 [20.8%], respectively) but without reaching statistical significance (P = 0.09, OR 2.25, CI 0.98–5.18).

Complete MBL2 diplotypes (exon 1 and promoter)

There were no statistical significant differences in MBL2 diplotype frequencies in either the SLE or the control population. HYPA and LXPA were the most frequent haplotypes (65 and 62 participants, respectively), followed by LYQA in 53 participants. LYQA/HYPA was the most frequent diplotype in SLE cases and in controls (6 [15.4%] and 17 [17.7%], respectively) with a median MBL level of 4160 ng/mL (range, 2520–5040 ng/mL) in cases and 3820 ng/mL (range, 1660–10,194 ng/mL) in controls.

The following diplotypes were only detected in patients with SLE: LYQC/HYPD (one case), LXPA/LYQC (one case) and LYPB/HYPD (two cases), and all of them had very low MBL levels (20, 20, 2 and 10 ng/mL, respectively).

MBL levels

Median MBL level in SLE patients and controls was 1675 ng/mL (range, 2–9726 ng/mL) and 2341 ng/mL (range, 4–11,534 ng/mL), respectively, showing no statistically significant difference (P = 0.89).

There was no statistically significant difference in the relationship between gender and MBL levels; SLE: 1898.5 (range, 10–3787) in two males versus 1675.1 (range, 2–9726) in 36 women, P = 0.08; controls: 3320 (range, 1590–6920) in seven men versus 2320 (range, 4–11,534) in 89 women, P = 0.19. Very low MBL levels (⩽100 ng/mL) were observed in 21 participants: 10 cases (25.6%) and 11 controls (11.46%), respectively (P = 0.07). Levels >1000 ng/mL were found in 22 (56.41%) SLE patients and 70 (79.92) healthy controls (P = 0.09).

In Caucasian participants, severe MBL deficiency was found in nine cases (25.7%) and one control (11.5%), finding no statistically significant difference (P = 0.07). Levels >1000 ng/mL were found in 19 cases (54.3%) and 70 controls (72.9%) (P = 0.01). As in the overall group, it was not possible to find statistically significant difference in median MBL levels comparing cases and controls.

MBL2 genotype and MBL levels

There was a correlation between exon 1 and promoter MBL2 genotype and MBL levels (Figure 1). When analysing together cases and controls, the wild-type genotype YA/YA showed very high MBL levels (>1000 ng/mL) except for one patient who had a moderate deficiency, and genotype O/O showed very low levels (<100 ng/mL).

Figure 1.

MBL2 genotype and MBL levels.

The same results were found in the subgroup of Caucasians.

MBL levels in patients and controls were compared based on genotype (YA/YA, YA/XA, XA/XA, YA/O, XA/O, O/O), but there was no statistically significant difference between cases and controls, nor was there one between cases and controls among Caucasians.

MBL levels and SLE activity, SLE manifestations, reactant phase proteins and complement (CH50, C3, C4) levels

SLE disease activity and clinical manifestations are shown in Table 2.

There was no correlation between SLE clinical manifestations and exon 1 variant alleles (A/A vs. A/O and O/O): cutaneous: 10 (28.57%) and 16 (76.19%), arthritis: 9 (25.71%) and 11 (52.38%), nephritis: 6 (17.14%) and 5 (23.81%), serositis: 2 (5.71%) and 12 (57.14%), neuropsychiatric: 4 (11.43%) and 3 (14.29%) and hematologic: 4 (11.43%) and 9 (42.86%), respectively. Neither could we find a correlation between MBL levels and reactant phase proteins (erythrocyte sedimentation rate [ESR] or C-reactive protein [CRP]). Complement deficiencies (C3, C4 and CH50) were analysed together and one by one to search for a relation with MBL genotypes (A/A versus A/O or O/O), and no statistically significant difference was found.

MBL levels and drugs

Thirty (76.9%) SLE patients were receiving immunosuppressive drugs. Ten (25.6%) patients were taking corticosteroids only, seven (17.9%) mycophenolate mofetil and corticosteroids, six (15.4%) azathioprine and corticosteroids, three (7.7%) azathioprine only, two (5.1%) methotrexate, one (2.6%) mycophenolate mofetil and one (2.6%) methotrexate and corticosteroids. There was no relation between MBL levels and the use of immunosuppressive drugs.

Discussion

In our study we evaluated MBL levels and complete genotype, including promoter polymorphisms, in patients with SLE and in a healthy control group. HYPA and LXPA were the most frequent haplotypes in both groups. Diplotypes that were only found in SLE patients (LYQC/HYPD, LXPA/LYQC and LYPB/HYPD) were associated with very severe MBL deficiency.

HYPA and LXPA were the most frequent haplotypes, which is in agreement with previous studies in the general European population, including a study in Caucasians (Spain).^25–27 Similarly, the wild-type genotype A/A was the most frequent genotype in both, the general population and the SLE patients as in other studies,³ but was even more frequent in controls compared to SLE cases in the subgroup of Caucasians. Several studies have suggested that genotypes related to very severe MBL deficiency are a risk factor for developing SLE in different ethnic populations;^14,15,28,29 however, the majority of previous studies did not report a complete MBL2 genotyping, promoter allelic variants were rarely analysed^20,28,30 and results are contradictory. An association between allele B in codon 54 of exon 1 and SLE has been extensively described.^14,15,28,31 In a recent study in Indian SLE patients, it was found that genotypes producing low MBL levels, especially the presence of allele B but also allele X at position -221, were significantly more frequent in SLE patients compared to healthy participants.²⁸ A meta-analysis confirmed identical results and added allele L at position −221 as a risk factor for developing SLE.^14,15 In our study, we also found in Caucasians a tendency to a higher incidence of allele B in the SLE group compared to the controls, although without reaching statistical significance. Other studies conducted in the Canary Islands, China and Hungary found no statistically significant differences for allele frequencies in exon 1 and promoter MBL2 genotype comparing SLE patients and a control group, but a trend towards a higher frequency of low MBL producer genotypes in SLE patients was generally found.^19,30,32

MBL serum levels largely depend on MBL2 exon 1 and promoter genotype, but there is no exact correlation between MBL2 genotypes and MBL levels probably because of other factors such as hormones or acute inflammation that modulate MBL production.²⁶ MBL has been considered an acute phase reactant, but studies have yielded conflicting results^26,29,33 and one study showed that MBL did not act as an acute phase reactant in pneumonia.³⁴ Likewise, our study did not find any relationship between MBL levels and acute phase reactants such as CRP and ESR.

MBL has been considered a modulator of disease activity in SLE. On the one hand, a higher prevalence of cardiovascular, renal or severe disease was observed in patients carrying low-MBL producer genotypes compared to those carrying high MBL producer genotypes^12,16 and some studies have also concluded that high MBL levels were associated with SLE nephritis and other SLE manifestations,^33,35 but on the other hand another article related low MBL levels with lupus nephritis.¹³ In our study, most of our patients had a low SLEDAI score (between 0 and 2) and we did not find any significant relationship between MBL levels and disease severity, nor between any allelic variants in exon 1 and clinical manifestations. Homozygosity for MBL variant alleles has been associated with infections in SLE patients,^18,36 but we could not evaluate infections in this transversal study. MBL is associated with complement activation through the lectin pathway and therefore low complement levels in patients with high MBL could be expected, as described in SLE in an Indian study,³³ but results are contradictory.^12,18 In our study, we did not find any associations between the complement system and MBL levels.

Our study has some important limitations. First of all, the number of SLE patients included was small, most of them had a low SLEDAI score and treatments were heterogeneous. MBL levels were obtained at the time of inclusion in the study but were not repeated during a flare or an infection. This would have allowed to determine the role of MBL as an acute phase reactant. Moreover, in our study, SLE participants were of multi-ethnic origin, whereas controls were exclusively Caucasians. This could affect the results which is why we created a subgroup of Caucasian participants. However, the results showed the same tendency in both analyses.

Nevertheless, our study has some important strengths: a complete MBL2 genotyping of exon 1 and promoter was done, an analysis which is not very commonly found in published studies; moreover, very few studies analysing a Caucasian population are found in the literature.

Conclusions

Although there is evidence that MBL levels and genotype are related to SLE, their exact role has to be elucidated. In our study, LYQC/HYPD, LXPA/LYQC and LYPB/HYPD diplotypes, which are associated with very low MBL levels, were only found in SLE patients. The wild-type genotype was more frequent in controls compared to SLE cases and there was a tendency to a higher incidence of allele B in the SLE group compared to the controls. Thus, MBL levels and some genetic variants could be a risk factor for developing SLE. The precise consequences of MBL deficiency in relation to the development of SLE and disease progression, as well as the role of MBL as a biomarker in assessing SLE activity remain unclear. Further studies including large longitudinal cohort studies with a complete genotype profile of the protein (MBL2 exon 1 and promoter) and MBL levels are needed, as well as studies of other components of the lectin pathway of the complement, including ficolins and MBL-associated serine-proteases (MASPs).

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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