WK Ip, SY Chan, YL Lau

Mannose binding lectin (MBL) is a calcium-dependent serum lectin secreted by the liver as an acute-phase protein. It binds mannose and N-acetylglucosamine terminated glycoproteins and plays a very important role in the innate immune defence by opsonizing various microorganisms for phagocytosis. Low levels of the MBL have been implicated in the pathogenesis of systemic lupus erythematosus. Three point mutations have been previously found in the human MBL gene, located in codons 52, 54 and 57 of exon 1. They are predicted to affect the tertiary structure of the collagenous region of the protein, and are known to be associated with low serum concentrations of MBL. Two positions in the promoter region of the gene are also shown to be polymorphic in controlling MBL expression level.

A group of Chinese patients with SLE were found to have persistently low MBL protein level by serial measurements using ELISA (Enzyme-linked-immunosorbent-assay). Studies such as by generation of DNA heteroduplexes using an universal heteroduplex generator (UHG) showed that they did not have the three previously reported mutations. The results from allelic specific oligonucleotide hybridization (ASO) to identify the promoter genotype also could not explain their low expression level. This study looked for unidentified polymorphism in the promoter or novel structural mutation of the gene which may be unique to the Chinese population.

Screening for mutations in the exon 1 and 2 with PCR-SSCP
Specific amplification of the target fragments with radioactively labeled primers was followed by non-denaturing polyacrylamide gel electrophoresis at room temperature. None of the PCR fragments of exon I and exon 2 from the 14 patients showed mobility shift.

DNA sequence analysis for the promoter region
The sequences of the promoter region of MBL gene in the DNA samples from the 14 patients and individuals who had normal MBL protein levels were amplified by PCR and subsequently sequenced directly. Eight hundred and fifty base-pair promoter region was sequenced and found to be the same as the published sequence with two polymorphic sites.

The persistently low MBL serum levels in our Chinese SLE patients were unlikely to be due to any novel structural mutation and other promoter polymorphism. However, based on the results of MBL promoter haplotyping, an observation has been made that the MBL levels of our Chinese SLE patients were significantly lower than those of controls in each haplotype. There may be other risk factors that affect the MBL phenotype and the frequency of each promoter haplotype may be different. We are now studying a larger pool of sample.