The objective of this study was to study the relationship between SP-D Met31Thr (T/C) polymorphism and the susceptibility of GDM in Chinese patients.
Surfactant collectin such as SP-D is well known to be involved in lung innate immunity to prevent respiratory infections (21). The SP-D Met31Thr polymorphism has been demonstrated correlating with susceptibility to increased incidence or risk of various disorders, including chronic obstructive pulmonary disease (22), allergic rhinitis (23), asthma (24), acute kidney injury (25), obesity and accompanying T2D (26). Interestingly, one C-allele carriers showed decreased circulating SP-D, fasting glucose and lower prevalence of T2D (26). However, in our study TT homozygotes had 11.6% (P<0.05) lower prevalence of GDM than C-allele carriers (OR = 0.473), and the T/C genotypes showed a higher risk of GDM (OR = 2.440). However, the levels of glucose homeostasis markers were not significantly different in women with GDM compared to matched healthy controls. Furthermore, compared with healthy controls, serum SP-D levels in GDM patients were significantly elevated (p<0.01), but no difference was observed in serum SP-D levels between GDM patients with Thr31Thr genotype and those with Met31Thr and Met31Met genotypes. To mitigate effects induced by race difference or heterogeneity in region and living environments, all GDM patients and healthy controls in our study were confined the Chinese Han nationality residing in similar geographic locations from Zhejiang Province of China. As a result, frequencies of genotypes at Met31Thr loci in healthy controls were similar to those reported in previous studies conducted in the Chinese population (25) while the distribution of genotypes in the Chinese population differed from the Western counterpart (26). Therefore, we speculated that the SP-D Met31Thr polymorphism might be used as a biomarker to predict patient susceptibility to Chinese GDM patients.
As an intraexonic polymorphism (rs721917) located in codon 11 in the SP-D N-terminal region, the Met31Thr results in changes in amino acid residues of SP-D protein. It has been reported that the polymorphic variation influences oligomerization, function and circulating concentrations (27). Leth-Larsen et al. (27) found that Thr31Thr genotype produces almost exclusively monomers of SP-D, whereas Met31Met genotype shows multimers such as dodecamers and trimers of subunits. These macromolecule polymers of SP-D may bind to viruses and bacteria, while the monomeric species seem to bind LPS almost exclusively. However, it is unclear how these different oligomeric protein molecules influence other biological function. It needs to be further studies in the future.
In the present study, we found that GDM subjects exhibited higher serum SP-D concentrations compared to healthy controls (p = 0.002), particularly among subjects with Thr31Thr genotype, which might contribute to individual susceptibility. However, no difference was observed in serum SP-D levels between GDM subjects with Thr31Thr genotype and those with Met31Thr and Met31Met genotypes. Neus Pueyo et al. (11) found cross-sectional and longitudinal associations of circulating SP-D concentrations within insulin resistance and T2D, and also described that the Met31Thr SP-D gene SNP rs721917 was associated with insulin resistance and the prevalence of T2D (28). In this study, GDM partially shares the same mechanism with T2D, and the possible influence of the Met31Thr SP-D gene SNP on GDM may arise from changes in the molecular structure and/or functional properties of SP-D protein rather than through changes in its circulating concentrations. Jie Yan et al. (29) verified twenty genes associated with T2D and obesity in a Chinese population and found only four SNPs were significantly correlated with GDM, and those risk alleles were associated with lipid profiles and glucose level. Ming et al. (30) genotyped one hundred and twelve susceptibility genetic variants confirmed by genome-wide association studies for T2D from two independent populations, and identified only eleven SNPs associated with the risk of GDM. These findings might be explained by two possible reasons. Firstly, mechanisms responsible for incidence or progression vary from gestational diabetes mellitus (GDM) and obesity or its accompanying T2D. Besides, as one of the metabolic diseases, GDM is involved in interactions between genetic and environmental or nutritional factors, which indicates that GDM pathogenesis could be attributed to various elements besides genetic polymorphisms. Secondly, our study was carried only in the Chinese Han population, which expressed different genetic variants due to race, region, and living environments.
In conclusion, we found at first time that SP-D gene polymorphism (rs721917) was associated with GDM and there is higher serum SP-D level in the GDM compared to matched healthy controls.