Genetic Association of Pulmonary Surfactant Protein Genes, SFTPA1, SFTPA2, SFTPB, SFTPC, and SFTPD With Cystic Fibrosis

Abstract

Surfactant proteins (SP) are involved in surfactant function and innate immunity in the human lung. Both lung function and innate immunity are altered in CF, and altered SP levels and genetic association are observed in Cystic Fibrosis (CF). We hypothesized that single nucleotide polymorphisms (SNPs) within the SP genes associate with CF or severity subgroups, either through single SNP or via SNP-SNP interactions between two SNPs of a given gene (intragenic) and/or between two genes (intergenic). We genotyped a total of 17 SP SNPs from 72 case-trio pedigree (SFTPA1 (5), SFTPA2 (4), SFTPB (4), SFTPC (2), and SFTPD (2)), and identified SP SNP associations by applying quantitative genetic principles. The results showed (a) Two SNPs, SFTPB rs7316 (p = 0.0083) and SFTPC rs1124 (p = 0.0154), each associated with CF. (b) Three intragenic SNP-SNP interactions, SFTPB (rs2077079, rs3024798), and SFTPA1 (rs1136451, rs1059057 and rs4253527), associated with CF. (c) A total of 34 intergenic SNP-SNP interactions among the 4 SP genes to be associated with CF. (d) No SNP-SNP interaction was observed between SFTPA1 or SFTPA2 and SFTPD. (e) Equal number of SNP-SNP interactions were observed between SFTPB and SFTPA1/SFTPA2 (n = 7) and SP-B and SFTPD (n = 7). (f) SFTPC exhibited significant SNP-SNP interactions with SFTPA1/SFTPA2 (n = 11), SFTPB (n = 4) and SFTPD (n = 3). (g) A single SFTPB SNP was associated with mild CF after Bonferroni correction, and several intergenic interactions that are associated (p < 0.01) with either mild or moderate/severe CF were observed. These collectively indicate that complex SNP-SNP interactions of the SP genes may contribute to the pulmonary disease in CF patients. We speculate that SPs may serve as modifiers for the varied progression of pulmonary disease in CF and/or its severity.

Keywords: SP-A; SP-B; SP-C; SP-D; cystic fibrosis; surfactant protein.

Figure 1

Genetic interactions between SP genes and associations with CF (A). Genetic association of SFTPB and SFTPC with CF by single SNP, and intragenic and intergenic SNP-SNP interactions between SFTPB and SFTPC, and with SFTPA1 and SFTPA2. On the left SNPs of the surfactant genes SFTPA1 and SFTPA2 encoding the hydrophilic surfactant proteins, SP-A1 and SP-A2 and on the right SNPs of the surfactant genes SFTPB and SFTPC, encoding the hydrophobic proteins, SP-B and SP-C, are shown (B). Genetic association of SFTPB and SFTPC with CF by single SNP, and intragenic and intergenic SNP-SNP interactions between SFTPB and SFTPC, and with SFTPD. On the left SNPs of SFTPD and on the right SNPs of SFTPB and SFTPC are shown: In Figure, the star depicts a single SNP association with CF, the black dash line depicts intragenic interactions associated with CF, and the black solid line depicts intergenic interactions associated with CF.

Figure 2

Summary of SNP interactions between genes encoding the surfactant proteins. The cartoon in this figure depicts the observed associations and reveals the following: (a) No association between SFTPD and SFTPA1 or SFTPA2. (b) Out of the 34 intergenic interactions, 28 are between genes encoding the hydrophobic and the hydrophilic surfactant proteins. Only four are between genes (SFTPB and SFTPC) encoding the hydrophobic surfactant proteins and two are between genes (SFTPA1 and SFTPA2) encoding the hydrophilic SP-A1 and SP-A2. (c) SFTPC has the highest number of interactions with SFTPA1 (n = 8), while SFTPB has the highest number of interactions with SFTPD (n = 7). (d) SFTPB has a total of 14 intergenic interactions with genes encoding hydrophilic proteins, seven with SFTPD, four with SFTPA2, and three with SFTPA1. SFTPC also has 14 intergenic interactions, eight with SFTPA1, three with SFTPA2 and only 3 with SFTPD. (e) SFTPA1 has two intergenic interactions with SFTPA2, as well as intragenic interactions. (f) SFTPB is the only other gene (other than SFTPA1) that has an intragenic interaction. (g) Only SFTPB and SFTPC have each a single SNP association with CF.

Figure 3

Interactions in mild and moderate/severe CF subgroups with p-value < 0.01; these interactions were significant in the entire CF group after Bonferroni correction. Mild CF: Significant interactions between SFTPA1, SFTPA2, SFTPB, and SFTPD are shown with black arrows. Green box indicates the only SNP that remained significant after Bonferroni correction. Star (*) indicates that this SNP is significant at p < 0.03 by itself. Moderate/severe CF: Red arrow shows the only significant interaction at p < 0.01 that is also significant in the entire CF groups after Bonferroni correction. No significant interactions that include SNPs of the SFTPC are observed in either severity group.