analysis of the structural and functional implications of R27H polymorphism.

In view of the documented association of solute carrier family 19 member 1 (SLC19A1) G80A (R27H) polymorphism with the risk for different types of cancers and systemic lupus erythematosus (SLE), we have reanalysed the case-control study on breast cancer to ascertain the conditions in which this polymorphic variant exerts the risk of breast cancer. Association statistics have revealed that this polymorphism exerts the risk for breast cancer under the conditions of low folate intake, and in the absence of well-documented protective polymorphism in cytosolic serine hydroxymethyltransferase. To substantiate this observation, we have developed a homology model of SLC19A1 using glycerol-3-phosphate transporter (d1pw4a) as a template where 73% of the residues were modelled at 90% confidence while 162 residues were modelled . The wild and mutant proteins shared same topology in S3, S5, S6, S7, S11 and S12 transmembrane domains. The topology varied at S1 (28-43 residue vs 28-44 residue), S2 (66-87 residue vs 69-87 residue), S4 (117-140 residue vs 117-139 residue), S8 (305-325 residue vs 305-324 residue), S9 (336-356 residue vs 336-355residue), and S10 (361-386 residue vs 361-385 residue) transmembrane domains between wild versus mutant proteins. S2 domain is shortened by three amino acid residues in the mutant while in other domains the difference corresponds to one amino acid residue. The 3DLigandSite analysis revealed that the metallic-ligand-binding sites at 273Trp, 277Asn, 379Leu, 439Phe and 442Leu are although unaffected, there is a loss of active sites corresponding to nonmetallic ligand binding. Tetrahydrofolate and methotrexate have lesser affinity towards the mutant protein than the wild protein. To conclude, the R27H polymorphism affects the secondary and tertiary structures of SLC19A1 with the significant loss in ligand-binding sites.