Introduction: Patients with differentiated thyroid cancer (DTC) are usually managed with total thyroidectomy and subsequent radioiodine ablation of the remnant thyroid tissue. Since these patients become athyrotic, L-throxine (L-T4) therapy is required for life, in order to replace the thyroid hormones and also to suppress the endogenous thyroid stimulating hormone (TSH) which may have a growth-promoting effect on any residual thyroid cancer cells. The approximate dose required to achieve this suppression is about 2 g/kg. However, there is wide variation between patients in their L-T4 requirement. Although factors such as the timing of the dose, compliance, weight and age play important roles, genetic factors are also thought to be important in this dose variability. Therefore, the aims of this study are to identify and evaluate the association of polymorphisms in six genes [(iodothyronine deiodinases (DIO) 1, 2 and 3, paired box gene 8 (PAX8), thyroid stimulating hormone subunit β (TSHβ), and sodium iodide symporter (NIS)], involved in thyroid hormone metabolic and functional pathways with DTC risk and L-T4 dose requirement.
Patients and Methods: Initially, 96 healthy Saudi individuals were recruited from the Family Medicine Polyclinics at KFSHRC to establish the single nucleotide polymorphisms (SNPs) present in genes of interest in the Saudi general population. This was followed by association studies in 507 patients and 597 controls. Candidate patients had undergone total thyroidectomy and received radioiodine ablation. They were on L-thyroxine suppressive therapy (Euthyrox, Merck Pharmaceuticals, NJ, USA), aiming at obtaining either suppressed (TSH <0.1 mU/L) or near-suppressed (0.1≤TSH<0.5 mU/L) serum TSH levels with FT4 in the normal range (12-22 pmol/L). Identification of SNPs was accomplished by sequencing all exons and exon-intron boundaries using MegaBACE DNA analysis system, and the association of the gene variants with risk of thyroid cancer and/or thyroxin dose requirement in the patient population was accomplished using real-time PCR with Taqman chemistry.
Result: Several novel and familiar SNPs were described in the studied genes with a minor allele frequency of 0.05 in the general population, of which 39 were selected for association studies. Following the adjustment for age, gender and smoking, the rs1321108, rs1321109 of the TSHβ gene, rs11123172, rs67776659 of the PAX8 gene and rs945006 of the DIO3 were associated with DTC. Interestingly, several haplotypes constructed from the studied SNPs in PAX8, TSHβ and NIS were associated with disease. The rs12084242, rs12095080, rs17109582 of DIO1, rs1321109 of TSHβ, rs2241975, and rs3738913 of PAX8 were associated with thyroxine dose requirement in the suppressed group, rs12095080 of DIO1 as well as rs2241975 and rs4849186 of PAX8 were associated with the difference in thyroxine dose requirement in the combined group, and rs4849186 and rs1478 of PAX8 were associated in the near- suppressed group.
Discussion and Conclusion: The study has revealed that TSHβ, NIS PAX8 and DIO3 are associated with DTC risk, and changes in DIO1, TSHβ and PAX8 may be important with respect to the requirement of altering thyroxine dose in therapy of DTC.