Abstract
Vitamin D (vitD) deficiency is a key public health issue worldwide. Despite the abundance of sunlight exposure in countries in the Middle East, including Saudi Arabia, vitD deficiency is still highly prevalent in these regions and particularly in postmenopausal women. VitD is mainly bound to vitD binding protein (VDBP), while the unbound form of vitD (˂1%) is said to be free. Free 25(OH)D is suggested to be more precise than total 25(OH)D in evaluating vitD status, especially in a population with different ethnic groups. Furthermore, the association between vitD and type 2 diabetes mellitus (T2DM), and the influence of genetic polymorphisms in genes involved in vitD metabolism on 25(OH)D levels seem to differ between the ethnic groups. Therefore, the aims of this PhD project were as follows: (i) Determine vitD status (total and free 25(OH)D) and its association with metabolic health parameters and specific single-nucleotide polymorphisms (SNP) in vitD related genes in a multi-ethnic vitD deficiency high risk cohort, particularly postmenopausal women in Saudi Arabia; (ii) Look for known or novel genetic variations in genes involved in vitD metabolism in families diagnosed with vitD deficiency in Saudi Arabia; (iii) Investigate the association between vitD and glycaemic parameters in a multi-ethnic cohort of postmenopausal women with T2DM in Saudi Arabia.
Study (i) included a multi-ethnic cohort of 459 postmenopausal healthy women aged between 50 and 81 years, randomly recruited from seven primary health care centers scattered in Jeddah, Saudi Arabia. Blood samples were collected from subjects for measurement of serum levels of total 25(OH)D, directly measured free 25(OH)D, VDBP, metabolic bone parameters, lipid profile and other serum biochemical tests .
Around 49% of the participants had optimal total 25(OH)D level (≥20 ng/ml) according to Institute of Medicine. Total 25(OH)D level was associated with rs7041 SNP in GC gene (P=0.023). A positive correlation was found between directly measured free and total 25(OH)D (r=0.61, P<0.0001). The total, but not free 25(OH)D showed significant association with serum intact parathyroid hormone (PTH), blood pressure, waist and hip circumferences (P<0.05); whilst free 25(OH)D but not total 25(OH)D showed a significant association with total cholesterol and LDL-C (P=0.027 and P=0.022; respectively). These observed total 25(OH)D significant associations were significantly affected by ethnic group.
Study (ii) involved 21 families with vitD deficiency (n=39) in Saudi Arabia. WES was performed for DNA samples, then resulting WES data was filtered and a number of variants were prioritized and validated by Sanger DNA sequencing. We were able to find a novel mutation in DHCR7 (rs143587828) in two subjects from one family, and a polymorphism in LRP2 (rs2075252) in 3 families (2 subjects from each family, n=6). When these variants were validated, they were not observed in controls (n=100) which may suggest that these genetic variants might affect vitD levels and influence vitD status. Further studies are now required to confirm the association of these variants with vitD deficiency.
Study (iii) involved a multi-ethnic cohort of postmenopausal females (n = 173, age ≥ 50 years) with T2DM. Several biochemical parameters were measured including total 25(OH)D, glycosylated hemoglobin, insulin, glucose, c-peptide and insulin sensitivity indices. VitD status was inversely associated with insulin resistance and anthropometric measures in this cohort [fasting glucose (r=-0.165, P=0.037), insulin (r=-0.184, P=0.02), C-peptide (r=-0.19, P=0.015), HOMA2- IR C-peptide (r=-0.23,P=0.004), body weight (r=-0.173 P=0.028), waist and hip circumferences (r=-0.167, P=0.033; r=-0.22, P=0.004 respectively)]. Our findings also showed that vitD associations with insulin resistance and anthropometric measures in women with white ethnicity were significant (P<0.05); unlike those from black/Asian ethnic backgrounds.