Genetic variation in GC and CYP2R1 affects 25-hydroxyvitamin D concentration and skeletal parameters: A genome-wide association study in 24-month-old Finnish children

Vitamin D is important for normal skeletal homeostasis, especially in growing children. There are no previous genome-wide association (GWA) studies exploring genetic factors that influence vitamin D metabolism in early childhood. We performed a GWA study on serum 25-hydroxyvitamin D (25(OH)D) and response to supplementation in 761 healthy term-born Finnish 24-month-old children, who participated in a randomized clinical trial comparing effects of 10 μg and 30 μg of daily vitamin D supplementation from age 2 weeks to 24 months. Using the Illumina Infinium Global Screening Array, which has been optimized for imputation, a total of 686085 markers were genotyped across the genome. Serum 25(OH)D was measured at the end of the intervention at 24 months of age. Skeletal parameters reflecting bone strength were determined at the distal tibia at 24 months using peripheral quantitative computed tomography (pQCT) (data available for 648 children). For 25(OH)D, two strong GWA signals were identified, localizing to GC (Vitamin D binding protein) and CYP2R1 (Vitamin D 25-hydroxylase) genes. The GWA locus comprising the GC gene also associated with response to supplementation. Further evidence for the importance of these two genes was obtained by comparing association signals to gene expression data from the Genotype-Tissue Expression project and performing colocalization analyses. Through the identification of haplotypes associated with low or high 25(OH)D concentrations we used a Mendelian randomization approach to show that haplotypes associating with low 25(OH)D were also associated with low pQCT parameters in the 24-month-old children. In this first GWA study on 25(OH)D in this age group we show that already at the age of 24 months genetic variation influences 25(OH)D concentrations and determines response to supplementation, with genome-wide significant associations with GC and CYP2R1. Also, the dual association between haplotypes, 25(OH)D and pQCT parameters gives support for vertical pleiotropy mediated by 25(OH)D.维生素D对于维持正常骨骼稳态至关重要，尤其在生长发育期的儿童群体中。此前尚无全基因组关联研究（Genome-Wide Association Study, GWA）探索影响幼儿期维生素D代谢的遗传因素。本研究针对761名健康足月出生的芬兰24月龄儿童，开展了血清25-羟维生素D（25(OH)D）水平及维生素D补充应答的全基因组关联研究，这些儿童参与了一项随机临床试验，旨在比较从2周龄至24月龄每日补充10μg与30μg维生素D的干预效果。研究采用经基因填充优化的Illumina Infinium全球筛查阵列（Illumina Infinium Global Screening Array），在全基因组范围内完成了总计686085个遗传标记的基因分型。干预结束时（即儿童24月龄时）检测血清25(OH)D水平；于24月龄时通过外周定量计算机断层扫描（peripheral quantitative computed tomography, pQCT）检测胫骨远端反映骨强度的骨骼参数，共计648名儿童具备该检测数据。针对血清25(OH)D水平，本研究鉴定出两个较强的全基因组关联信号，分别定位于GC（维生素D结合蛋白）与CYP2R1（维生素D 25-羟化酶）基因；包含GC基因的全基因组关联位点同时与维生素D补充应答存在关联。通过将关联信号与基因型组织表达（Genotype-Tissue Expression）项目的基因表达数据进行对比，并开展共定位分析，本研究进一步验证了这两个基因的重要性。研究人员通过鉴定与低或高25(OH)D浓度相关的单体型，采用孟德尔随机化（Mendelian randomization）方法证实，与低25(OH)D水平相关的单体型，同样与24月龄儿童的pQCT检测参数偏低存在关联。作为该年龄组首个针对25(OH)D水平的全基因组关联研究，本研究证实，早在24月龄时，遗传变异即可影响血清25(OH)D浓度，并决定个体对维生素D补充的应答情况，且与GC及CYP2R1基因存在全基因组显著关联。此外，单体型、25(OH)D水平与pQCT参数三者间的双重关联，为25(OH)D介导的垂直多效性提供了支持证据。