Medical Genomics and Metabolic Genetics Branch, National Human Genome Research Institute, Bethesda, MD 20892, USA.
Department of Clinical Medicine, School of Medicine, Trinity College Dublin, Dublin 2, Ireland.
Hum Mol Genet. 2017 Dec 15;26(24):4975-4988. doi: 10.1093/hmg/ddx369.
Vitamin B12 deficiency is common in older individuals. Circulating vitamin B12 concentration can be used to diagnose deficiency, but this test has substantial false positive and false negative rates. We conducted genome-wide association studies (GWAS) in which we resolved total serum vitamin B12 into the fractions bound to transcobalamin and haptocorrin: two carrier proteins with very different biological properties. We replicated reported associations between total circulating vitamin B12 concentrations and a common null variant in FUT2. This allele determines the secretor phenotype in which blood group antigens are found in non-blood body fluids. Vitamin B12 bound to haptocorrin (holoHC) remained highly associated with FUT2 rs601338 (p.Trp154Ter). Transcobalamin bound vitamin B12 (holoTC) was not influenced by this variant. HoloTC is the bioactive the form of the vitamin and is taken up by all tissues. In contrast, holoHC is only taken up by the liver. Using holoHC from individuals with known FUT2 genotypes, we demonstrated that FUT2 rs601338 genotype influences the glycosylation of haptocorrin. We then developed an experimental model demonstrating that holoHC is transported into cultured hepatic cells (HepG2) via the asialoglycoprotein receptor (ASGR). Our data challenge current published hypotheses on the influence of genetic variation on this clinically important measure and are consistent with a model in which FUT2 rs601338 influences holoHC by altering haptocorrin glycosylation, whereas B12 bound to non-glycosylated transcobalamin (i.e. holoTC) is not affected. Our findings explain some of the observed disparity between use of total B12 or holoTC as first-line clinical tests of vitamin B12 status.
维生素 B12 缺乏在老年人中很常见。循环中维生素 B12 的浓度可用于诊断缺乏症，但该检测存在大量的假阳性和假阴性。我们进行了全基因组关联研究（GWAS），其中我们将总血清维生素 B12 分为与转钴胺素和触珠蛋白结合的部分：两种具有非常不同生物学特性的载体蛋白。我们复制了总循环维生素 B12 浓度与 FUT2 中常见的无效变异之间的关联。该等位基因决定了分泌表型，其中血型抗原存在于非血液体液中。与 FUT2 rs601338（p.Trp154Ter）相关的结合触珠蛋白的维生素 B12（全同型 HC）仍然高度相关。该变体不影响转钴胺素结合的维生素 B12（全同型 TC）。全同型 TC 是维生素的生物活性形式，被所有组织吸收。相比之下，全同型 HC 仅被肝脏吸收。使用已知 FUT2 基因型个体的全同型 HC，我们证明了 FUT2 rs601338 基因型影响触珠蛋白的糖基化。然后，我们开发了一种实验模型，证明全同型 HC 通过去唾液酸糖蛋白受体（ASGR）被转运到培养的肝细胞（HepG2）中。我们的数据挑战了目前关于遗传变异对这一临床重要指标影响的假设，并与 FUT2 rs601338 通过改变触珠蛋白糖基化来影响全同型 HC 的模型一致，而与非糖基化转钴胺素（即全同型 TC）结合的 B12 不受影响。我们的研究结果解释了一些观察到的总 B12 或全同型 TC 作为维生素 B12 状态的一线临床检测之间的差异。
