Mutations of TCN1 Cause Transcobalamin I Deficiency with Low Serum Cobalamin Levels That Are Indistinguishable From Cobalamin Deficiency.

Abstract 1989

Poster Board I-1011

Low serum cobalamin levels have diverse origins. Indeed, in 21-22% of cases low cobalamin levels do not even reflect cobalamin deficiency, as judged by metabolic criteria, and in most of the rest they reflect subclinical, purely biochemical cobalamin insufficiency. These complicate the interpretation of low cobalamin levels and have engendered controversies. Transcobalamin (TC) I deficiency causes spuriously low cobalamin levels because this binding protein normally carries >70% of the cobalamin in the blood, yet cellular cobalamin status remains normal despite the low plasma cobalamin levels. The entity is often misdiagnosed as cobalamin deficiency and incorrectly treated with cobalamin, and it is thought to be rare. Diagnosis is especially elusive if TC I deficiency is mild. We recently described two TCN1 nonsense mutations, 270delG and 315C>T, in 8 affected members of 3 non-Caucasian families, that caused TC I deficiency that was severe (absence of TC I and very low cobalamin) when both alleles are affected or mild (low plasma TC I and low or low-normal cobalamin) when only one allele was affected (Carmel et al, Br J Haematol, in press). We now describe the new association between TC I deficiency and a TCN1 polymorphism known to be more common in Caucasians. Patients and Methods: Plasma total TC I was assayed by radioimmunassay. TCN1 was sequenced as previously described, using 9 primer sets to sequence each of the 9 exons, in 4 unrelated patients with unexplained low cobalamin levels, in all of whom cobalamin deficiency and malabsorption were ruled out by metabolic and absorption studies. Available relatives were similarly tested. Results: All 4 unrelated propositi with mild TC I deficiency (plasma TC I 87-136 pmol/l, normal 165-454; serum cobalamin 94-155 pmol/l, normal 162-664) were found to be heterozygotes for a 999G>T missense mutation that causes substitution of tyrosine, a large hydrophobic amino acid, for the smaller, negatively charged aspartic acid at position 301. These 4 families of Caucasian ancestry also included 3 relatives with equally mild TC I deficiency (TC I 117-185; cobalamin 111-271), all of whom had the same mutation. In contrast, all 4 relatives with normal TC I levels (TC I 205-367; cobalamin 210-345) had no mutations. Conclusions: The association of heterozygosity for 999G>T with mild TC I deficiency in 7 affected members of 4 Caucasian families and its absence in all 4 unaffected members of these families suggest that this common mutation is responsible for TC I deficiency. It also adheres to the association of heterozygosity for our previously identified TCN1 mutations (270delG and 315C>T) with mild TC I-deficiency. The former occurred in non-Caucasian families, particularly of African ancestry, and were often linked in unexplained, possibly indirect ways with Hemoglobin S. The findings also suggest TCN1 mutational diversity among different ethnic groups. The known preponderance of the 999G>T polymorphism in European subjects is compatible with our ethnic findings. It may also explain why whites have significantly lower cobalamin and TC I levels than do blacks. Finally, the relatively high frequency of 999G>T polymorphism (11.6% + 0.02% heterozygosity frequency; NCBI website) resembles the 15% frequency of mild TC I deficiency reported in a TC I survey that suggested that TC I deficiency is a surprisingly common cause of low serum cobalamin levels and may often be hereditary. These TCN1 mutations should expand the ability to diagnose TC I deficiency, which can be a difficult diagnosis to make because TC I immunoassay is rarely available. They should also allow more reliable study of hereditary TC I deficiency and its frequency as a cause of low cobalamin levels. A better understanding of the entity may help clarify the currently mysterious role of TC I.