In humans, poor nutrition, malabsorption and variation in
cobalamin (
vitamin B12) metabolic genes are associated with hematological, neurological and developmental pathologies.
Cobalamin is transported from blood into tissues via the
transcobalamin (TC) receptor encoded by the CD320 gene. We created mice carrying a targeted deletion of the mouse ortholog, Cd320. Knockout (KO) mice lacking this TC receptor have elevated levels of plasma
methylmalonic acid and
homocysteine but are otherwise healthy, viable, fertile and not anemic. To challenge the Cd320 KO mice we maintained them on a
vitamin B12-deficient diet. After 5 weeks on this diet, reproductive failure develops in Cd320 KO females but not males. In vitro, homozygous Cd320 KO embryos from
cobalamin-deficient Cd320 KO dams develop normally to embryonic day (E) 3.5, while in vivo, few uterine decidual implantation sites are observed at E7.5, suggesting that embryos perish around the time of implantation.
Dietary restriction of
vitamin B12 induces a severe
macrocytic anemia in Cd320 KO mice after 10-12 months while control mice on this diet are
anemia-free up to 2 years. Despite the severe
anemia,
cobalamin-deficient KO mice do not exhibit obvious neurological symptoms. Our results with Cd320 KO mice suggest that an alternative mechanism exists for mice to transport
cobalamin independent of the Cd320 encoded receptor. Our findings with deficient diet are consistent with historical and epidemiological data suggesting that low
vitamin B12 levels in humans are associated with
infertility and developmental abnormalities. Our Cd320 KO mouse model is an ideal model system for studying
vitamin B12 deficiency.