Loss of the Vitamin B-12 Transport Protein Tcn2 Results in Maternally Inherited Growth and Developmental Defects in Zebrafish

Background: In humans, vitamin B-12 (cobalamin) transport involves 3 paralogous proteins: transcobalamin, haptocoi tin, and intrinsic factor. Zebrafish (Dania rerio) express 3 genes that encode proteins homologous to known B-12 carrier proteins: tcn2 (a transcobalamin ortholog) and 2 atypical beta-domain-only homologs, tenba and tcnbb. Objectives: Given the orthologous relation between zebrafish Tcn2 and human transcobalamin, we hypothesized that zebrafish carrying null mutations of tcn2 would exhibit phenotypes consistent with vitamin B-12 deficiency. Methods: First-generation and second-generation tcn2(-/-) zebrafish were characterized using phenotypic assessments, metabolic analyses, viability studies, and transcriptomics. Results: Homozygous tcn2(-/-) fish produced from a heterozygous cross are viable and fertile but exhibit reduced growth, which persists into adulthood. When first-generation female tcn2(-/-) fish are bred, their offspring exhibit gross developmental and metabolic defects. These phenotypes are observed in all offspring from a tcn2(-/-) female regardless of the genotype of the male mating partner, suggesting a maternal effect, and can be rescued with vitamin B-12 supplementation. Transcriptome analyses indicate that offspring from a tcn2(-/-) female exhibit expression profiles distinct from those of offspring from a tcn2(+/+) female, which demonstrate dysregulation of visual perception, fatty acid metabolism, and neurotransmitter signaling pathways. Conclusions: Our findings suggest that the deposition of vitamin B-12 in the yolk by tcn2(-/-) females may be insufficient to support the early development of their offspring. These data present a compelling model to study the effects of vitamin B-12 deficiency on early development, with a particular emphasis on transgenerational effects and gene-environment interactions.