Hematologic characterization and chromosomal localization of the novel dominantly inherited mouse hemolytic anemia, neonatal anemia (Nan).

One of the most commonly inherited anemias in man is Hereditary Spherocytosis (HS) with an incidence of 1 in 2000 for persons of Northern European descent. Mouse models of HS include spontaneous inherited hemolytic anemias and those generated by gene targeting. The Neonatal anemia (Nan) mouse is a novel model of HS generated by N-ethyl-N-nitrosurea mutagenesis and suffers from a severe neonatal anemia. Adult Nan mice have a lifelong hemolytic anemia with decreased red blood cell numbers, hematocrit, and hemoglobin, but elevated zinc protoporphyrin levels. Blood smears taken from Nan mice show a hypochromic anemia characterized by poikilocytosis, anisocytosis and polychromasia. The Nan phenotype can be transferred by bone marrow transplantation indicating that the defect is intrinsic to bone marrow. The hemolytic anemia in adult Nan mice can be identified by osmotic fragility testing. Examination of the erythrocyte membrane skeleton proteins (EMS) reveals a global deficiency of these proteins with protein 4.1a being completely absent. The Nan locus maps to mouse Chromosome 8 and does not co-localize with any known EMS genes. The identification of the Nan gene will likely uncover a novel protein that contributes to the stability of the EMS and may identify a new mutation for HS.

Iron metabolism mutant hbd mice have a deletion in Sec15l1, which has homology to a yeast gene for vesicle docking.

Defects in iron absorption and utilization lead to iron deficiency and anemia. While iron transport by transferrin receptor-mediated endocytosis is well understood, it is not completely clear how iron is transported from the endosome to the mitochondria where heme is synthesized. We undertook a positional cloning project to identify the causative mutation for the hemoglobin-deficit (hbd) mouse mutant, which suffers from a microcytic, hypochromic anemia apparently due to defective iron transport in the endocytosis cycle. As shown by previous studies, reticulocyte iron accumulation in homozygous hbd/hbd mice is deficient despite normal binding of transferrin to its receptor and normal transferrin uptake in the cell. We have identified a strong candidate gene for hbd, Sec15l1, a homologue to yeast SEC15, which encodes a key protein in vesicle docking. The hbd mice have an exon deletion in Sec15l1, which is the first known mutation of a SEC gene homologue in mammals.

Positional cloning of the Ttc7 gene required for normal iron homeostasis and mutated in hea and fsn anemia mice.

Genes playing essential roles in iron homeostasis have yet to be identified. We report the discovery of a strong candidate gene affecting iron homeostasis in two allelic anemia mouse mutants: hea (hereditary erythroblastic anemia) and fsn (flaky skin). To clone this novel gene positionally, we established a large backcross, which generated a critical region of seven genes from which only one gene exhibited a mutation in hea mice. This was a deletion in Ttc7 (tetratricopeptide repeat domain 7) extending from exon 1 to exon 14. Correspondingly, the allelic variant fsn mice showed a mutation of an ETn retrotransposon integration into intron 14 of the Ttc7 gene, which results in an abnormal Ttc7 RNA transcript. TTC7 is a member of the TPR repeat protein family known to interact with other proteins, to facilitate transport, and to act as chaperone or scaffolding proteins. We speculate that TTC7 plays an important role in iron transport.