A New Model of Hereditary Spherocytosis Demonstrates Profound Homeostatic Compensation in Severely Anemic Mice.

As part of a concerted program to generate new models of human disease we have generated multiple heritable mutant mouse strains using ethylnitrosourea (ENU) random mutagenesis combined with a dominant blood screen. Here we describe a mutant strain with dominantly-inherited red blood cell (RBC) low mean corpuscular volume. Preliminary phenotypic analysis of affected mice demonstrated spherical RBC morphology, increased osmotic fragility, mild reticulocytosis and a reduction in circulating lifetime - features characteristic of human hereditary spherocytosis. Gene mapping revealed the causative point mutation to be in the mouse ankyrin-1 gene (Ank1) locus at exon 26 - a G to T transversion causing substitution of a glutamate codon with a premature stop (Ank1* 895 ). The resulting gene product is a truncated ankyrin-1 protein (terminating at amino acid 894) consisting primarily of the N-terminal band 3-binding domain without a functional ZU5 domain (spectrin-binding) or regulatory/death domain (Fig.1). The Ank1* 895 allele results in a hypomorph such that stably-expressed protein isolated from RBC ghost preparations is undetectable by Coomasie stain and only barely detectable by Western. Our ENU-generated mutation is distinct from the spontaneous mutation identified in the normoblastosis (nb) mouse (Peters et al. 1991. J. Cell Biol; Birkenmeier et al. 2003. Hematol J.) which yields a functional protein product with intact band 3- and spectrin-binding domains (Fig.1).