Mutations in the human ALMS1 gene cause Alström syndrome (AS), a progressive disease characterized by neurosensory deficits and by metabolic defects including childhood obesity, hyperinsulinemia and Type 2 diabetes. Other features that are more variable in expressivity include dilated cardiomyopathy, hypertriglyceridemia, hypercholesterolemia, scoliosis, developmental delay and pulmonary and urological dysfunctions. ALMS1 encodes a ubiquitously expressed protein of unknown function. To obtain an animal model in which the etiology of the observed pathologies could be further studied, we generated a mouse model using an Alms1 gene-trapped ES cell line. Alms1-/- mice develop features similar to patients with AS, including obesity, hypogonadism, hyperinsulinemia, retinal dysfunction and late-onset hearing loss. Insulin resistance and increased body weight are apparent between 8 and 12 weeks of age, with hyperglycemia manifesting at approximately 16 weeks of age. In addition, Alms1-/- mice have normal hearing until 8 months of age, after which they display abnormal auditory brainstem responses. Diminished cone ERG b-wave response is observed early, followed by the degeneration of photoreceptor cells. Electron microscopy revealed accumulation of intracellular vesicles in the inner segments of photoreceptors, whereas immunohistochemical analysis showed mislocalization of rhodopsin to the outer nuclear layer. These findings suggest that ALMS1 has a role in intracellular trafficking.

The function of the retinitis pigmentosa GTPase regulator interacting protein 1 (RPGRIP1) gene is currently not known. However, mutations within the gene lead to Leber Congenital Amaurosis and autosomal recessive retinitis pigmentosa in human patients. In a previously described knockout mouse model of the long splice variant of Rpgrip1, herein referred to as Rpgrip1(tm1Tili) mice, mislocalization of key outer segment proteins and dysmorphogenesis of outer segment discs preceded subsequent photoreceptor degeneration. In this report, we describe a new mouse model carrying a splice acceptor site mutation in Rpgrip1, herein referred to as Rpgrip1(nmf247) that is phenotypically distinct from Rpgrip1(tm1Tili) mice. Photoreceptor degeneration in homozygous Rpgrip1(nmf247) mice is earlier in onset and more severe when compared with Rpgrip1(tm1Tili) mice. Also, ultrastructural studies reveal that whereas Rpgrip1(nmf247) mutants have a normal structure and number of connecting cilia, unlike Rpgrip1(tm1Tili) mice, they do not elaborate rod outer segments (OS). Therefore, in addition to its role in OS disc morphogenesis, RPGRIP1 is essential for rod OS formation. Our study indicates the absence of multiple Rpgrip1 isoforms in Rpgrip1(nmf247) mice, suggesting different isoforms may play different roles in photoreceptors and underscores the importance of considering splice variants when generating targeted null mutations.

The nmf240 phenotype closely resembles that reported for Clcn2 knockout mice. The observation that heterozygous nmf240 mice present with a reduced ERG light peak component suggests that CLCN2 is necessary for the generation of this response component.

No abstract

Ankyrin deficiency is one of the most common causes of hereditary spherocytosis in humans. A spontaneous mutation, normoblastosis (Ank1nb), discovered in 1969 in a mouse stock maintained at the Jackson Laboratory, provides an important animal model for these human ankyrin-deficient anemias. Study of this model has led to the finding of multiple isoforms of Ank1 as well as Ank1nb-related pathology in nonerythroid tissues. To enhance the usefulness of this model, we have identified the Ank1nb mutation as the deletion of a guanosine residue in exon 36 of the erythroid ankyrin gene (Ank1). This results in a frame shift that introduces a stop 13 codons downstream and predicts a 157 kDa nb-ankyrin lacking the regulatory domain but including intact membrane- and spectrin-binding domains. By epitope scanning on immunoblots, we show that a previously reported protein (p150) found in nb reticulocytes is the predicted nb-ankyrin. Existing evidence indicates that this protein is functional, making the normoblastosis mutation a hypomorph rather than a null as originally thought. The nb-ankyrin provides an explanation for the milder phenotype displayed by nb/nb animals relative to the murine spectrin-deficient anemias, spherocytosis (Spna1(sph), Spna1(sph-1J), Spna1(sph-2BC), Spna1(sph-DEM)) and jaundiced (Spnb1(ja)), and suggests that truncated ankyrins could be useful in gene replacement therapy.

These results predict the heart as an additional site where alpha-spectrin mutations may produce a phenotype and raise the possibility that a novel functional class of small alpha-spectrin isoforms may exist.