Epigenetic and phenotypic consequences of a truncation disrupting the imprinted domain on distal mouse chromosome 7.

The distal end of mouse chromosome 7 (Chr 7) contains a large cluster of imprinted genes. In this region two cis-acting imprinting centers, IC1 (H19 DMR) and IC2 (KvDMR1), define proximal and distal subdomains, respectively. To assess the functional independence of IC1 in the context of Chr 7, we developed a recombinase-mediated chromosome truncation strategy in embryonic stem cells and generated a terminal deletion allele, DelTel7, with a breakpoint in between the two subdomains. We obtained germ line transmission of the truncated Chr 7 and viable paternal heterozygotes, confirming the absence of developmentally required paternally expressed genes distal of Ins2. Conversely, maternal transmission of DelTel7 causes a midgestational lethality, consistent with loss of maternally expressed genes in the IC2 subdomain. Expression and DNA methylation analyses on DelTel7 heterozygotes demonstrate the independent imprinting of IC1 in absence of the entire IC2 subdomain. The evolutionarily conserved linkage between the subdomains is therefore not required for IC1 imprinting on Chr 7. Importantly, the developmental phenotype of maternal heterozygotes is rescued fully by a paternally inherited deletion of IC2. Thus, all the imprinted genes located in the region and required for normal development are silenced by an IC2-dependent mechanism on the paternal allele.

Selective striatal neuronal loss in a YAC128 mouse model of Huntington disease.

An expanded CAG repeat is the underlying genetic defect in Huntington disease, a disorder characterized by motor, psychiatric and cognitive deficits and striatal atrophy associated with neuronal loss. An accurate animal model of this disease is crucial for elucidation of the underlying natural history of the illness and also for testing experimental therapeutics. We established a new yeast artificial chromosome (YAC) mouse model of HD with the entire human HD gene containing 128 CAG repeats (YAC128) which develops motor abnormalities and age-dependent brain atrophy including cortical and striatal atrophy associated with striatal neuronal loss. YAC128 mice exhibit initial hyperactivity, followed by the onset of a motor deficit and finally hypokinesis. The motor deficit in the YAC128 mice is highly correlated with striatal neuronal loss, providing a structural correlate for the behavioral changes. The natural history of HD-related changes in the YAC128 mice has been defined, demonstrating the presence of huntingtin inclusions after the onset of behavior and neuropathological changes. The HD-related phenotypes of the YAC128 mice show phenotypic uniformity with low inter-animal variability present, which together with the age-dependent striatal neurodegeneration make it an ideal mouse model for the assessment of neuroprotective and other therapeutic interventions.

Disruption of the endocytic protein HIP1 results in neurological deficits and decreased AMPA receptor trafficking.

Huntingtin interacting protein 1 (HIP1) is a recently identified component of clathrin-coated vesicles that plays a role in clathrin-mediated endocytosis. To explore the normal function of HIP1 in vivo, we created mice with targeted mutation in the HIP1 gene (HIP1(-/-)). HIP1(-/-) mice develop a neurological phenotype by 3 months of age manifest with a failure to thrive, tremor and a gait ataxia secondary to a rigid thoracolumbar kyphosis accompanied by decreased assembly of endocytic protein complexes on liposomal membranes. In primary hippocampal neurons, HIP1 colocalizes with GluR1-containing AMPA receptors and becomes concentrated in cell bodies following AMPA stimulation. Moreover, a profound dose-dependent defect in clathrin-mediated internalization of GluR1-containing AMPA receptors was observed in neurons from HIP1(-/-) mice. Together, these data provide strong evidence that HIP1 regulates AMPA receptor trafficking in the central nervous system through its function in clathrin-mediated endocytosis.