Quantitative trait loci associated with susceptibility to therapy-related acute murine promyelocytic leukemia in hCG-PML/RARA transgenic mice.

Therapy-related acute myelogenous leukemia (t-AML) is an important late adverse effect of alkylator chemotherapy. Susceptibility to t-AML has a genetic component, yet specific genetic variants that influence susceptibility are poorly understood. We analyzed an F(2) intercross (n = 282 mice) between mouse strains resistant or susceptible to t-AML induced by the alkylator ethyl-N-nitrosourea (ENU) to identify genes that regulate t-AML susceptibility. Each mouse carried the hCG-PML/RARA transgene, a well-characterized initiator of myeloid leukemia. In the absence of ENU treatment, transgenic F(2) mice developed leukemia with higher incidence (79.4% vs 12.5%) and at earlier time points (108 days vs 234 days) than mice in the resistant background. ENU treatment of F(2) mice further increased incidence (90.4%) and shortened median survival (171 vs 254 days). We genotyped F(2) mice at 384 informative single nucleotide polymorphisms across the genome and performed quantitative trait locus (QTL) analysis. Thirteen QTLs significantly associated with leukemia-free survival, spleen weight, or white blood cell count were identified on 8 chromosomes. These results suggest that susceptibility to ENU-induced leukemia in mice is a complex trait governed by genes at multiple loci. Improved understanding of genetic risk factors should lead to tailored treatment regimens that reduce risk for patients predisposed to t-AML.

The impact of copy number variation on local gene expression in mouse hematopoietic stem and progenitor cells.

The extent to which differences in germline DNA copy number contribute to natural phenotypic variation is unknown. We analyzed the copy number content of the mouse genome to sub-10-kb resolution. We identified over 1,300 copy number variant regions (CNVRs), most of which are <10 kb in length, are found in more than one strain, and, in total, span 3.2% (85 Mb) of the genome. To assess the potential functional impact of copy number variation, we mapped expression profiles of purified hematopoietic stem and progenitor cells, adipose tissue and hypothalamus to CNVRs in cis. Of the more than 600 significant associations between CNVRs and expression profiles, most map to CNVRs outside of the transcribed regions of genes. In hematopoietic stem and progenitor cells, up to 28% of strain-dependent expression variation is associated with copy number variation, supporting the role of germline CNVs as key contributors to natural phenotypic variation in the laboratory mouse.

Ammonia-mediated LTP inhibition: effects of NMDA receptor antagonists and L-carnitine.

Because hyperammonemia is thought to contribute to the pathogenesis of hepatic encephalopathy, we examined the effects of ammonia on ATP levels, neuronal morphology, and synaptic function in rat hippocampal slices. Although ammonia did not alter ATP levels supported by 10 mM glucose, ammonia significantly depressed ATP levels in the presence of 3.3 mM glucose or 10 mM pyruvate, suggesting effects on respiratory energy metabolism. Ammonia also impaired synaptic function and neuronal integrity sustained by pyruvate. In 10 mM glucose, ammonia inhibited the induction and maintenance of long-term potentiation (LTP) in a concentration-dependent fashion. These inhibitory effects of ammonia were overcome by L-carnitine. DL-APV, an antagonist of NMDA receptors, also diminished the effects of ammonia on ATP levels and LTP induction, indicating that ammonia impairs neuronal function via altered metabolism and untimely NMDA receptor activation. These results suggest that L-carnitine and NMDA receptor antagonists have the potential to preserve neuronal function during hyperammonemia.

Acute vigabatrin retinotoxicity in albino rats depends on light but not GABA.

PURPOSE: Vigabatrin (VGB) is an irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase. Its use as an antiepileptic drug (AED) has been limited because it causes retinal dysfunction, leading to visual field defects (VFDs). We performed this study to identify factors contributing to acute VGB retinotoxicity. METHODS: In ex vivo experiments, Sprague-Dawley rat retinas were isolated and incubated with VGB or GABA in the presence or absence of light. In in vivo experiments, Sprague-Dawley rats were given intraperitoneal injections of VGB and then exposed to light or kept in the dark. The retinas were analyzed histologically by using both light and electron microscopy. RESULTS: Incubating retinas with 50-500 microM VGB under 20,000 Lux white light for < or = 20 h caused a characteristic time- and dose-dependent degeneration limited to the outer retina. Incubating retinas with 500 microM VGB in darkness for 20 h caused no damage. Five hundred micromolar GABA and 50 microM tiagabine were not toxic in the presence or absence of light. Sprague-Dawley rats exposed to an intense white light for 20 h after a 1,000-mg/kg intraperitoneal injection of VGB showed damage in the outer retina, whereas those kept in the dark did not. CONCLUSIONS: Direct exposure of the retina to VGB causes acute retinotoxicity that depends on light exposure rather than GABA accumulation.

Role of ammonia in reversal of glutamate-mediated Müller cell swelling in the rat retina.

Glutamate is thought to participate in a variety of retinal degenerative disorders. However, when exposed to glutamate at concentrations up to 1 mM, ex vivo rat retinas typically exhibit Müller cell swelling, but not excitotoxic neuronal damage. This Müller cell swelling is reversible following glutamate washout, indicating that the glial edema is not required for glutamate-induced neuronal injury. It is unclear whether glutamate directly induces the Müller cell swelling or whether a metabolite of glutamate such as glutamine acts as an osmolyte to generate the cellular edema. To examine this issue, ex vivo rat retinas were exposed to 1 mM glutamate or 1 mM glutamine and were evaluated histologically. Glutamate was also combined with 1 mM ammonia or with methionine sulfoximine (MSO), an inhibitor of glutamine synthetase, the enzyme that catalyzes the synthesis of glutamine from glutamate and ammonia. Glutamate-mediated Müller cell swelling was blocked by co-administration of ammonia and the reversibility of Müller cell swelling was inhibited by MSO administered following glutamate exposure. Glutamine alone failed to induce Müller cell swelling. These results indicate that glutamate-mediated Müller cell swelling is unlikely to result from glutamine accumulation. Rather, conversion of glutamate to glutamine in a reaction involving ammonia helps reverse Müller cell swelling following exposure to exogenous glutamate.