Stem Cell Therapies and Radiological Imaging in Ataxia Telangiectasia

Abstract

Ataxia telangiectasia (A-T) is an autosomal recessive disorder associated with earlychildhood ataxia, neurodegeneration, immune deficiency, radiosensitivity, sterility, and anincreased susceptibility to cancer. In spite of numerous years of research, there is still nocurative treatment for A-T, nor a reliable animal model that recapitulates the prominentneurological phenotypes observed in patients. To address these deficiencies and investigatepotential therapeutic approaches I have attempted to generate a universal donor pluripotentstem cell line, utilized radiological imaging to investigate a novel rat model of A-T, andtested whether bone marrow transplantation (BMT) could provide benefit in the treatment ofhematological defects associated with A-T.Cells derived from pluripotent stem cell populations may provide a novel approach to therapyin this disease. Universal embryonic stem cell donor lines would greatly facilitate stem cellbased regenerative medicine and permit facile testing of human pluripotent stem cell derivedgrafts in xenogeneic settings. Because HLA-G overcomes immune cell mediated attack offoetal tissues during pregnancy and inhibits T-cell responses and dendritic cell antigenmaturation in vitro and in vivo, it is an attractive candidate molecule for achieving this goal.Here we investigated whether enforced expression of either the soluble or the membrane boundform of the HLA-G mouse homologue, H2-Bl, in human and mouse ES cell lines would allowengraftment in immunocompetent mice. Despite evidence for robust expression of soluble ormembrane bound H2-Bl molecules and effective inhibition of CD8+ T-cell proliferation by allH2-Bl engineered ES cell lines, all failed to generate teratomas in immunocompetent mice,despite doing so in NOD-SCID mice. We conclude that expression of H2-Bl in human andmouse embryonic stem cells alone is insufficient to overcome xenogeneic rejection.Animal models are likely to play a key role in the development of therapeutic strategies for thetreatment of A-T. While multiple mouse models of A-T currently exist and recapitulate someof the phenotypes attributed to the human disease, prominent neurodegeneration has not beenobserved in these animal. As such, our research group has produced a gene knock-out modelin the rat, utilizing Zinc Finger Nuclease technology. Approximately 50% of the Atm -/- ratsshow dilated blood vessels in the eyes, microgliosis, and develop hind limb paralysis,suggesting this model may recapitulate some of the neurological defect observed in A-T patients. Magnetic resonance imaging (MRI) was utilized to assess structural changes in thebrain and spinal cord. Surprisingly, no significant differences in cerebellar volume orgray/white matter ratios were observed in the brain or spinal cord of healthy animals.However, lymphomas were detected in the T12-L3 region of the spine in all of theparalysed animals tested. I utilized PET/MRI in conjunction with the radiotracer(18F)PBR111, which is highly selective for the TSPO receptor of activated microglia in thebrain, to assess microglial activation in vivo in 5 month old control and ATM null non nparalysed rats, three of each. No differences were observed in the cerebellum of theserats relative to WT rats, as well as multiple other brain regions assessed in healthyanimals. These data provide evidence that paralysis in the A-T rat model is associatedwith the presence of lymphomas within the spinal column.The Atm -/- rat has proven to be a reliable model for some of the hematopoietic phenotypesobserved in A-T, including the presence of aberrant T-cell development, as well as anincreased frequency in development of lymphoid malignancies. To test whether delivery ofwild type (WT) hematopoietic stem cell populations could provide therapeutic benefits, Iperformed BMT of WT bone marrow into Atm -/- rats. Importantly, I found bone marrowtransplantations significantly delayed the onset of lymphoma within the recipients.These data reveal that despite the fact that the rat recapitulates some aspect of A-T, such aslymphomagenesis, it does not reliably phenocopy the human disease in terms of cerebellardegeneration. Utilizing advanced imaging technologies such as those employed in thisproject, could provide a valuable model to investigate potential therapeutic approaches fortreatment in A-T, such as BMT as well as human pluripotent stem cell based therapies.