Project Summary
Friedreich’s ataxia (FRDA) is a multi-systemic autosomal recessive disorder that is predominantly caused by a
homozygous GAA repeat expansion mutation within the first intron of the frataxin (FXN) gene leading to a
decrease of protein expression. Frataxin is a mitochondrial protein involved in iron metabolism. FRDA is
characterized by ataxia, neurodegeneration, muscle weakness, and cardiomyopathy. There is no treatment for
this lethal disease. We tested a new therapy for this disease consisting of wildtype (WT) hematopoietic stem
and progenitor cell (HSPC) transplantation in the Y8GR mouse model of FRDA. This therapy worked beyond
our expectation in FRDA completely correcting the neurologic, muscular and cardiac complications after a single
infusion of HSPCs in lethally irradiated Y8GR mice. We optimized an autologous CRISPR/Cas9-mediated gene
correction HSPC approach for FRDA and we are now conducting the investigational New Drug-enabling studies
for the clinical translation of this approach. Addressing the mechanism of action, we previously showed that
tissue rescue was at least partly mediated by transfer of frataxin from HSPC-derived microglia-like cells to
diseased neurons. The exact mechanism of rescue including how microglial replacement and frataxin transfer
from microglia-like cells to neurons contributes to this rescue are still unknown. Very few studies investigated
the role of microglia in the pathogenesis of FRDA. We believe that the impressive response of the FXN-
expressing HSPC transplant is due to, not only the transfer of FXN into neurons, but also to the rescue of the
microglial phenotype. The central hypothesis is that these findings in the mouse model are translatable such
that CRISPR-editing of the GAA repeat expansion in HSPCs in FRDA will lead to both functional rescue of
microglia-like cells and transfer of FTX to neurons. Applying a novel cohort of induced pluripotent stem cells from
FRDA patients, controls, and importantly lines created with the proposed gene editing, we find a dramatic
phenotype in FRDA microglia that is corrected by editing. We will utilize novel in vitro and in vivo methods
including patient-derived organoids (mini-brains) and a murine xenotransplantation model of human microglia to
ascertain the microglial contribution to FRDA pathogenesis, the potential for gene editing to correct this
phenotype, and the mechanisms and functional ramifications of FTX transfer to neurons from microglia. . This
project is significant because it provides a human specific platform for disease modeling and validating novel
therapeutic approaches and provides supporting evidence that CRISPR HPSC transplantation can be used to
treat FRDA. It will also advance the understanding of microglia-neuron interactions, and open new perspectives
for the treatment of neurodegenerative diseases due to mitochondrial dysfunction.