Exploring The Relationship Between Autophagy and Mitochondrial Dysfunction In a Mouse Model Of MDS

Although myelodysplastic syndrome (MDS) is uncommon in children, with an annual estimated incidence of 0.5 to 4 cases per million children compared to 20 to 40 cases per million in adults, the incidence of MDS appears to be increasing in the United States with the improvements in survival rates of childhood cancers, at least in part because the intensive chemo- and radiotherapy regimens that cure pediatric patients with cancer may also cause therapy-related MDS. Unlike in other blood-related cancers, the disease-causing mutations in pediatric MDS and t-MDS/AML are unknown. Nevertheless, recent studies have suggested that the mitochondrial dysfunction associated with MDS results from the inability of hematopoietic stem/progenitor cells to appropriately cope with mitochondrial damage andmitochondrial dysfunction appears to be an early event in the development of t-MDS/AML.

Autophagy is a catabolic process involved in the sequestration and breakdown of cellular components, including organelles such as mitochondria and long-lived or misfolded proteins. By sequestering and degrading depolarized fragments of mitochondria that result from cycles of mitochondrial fusion and fission, autophagy plays a critical role in the maintenance of healthy pools of mitochondria. Given the importance of autophagy in mitochondrial quality control and increasing evidence that mitochondrial dysfunction plays a role in the pathogenesis of MDS, we sought to explore the role of autophagy in the pathogenesis of MDS associated with mitochondrial dysfunction using the previously established PolgA^mt/mt mouse model (a.k.a. mtDNA mutator mice).

The mtDNA mutator mice express a proof-reading-deficient mutant of mtDNA polymerase (PolgA-D257A), which results in increased mitochondrial mutation frequencies in multiple tissues, and a decline in respiratory function of mitochondria-encoded complexes. The delayed onset of an MDS-like phenotype in these mice provides a window of time to interrogate the role of autophagy in the progression of disease symptoms. Disruption of autophagy in erythroid progenitors did not cause a significant decrease in hemoglobin or red blood cell count in PolgA^wt/wt mice, however, it accelerated the onset of anemia and morbidity significantly in PolgA^mt/mt mice, suggesting that autophagy delays the onset of symptoms associated with pathologic mtDNA mutations. Flow cytometric evaluation of peripheral blood of PolgA^mt/mt mice and wild-type littermates revealed an unexpected defect in RBC maturation in PolgA^mt/mt animals, which became apparent with age or following serial phlebotomy. The defect was similar to that observed in autophagy-defective Ulk1-deficient mice suggesting that pathogenic mtDNA mutations may cause an age-dependent delay in autophagy. Transformed PolgA^mt/mt murine embryonic fibroblasts (MEFs) with mitochondrial dysfunction also showed a defect in basal LC3 conversion and p62 accumulation, consistent with impaired flux through the autophagy pathway. Together, our data indicates that although autophagy initially combats accumulation of dysfunctional mitochondria resulting from an increased mtDNA mutation burden, the latter eventually leads to a decline in basal autophagy and impaired clearance of mitochondria, which contributes to the decline in mitochondrial function and progression of MDS-like disease in mice.