Proteostasis failure and mitochondrial dysfunction contribute to chromosomal instability-induced microcephaly.

Mosaic variegated aneuploidy (MVA), a rare human congenital disorder that causes microcephaly, is characterized by extensive abnormalities in chromosome number and results from mutations in genes involved in accurate mitotic chromosome segregation. To characterize the cellular mechanisms underlying this disease, here we generated a Drosophila model of microcephaly caused by the depletion of a single spindle assembly checkpoint (SAC) gene in the neural stem cell (NSC) compartment. We present evidence that loss of stemness - compromised identity and proliferative capacity of NSCs- plays an important role in MVA and results in a reduced number of neurons and glial cells. We show that loss of stemness arises from the accumulation over time of an unbalanced number of gains and losses of more than one chromosome, rather than a direct consequence of chromosomal instability-induced DNA damage or the production of simple aneuploidies. We unravel a contribution of proteostasis failure and mitochondrial dysfunction to the negative impact of complex aneuploidies on stemness, a highly energy demanding cellular state. We identify overexpression of Radical Oxygen Species scavengers, mitochondria chaperones and apoptosis inhibition as genetic interventions capable of dampening the deleterious effects of aneuploidy on brain size.