Astrocyte-induced DNA damage as a mechanism of motor neuron death in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that affects motor neurons in the
brain and spinal cord, leading to paralysis and death. 95% of ALS cases are classed as sporadic and
5% occur due to an inherited mutation in an ALS-linked gene, the most common of which are the
C9ORF72 and SOD1 genes. The cause of motor neuron degeneration remains unclear, however,
several studies have shown increased DNA damage and astrocyte toxicity as consistent features of
sporadic and familial ALS. ALS astrocyte toxicity is transmitted through secreted factors, but the
mechanisms and molecules involved remain elusive. It was hypothesised that astrocyte-induced
DNA damage could contribute to motor neuron death in ALS.

To test this, induced astrocytes (iAstrocytes), which retain hallmarks of ageing, were obtained from
healthy controls, and patients with sporadic ALS, SOD1-ALS or C9ORF72-ALS. Increased DNA
damage and impaired DNA repair was observed in specific ALS astrocyte lines but was not
consistently altered in any genetic subgroup, indicating some ALS astrocytes may be affected by the
same cell-autonomous DNA damage observed in motor neurons. Conditioned medium derived
from C9ORF72-ALS and sporadic ALS astrocytes, but not SOD1-ALS astrocytes, was found to induce
an increase in DNA damage and an impairment in DNA damage signalling in healthy iPSC-derived
motor neurons. The mechanism for astrocyte-induced DNA damage remains unclear. C9-ALS
astrocytes were found to express and secrete dipeptide repeat proteins, which have previously
been shown to induce DNA damage. ALS astrocyte conditioned medium was also found to induce
p62 aggregation, which may lead to DNA damage and repair impairment, however further work is
needed to verify this. In summary, our work identifies a new mechanism by which ALS astrocytes
may induce cell death in motor neurons and suggests that DNA damage signalling or p62 may be a
potential therapeutic target.