Histone H2A ubiquitination resulting from Brap loss of function connects multiple aging hallmarks and accelerates neurodegeneration

Abstract

Aging is an intricate process that is characterized by multiple hallmarks including stem cell exhaustion, genome instability, epigenome alteration, impaired proteostasis, and cellular senescence. While each of these is detrimental at the cellular level, it remains unclear how they are interconnected to cause systemic organ deterioration. Here we show that abrogating Brap, a BRCA1 associated protein, results in cellular senescence with persistent DNA double-strand breaks and elevation of histone H2A mono- and poly-ubiquitination (H2Aub). The high H2Aub initiates proteasome-dependent histone proteolysis, leading to global epigenetic alteration, ubiquitinated protein accumulation, and senescence reinforcement. When these defects occur in mice carrying Brap deletions in cerebral cortical neural progenitors or postnatal neurons, they accelerate brain aging, induce neurodegeneration, and shorten lifespan. As we show H2Aub is also increased in human brain tissues of Alzheimer disease, these data together suggest that chromatin aberrations mediated by H2Aub act as a nexus of multiple aging hallmarks.

Nde1 is Required for Heterochromatin Compaction and Stability in Neocortical Neurons

“…Consistent with γH2A.X, 53BP1 and Brca1, the two essential molecules for DSB repair, were also upregulated in Nde1 -/and Nde1 -/-p53 -/than in WT and p53 -/cortical tissues (Figure 5C). Moreover, IB of cortical histone extracts not only confirmed the elevation of γH2A.X in the cortical tissue of Nde1 -/and Nde1 -/-p53 -/mice but also showed that the increased γH2A.X in Nde1 -/and Nde1 -/-p53 -/mice were predominantly penta-ubiquitinated (Figure 5D, E ), a characteristic DNA damage response that we observed in Brap cKO mice and postmortem brains of patients with Alzheimer's disease (Guo et al, 2020). These data together demonstrated that Nde1 LOF increased DSBs in adult cortical neurons.…”

“…Moreover, IB of cortical histone extracts not only confirmed the elevation of γH2A.X in the cortical tissue of Nde1 -/- and Nde1 -/- p53 -/- mice but also showed that the increased γH2A.X in Nde1 -/- and Nde1 -/- p53 -/- mice were predominantly penta-ubiquitinated (Figure 5D, E). The dual PTM of phosphorylation and ubiquitination of histone H2A.X is a characteristic DNA damage response that we observed in Brap cKO mice and postmortem brains of patients with Alzheimer’s disease(Guo et al, 2020). Therefore, heterochromatin aberrations resulting from Nde1 LOF can lead to genome instability and DSBs in adult cortical neurons with age-dependent progression and pathological impacts.…”

“…However, nuclear architecture aberration and DSBs were specifically observed in neocortical neurons and we have not seen tumor-like brain phenotypes after analyzing over a hundred Nde1 -/and Nde1 -/-p53 -/mice, though some mutant mice that died between 3 and 12 months showed tumors outside of the CNS. Meanwhile, our study of Brap cKO mice revealed a significant lifespan shortening along with accelerated cellular senescence and neurodegeneration (Guo et al, 2020). Given the synergistic function of Nde1 with Brap and persistent DSBs in cortical neurons of Nde1 -/-, Nde1 -/-p53 -/-, and Brap cKO brains, we next sought to determine whether Nde1 LOF shares the phenotypes of Brap LOF and progeria syndromes by examining their common histone modifications.…”

“…The multiple aging promoting phenotypes resulting from Brap LOF was initiated by increased mono-and poly-ubiquitination of histone H2A including on sites other than K119. Increased H2Aub in Brap cKO cortices starts from early development, accumulates overtime, and leads to a substantial backlog of poly-ubiquitinated proteins in the cortical tissue even though proteasome remained fully active (Guo et al, 2020). In the cortical tissue of Nde1 -/mice, a change in H2Aub was not evident in embryonic and neonatal development, yet it was significantly elevated along with penta-ubiquitinated γH2A.X at ages older than 3-4 months (Figure 6A, 5E).…”

“…Thus, the aging and HGPS associated H4K20me3 and H2Aub are likely to reflect a compensatory mechanism of coping with recurrent and irreparable DSBs in the heterochromatic repetitive DNA. These histone PTM changes condense heterochromatin and may also cause secondary epigenetic and proteomic alterations, leading to premature brain aging as observed in Brap cKO mice (Guo et al, 2020).…”