Implications of histone H2AX abundance in breast cancer

Histones are responsible for the packaging of DNA into a eukaryotic cell nucleus, and the H2A variant H2AX plays an important role in the DNA damage response (DDR). The phosphorylated form of H2AX, known as γH2AX, has been suggested as a prognostic cancer biomarker although it is unclear how variations in H2AFX copy number, mRNA and protein abundance contribute to genome instability. Analysis of copy number alterations (CNAs) in large human cancer datasets has revealed that H2AFX seldom undergoes amplification or homozygous deletions and is rarely mutated. In contrast, loss of one copy of H2AFX is very common and 41% of samples in the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) dataset exhibit loss of one H2AFX allele. This loss is most frequent in the hormone receptor positive luminal B subtype (LumB, 63%), which is characterised by rapid cell proliferation, genome instability and overall poor prognosis. No homozygous deletions or mutations of H2AFX are observed in LumB tumours and only 2% of samples show H2AFX amplifications, suggesting that H2AFX could be essential. Four cell lines which represent H2AFX copy number status and luminal subtypes were selected from the Cancer Cell Line Encyclopaedia (CCLE) to investigate the effects of loss of one H2AFX allele. This revealed that H2AFX CNAs have little impact on transcript or protein abundance. Measurement of γH2AX foci formation and disappearance after ionising irradiation show that H2AX is rapidly phosphorylated after damage and repair is facilitated without correlation to the molecular subtype or H2AFX gene dosage. In summary, we propose that H2AFX is essential in breast cancer but that loss of one H2AFX allele is well tolerated and leaves near-normal abundance of mRNA and protein. This suggests a regulatory mechanism to maintain H2AX abundance and a robust DDR irrespective of H2AFX copy number or breast cancer subtype.2021-02-1