New Δ246p53 isoform responds to DNA damage to enhance p53 functions in senescence and tumour suppression

ABSTRACT p53 is with little doubt one of the most powerful genes in our genome, as it makes growth vs arrest, repair vs replacement, catabolism vs anabolism, life vs death decisions in the cell. An alteration or malfunction in p53 may lead to cancer, degeneration or premature ageing. So, it is not surprising that p53 is also one of the most complex and tightly regulated genes, encoding for at least 10 RNA variants and 12 widely accepted protein forms. Here we identify one new p53 protein isoform of approximately 18 kDa that we termed Δ246p53. Δ246p53 is translated from an alternative translation initiation site (TIS) in codon 246. TIS-246 is preceded by a strong Kozak sequence and appears conserved in vertebrates, from sea lamprey to humans. Δ246p53’s origin and expression in cells were confirmed by frameshift and start codon mutations; five different antibodies against several epitopes from its N-terminus down to its C-terminus and one against the DNA binding domain region just upstream of its initiator methionine working as negative control; as well as siRNAs and an antisense oligo targeting TIS-246, which knocked-down Δ246p53 with little or no effect on full-length (FL) p53 protein levels. Δ246p53 was induced by DNA damage and triggered senescence and impaired tumour formation/growth in colony formation assays. Mechanistically, Δ246p53 interacted with FLp53, leading to a decrease in the expression of downstream target genes HDM2 and p21 . Concurrently, it also interacted with ΔNp63, leading to p53 -independent p63 -mediated activation of p21 , a known senescence activator. Our results unveil a new naturally occurring and tightly controlled factor with specific functions in p21 regulation and senescence. Future studies on Δ246p53 are likely to help us better understand and control the processes of tumour suppression and ageing.