Evolution of p53 in hypoxia-stressedSpalaxmimics human tumor mutation

Abstract

The tumor suppressor gene p53 controls cellular response to a variety of stress conditions, including DNA damage and hypoxia, leading to growth arrest and͞or apoptosis. Inactivation of p53, found in 40 -50% of human cancers, confers selective advantage under hypoxic microenvironment during tumor progression. The mole rat, Spalax, spends its entire life cycle underground at decidedly lower oxygen tensions than any other mammal studied. Because a wide range of respiratory adaptations to hypoxic stress evolved in Spalax, we speculated that it might also have developed hypoxia adaptation mechanisms analogous to the genetic͞epige-netic alterations acquired during tumor progression. Comparing Spalax with human and mouse p53 revealed an arginine (R) to lysine (K) substitution in Spalax (Arg-174 in human) in the DNAbinding domain, identical to known tumor associated mutations. Multiple p53 sequence alignments with 41 additional species confirmed that Arg-174 is highly conserved. Reporter assays uncovered that Spalax p53 protein is unable to induce apoptosisregulating target genes, resulting in no expression of apaf1 and partial expression of puma, pten, and noxa. However, cell cycle arrest and p53 stabilization͞homeostasis genes were overactivated by Spalax p53. Lys-174 was found critical for apaf1 expression inactivation. A DNA-free p53 structure model predicts that Arg-174 is important for dimerization, whereas Spalax Lys-174 prevents such interactions. Similar neighboring mutations found in human tumors favor growth arrest rather than apoptosis. We hypothesize that, in an analogy with human tumor progression, Spalax underwent remarkable adaptive p53 evolution during 40 million years of underground hypoxic life.

P53 in blind subterranean mole rats – loss-of-function versus gain-of-function activities on newly cloned Spalax target genes

“…Concerning mdm2 expression activation, Spalax p53 exhibited a gain-of-function activity on both human and Spalax mdm2 intronic regions. The pattern of expression of both p53-target genes corroborates our previous results (Ashur-Fabian et al, 2004).…”

“…This observation, in fact, triggered our interest in studying Spalax p53 in an effort to use this non-tumoral, hypoxia-adaptive model to study evolutionary changes in p53, analogous to acquired tumoral p53 mutations. Indeed, cloning of Spalax p53 (Ashur-Fabian et al, 2004;Avivi et al, 2005a) revealed two Spalax-specific substitutions in the p53 DNA-binding domain, exchanging arginines (R) for lysine (K) at codons corresponding to position 174 and 209 in humans. Both amino-acid changes are identical to known human tumor-associated mutations.…”

p53 in mitochondria enhances the accuracy of DNA synthesis

“…Notably, arginine 174 is not located in or near the DNA-binding site. 33 Irradiation (IR) induces the mitochondrial translocation of p53 in various cell lines (e.g., MCF-7 and HCT116) that are highly resistant to apoptosis. 6 Mitochondrial p53 comprised small amount (B2%) of the total induced p53.…”

Evolutionary biology of cancer

“…tumors often involves a specific amino acid change in the p53 tumor suppressor gene during somatic evolution, a molecular adaptation that has evolved convergently in the p53 gene of hypoxia-stressed Spalax mole rats during evolution at the level of populations [21].…”