Small mitochondrial Arf (smArf) protein corrects p53-independent developmental defects of Arf tumor suppressor-deficient mice

The mouse p19(Arf) (human p14(ARF)) tumor suppressor protein, encoded in part from an alternative reading frame of the Ink4a (Cdkn2a) gene, inhibits the Mdm2 E3 ubiquitin ligase to activate p53. Arf is not expressed in most normal tissues of young mice but is induced by high thresholds of aberrant hyperproliferative signals, thereby activating p53 in incipient tumor cells that have experienced oncogene activation. The single Arf mRNA encodes two distinct polypeptides, including full-length p19(Arf) and N-terminally truncated and unstable p15(smArf) (small mitochondrial Arf) initiated from an internal in-frame AUG codon specifying methionine-45. Interactions of p19(Arf) with Mdm2, or separately with nucleophosmin (NPM, B23) that localizes and stabilizes p19(Arf) within the nucleolus, require p19(Arf) N-terminal amino acids that are not present within p15(smArf). We have generated mice that produce either smARF alone or M45A-mutated (smArf-deficient) full-length p19(Arf) proteins. BCR-ABL-expressing pro/pre-B cells producing smArf alone are as oncogenic as their Arf-null counterparts in generating acute lymphoblastic leukemia when infused into unconditioned syngeneic mice. In contrast, smArf-deficient cells from mice of the Arf(M45A) strain are as resistant as wild-type Arf(+/+) cells to comparable oncogenic challenge and do not produce tumors. Apart from being prone to tumor development, Arf-null mice are blind, and their male germ cells exhibit defects in meiotic maturation and sperm production. Although ArfM45A mice manifest the latter defects, smArf alone remarkably rescues both of these p53-independent developmental phenotypes.