Tissue-specific alternative splicing in the human INK4a/ARF cell cycle regulatory locus

The INK4a/ARF locus on human chromosome 9p resides at the nexus of two crit ical cell cycle regulatory pathways, the p53 pathway and the retinoblastoma (pRb) gene pathway, Through the use of shared coding regions and alternati ve reading frames two distinct proteins are produced: INK4a is a cyclin-dep endent kinase inhibitor whereas ARF binds the MDM2 protooncogene and stabil izes p53. We have examined the expression patterns of the INK4a/ARF locus a t the RNA level in normal human and murine tissues to determine if these ge nes are coordinately regulated. We found that both INK4a and ARF were expre ssed in most tissues at lon levels detectable only by RT-PCR, The pancreas was an exception in that it expressed no detectable ARF mRNA but expressed high levels of INK4a mRNA, Furthermore, human pancreas expressed an additio nal previously unrecognized splice variant of INK4a, termed p12, through th e use of an alternative splice donor site within intron 1. The p12 transcri pt produced a 12 kD protein composed of INK4a exon 1 alpha and a novel intr on-derived C-terminus. This novel protein did not interact with cdk4 but wa s capable of suppressing growth in a pRb-independent manner. The implicatio ns of the capacity of the INK4a/ARF locus to encode a third transcript, and for pancreatic cancer, in which the INK4a/ARF locus is nearly always alter ed, are considered.