Homozygous codeletion and differential decreased expression of p15(INK4b), p16(INK4a)-α and p16(INK4a)-β in mouse lung tumor cells

The genes of murine cyclin D-dependent kinase inhibitors, p15(INK4b) and p16(INK4a), are located in a region of chromosome 4 where overlapping deletions were found in lung adenocarcinomas. The p16(INK4a) gene uniquely consists of alternative first exons (E1α and E1β), which are spliced to exon 2 in alternative reading frames to either encode p16(INK4a) (α form) or another potential tumor suppressor, pl9(ARF) (β form). We examined 99 lung adenocarcinomas of C3H/HeJ x A/J F₁(C3AF₁) and A/J x C3H/HeJ F₁(AC3F₁) mouse hybrids and 18 (13 metastatic, 5 nonmetastatic) tumorigenic mouse lung epithelial cell lines for p15(INK4b) p16(INK4a) gene inactivation. Homozygous codeletion occurred in eight of the 13 (62%) metastatic, four of the five (80%) nonmetastatic cell lines, but in only six of 99 (6%) adenocarcinomas. Neither p15(INK4b) nor p16(INK4a) gene was individually deleted in any of the tumors or cell lines, and all deletions of the pl6(INK4a) gene extended into exon 2, which would be expected to disrupt the functions of both p16(INK4a) and p19(ARF). We also detected no intragenic mutations of either gene in 44 tumors that displayed loss of heterozygosity at the p16(INK4a) locus or in any of the cell lines. Transcript levels of p16(INK4a)-α, p16(INK4a)-β and p15(INK4b) also were examined in each of the cell lines that retained copies of these genes. Whereas an immortal mouse lung epithelial cell line (E10) and two metastatic tumor cell lines (LM1 and E9) expressed p16(INK4a)-β and p15(INK4b) mRNA, the a transcript of p16(INK4a) was detected in only the LM1 cell line. These results suggest that both p15(INK4b) and p16(INK4a) (α and β) are targets of inactivation in mouse lung tumorigenesis.