The INK4a/ARF locus encodes two tumour suppressor proteins, p16INK4a and p14ARF, which act in the two main cell-cycle control pathways, p16-Rb and p14-p53 respectively. The present study examined the mRNA expression of these genes by reverse transcription-polymerase chain reaction (RT-PCR), and the inactivation mechanisms that alter these levels, in 100 primary breast carcinomas. Furthermore, the interdependence of these mechanisms was examined, since it has been reported that p14ARF is altered in most tumours in concordance with p16INK4a. The results show that promoter hypermethylation, tested by methylation-specific PCR (MSP), was the major mechanism of inactivation of these genes and was present in 31 (31%) and 50 (50%) of the tumours that showed decreased p16INK4a and p14ARF expression, respectively. Hemizygous deletion was the second cause of down-regulation. Homozygous deletion was rare and mutation was absent. In most tumours overexpressing p16INK4a or p14ARF, no detectable inactivation mechanisms were observed. Finally, the results indicate that these proteins are often co-altered in primary breast tumours and that p16INK4a and p14ARF had non-independent behaviour, since they were silenced or overexpressed concomitantly with a significant correlation (p < 0.05).
Copyright 2003 John Wiley & Sons, Ltd.