Components of cyclinD1/cyclin-dependent kinase 4 (CDK4)/p16
INK4a/pRb pathway are the frequent target of many tumor types. We examined the role of retinoblastoma susceptibility gene (
RB1) and the
CDK4 gene in human pituitary tumorigenesis. For the
RB1 gene, pRb expression and loss of heterozygosity (LOH) on 13q in pituitary adenomas were analysed. Immunostaining of pRb revealed lack of expression in 1 of 29 pituitary adenomas. In 4 of 31 pituitary adenomas, allelic imbalances including LOH of
RB1 on 13q14 were detected. Three of 4 pituitary adenomas, in which one adenoma lacked pRb expression, had a common LOH region at least from
D13S219 on 13q12.3-q13 to
D13S265 on 13q31-32. Interphase fluorescence
in situ hybridization with a probe of
RB1 showed 2 copies of
RB1 gene suggesting that mitotic recombination events, not deletion or chromosome loss, led to LOH in the 3 pituitary adenomas analyzed. All 27 exons, intron-exon boundaries, and essential promoter region of
RB1 gene were then sequenced in genomic DNA from 4 pituitary adenomas with allelic imbalance on 13q14 including one adenoma without pRb expression and 3 adenomas with pRb expression. Any somatic mutations, insertions, or microdeletions in the
RB1 gene were not detected in 4 pituitary adenomas. Methylation sensitive (MS)-polymerase chain reaction (PCR) and bisulfite sequencing analysis revealed hypomethylated status of CpG islands in the promoter region of the
RB1 genes of 4 pituitary adenomas. In addition, activating mutations of
CDK4 gene, which is a component of cyclinD1/CDK4/p16
INK4a/pRb pathway, were not detected in 31 pituitary adenomas. Based on these results, it is concluded that somatic mutations of the
RB1 gene or
CDK4 gene do not appear to play a major role in pituitary tumorigenesis. This supports the presence of potential tumor suppressor gene(s) on 13q12.3-q13 to 13q31-32 in pituitary adenomas.
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