CDKN2A: two tumour-suppressor programs in one locus
Function
The CDKN2A locus encodes distinct proteins through alternative reading frames. p16INK4a inhibits CDK4/6 to keep RB growth control engaged; p14ARF helps stabilize p53 by opposing MDM2.
CDKN2A programs active
Damaged or stressed cells face senescence, arrest, or p53-linked safeguards.
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Promoter/locus disabled
RB and/or p53 safeguards weaken, making inappropriate proliferation easier.
Deletion, mutation, and promoter methylation can disable this locus in cancer. In long-standing UC, methylation can be a field-change marker, but a methylated read does not prove both products are lost or that a clone will become malignant.
Evidence across the UC field
An early colectomy study sampled 89 sites from three patients with long-standing extensive UC. p16INK4a promoter hypermethylation was detected in 12.7% of nondysplastic samples, 70% of dysplastic samples, and every sampled carcinoma (Hsieh, Klump, Holzmann, Borchard, Gregor, and Porschen, 1998). The gradient is consistent with an early field alteration, but the patient count was only three and multiple sites from one colon are not independent patients.
The biology is also stage-dependent: regenerative UC crypts can induce p16 as a stress-associated “emergency brake” before epigenetic loss occurs. That makes protein interpretation non-monotonic—high p16 may mark stress or senescence, whereas promoter-silenced clones may later lose the safeguard. Exact promoter and isoform resolution is essential.