Keratin 20 is a specific marker of submicroscopic lymph node metastases incolorectal cancer: validation by K-RAS mutations

Lymph node status has major prognostic importance in colorectal cancer and greater precision in the diagnosis of lymph node metastases should provide better prognostic and therapeutic guidance. Keratin 20 (K20) gene expressio n has been used as a marker of lymph node metastases, but the evidence for this remains circumstantial. This study has therefore sought to determine K 20 specificity and to correlate K20 expression with mutant K-RAS expression , in order to provide direct evidence that K20 expression in lymph nodes of colorectal cancer patients genuinely reflects metastatic disease. Specific ity of K20 expression was established against a range of tissue types and 2 89 lymph nodes from 41 non-cancer control patients. K20 expression was rest ricted to gastrointestinal epithelia and was only present in one of the 289 control lymph nodes, giving a calculated specificity of 97.6% (95% confide nce limits: 87.1-99.9%). Forty-two tumour samples were analysed for the pre sence of K-RAS codon 12 gene mutations using a RT-PCR mutant allele-specifi c amplification (MASA) technique. Thirteen tumours (31%) had codon 12 mutat ions detected by MASA and these were further analysed to determine the exac t nature of the mutation. MASA was then used to screen the lymph nodes from these patients for the presence of the tumour-specific K-RAS transcript an d the results were compared with K20 RT-PCR and histopathology from the sam e samples. Whilst K-RAS MASA was not as sensitive as K20 RT-PCR, there was substantial agreement between the assays. There were no K20-negative lymph nodes which were found to be K-RAS MASA-positive, whereas seven nodes in fo ur patients were K20-positive and K-RAS-negative, in keeping with the diffe rences in assay sensitivity. These results further validate K20 as a marker by providing greater certainty that what is being detected represents occu lt metastatic disease. Copyright (C) 2000 John Wiley & Sons, Ltd.