Background: Long interspersed nucleotide element 1 (LINE-1) hypomethylation is suggested to play a role in the progression of colorectal cancer (CRC). To assess intra-patient heterogeneity of LINE-1 methylation in CRC and to understand its biological relevance in invasion and metastasis, we evaluated the LINE-1 methylation at multiple tumor sites. In addition, the influence of stromal cell content on the measurement of LINE-1 methylation in tumor tissue was analyzed.
Methods: Formalin-fixed paraffin-embedded primary tumor tissue was obtained from 48 CRC patients. Matched adjacent normal colon tissue, lymph node metastases and distant metastases were obtained from 12, 18 and 7 of these patients, respectively. Three different areas were microdissected from each primary tumor and included the tumor center and invasive front. Normal mucosal and stromal cells were also microdissected for comparison with the tumor cells. The microdissected samples were compared in LINE-1 methylation level measured by multicolor MethyLight assay. The assay results were also compared between microdissected and macrodissected tissue samples.
Results: LINE-1 methylation within primary tumors showed no significant intra-tumoral heterogeneity, with the tumor center and invasive front showing identical methylation levels. Moreover, no difference in LINE-1 methylation was observed between the primary tumor and lymph node and distant metastases from the same patient. Tumor cells showed significantly less LINE-1 methylation compared to adjacent stromal and normal mucosal epithelial cells. Consequently, LINE-1 methylation was significantly lower in microdissected samples compared to macrodissected samples. A trend for less LINE-1 methylation was also observed in more advanced stages of CRC.
Conclusions: LINE-1 methylation shows little intra-patient tumor heterogeneity, indicating the suitability of its use for molecular diagnosis in CRC. The methylation is relatively stable during CRC progression, leading us to propose a new concept for the association between LINE-1 methylation and disease stage.