Impact of tissue processing, archiving and enrichment techniques on DNA methylation yield in rectal carcinoma.

Background: Formalin fixation, duration of tissue storage and tissue enrichment techniques can affect DNA methylation yield but these effects have not been quantitatively measured. The aim is to investigate the relative impact of these conditions on DNA methylation in rectal cancer.

Methods: 10 rectal cancers with matched undissected fresh frozen tissues, laser capture microdissected (LCM) formalin-fixed paraffin-embedded (FFPE) tissues, manual macrodissected FFPE tissues, adjacent normal mucosa and stromal tissues were analysed for APC and LINE-1 methylation using bisulphite pyrosequencing.

Results: FFPE cancer tissues, which had been stored for at least 4 years showed similar APC and LINE-1 methylation changes to matched fresh frozen cancer tissues. Laser capture microdissection did not increase the degree of methylation detected compared to manual macrodissection. Analysis of stromal tissues showed that they had undergone significant methylation changes compared to adjacent macroscopically normal mucosa, but not to the same extent as cancer tissues.

Conclusion: Reliable DNA methylation results can be obtained from FFPE rectal cancer tissues, which have been in long-term storage. Because only minor differences in methylation between macrodissected and LCM cancer tissues were found, our results do not support the routine use of LCM to enrich for cancer cells for DNA methylation studies.