Specificity of the methylation-suppressed A isoform of candidate tumor suppressor RASSF1 for microtubule hyperstabilization is determined by cell death inducer C19ORF5.

Isoform-specific epigenetic silencing of RASSF1A (3p21.3) by promoter-specific CpG island hypermethylation occurs at high frequency in human tumors, whereas the closely related product of the same gene, RASSF1C, continues to be expressed. Both isoforms in isolation exhibit tumor suppressor properties and we show here similar cellular locations on mitochondria and microtubules, paclitaxel-like microtubule hyperstabilization, disruption of mitosis, and interaction with C19ORF5. We show both have identical but distinct sequence domains for microtubule association and hyperstabilization. C19ORF5 is a hyperstabilized microtubule-specific binding protein of which accumulation causes mitochondrial aggregation and cell death. We report herein that when A or C isoforms of RASSF1 are coexpressed with C19ORF5, the unique N-terminal sequence of RASSF1C prevents it from hyperstabilizing microtubules. This confers specificity on RASSF1A in microtubule hyperstabilization and accumulation of C19ORF5 on microtubules and could underlie a specific effect of hypermethylation-suppressed RASSF1A in tumor suppression.