NEW YORK (GenomeWeb) – A team of researchers in California has uncovered a microRNA that inhibits the growth of tumors with KRAS mutations.
KRAS mutations — which are found in 30 percent of non-small cell lung cancers and in 40 percent to 50 percent of colorectal cancers — are linked to poor survival and chemotherapy resistance. Using synthetic microRNAs, the University of California, San Diego's Tariq Rana and his colleagues screened KRAS mutant cancer cells for synthetic lethal interactions. As they reported today in Cancer Research, they found that miR-1298 inhibited the growth of KRAS-mutated cells, both in vitro and in vivo.
"For decades researchers have tried to directly inhibit KRAS activity, but there are no well-defined binding pockets in the protein that we can target with small-molecule drugs," Rana said in a statement. "Instead of trying to deter KRAS itself, we took the approach of looking for other molecules that, when inhibited, are lethal to cells only when KRAS is also mutated."
They further reported that miR-1298 works by targeting the tyrosine kinase FAK and the laminin subunit LAMB3, which itself could be a prognostic biomarker or a therapeutic target.
For this study, Rana and his colleagues screened a library containing some 850 miRNA mimics in an isogenic colon cancer cell line with wild-type KRAS and one with mutant KRAS, and assayed the viability of those cells. Three miRNA mimics — hsa-miR-618, hsa-miR-1298, and hsa-354 miR-512-5p — affected the viability of cancer cells with KRAS mutations. The researchers then tested these miRNA mimics in a panel of four NSCLC lines with wild-type KRAS and five with mutant KRAS, and found that miR-1298 exhibited selective lethality in all the KRAS-mutated cell lines.
Through a combination of microarray and miR-TRAP analyses, the researchers homed in on focal adhesion kinase (FAK), laminin b3 subunit (LAMB3), plakophilin-4 (PKP4), and E3 ubiquitin (WWP1) as targets of miR-1298. They knocked down these targets in mutated KRAS cell lines using siRNAs, and found that the downregulation of FAK and LAMB3 affected those cells' viability while the downregulation of PKP4 and WWP1 did not.
In addition, the team noted that miR-1298 significantly and selectively decreased FAK and LAMB3 mRNA levels in KRAS-mutated cells, and found that co-expression of FAK and LAMB3 rescued KRAS-mutated cells that had been treated with miR-1298. At the same time, they uncovered putative miR-1298 binding sites on both FAK and LAMB3 mRNA. Rana and his colleagues also reported that LAMB3 is a downstream transcriptional target of KRAS, while FAK is not a direct target.
In a cohort of 259 lung cancer patients with and without a KRAS transcriptional signature, Rana and his colleagues examined the expression levels of FAK and LAMB3. From this, they found that high LAMB3 expression was associated with poorer overall survival in patients with the KRAS transcriptional signature. FAK expression had no prognostic value, however, and neither did LAMA3 or LAMC2, the other subunits of the laminin 332 heterotrimer.
"This clinical finding suggests LAMB3 could be used as a prognostic biomarker, and underscores LAMB3's potential as a therapeutic target for KRAS-driven cancers," Rana added. "What's more, it highlights miRNAs as important tools for probing complex biological processes, identifying new therapeutic targets and developing potential new RNA-based therapeutics."