Could Siglec-12 cause cancers to advance?
People who produce this protein may be at twice the risk for advanced cancer
HOBOKEN, NJ—New research, recently published in FASEB BioAdvances, has revealed a human-specific connection between advanced carcinomas and a gene called SIGLEC12. Such cancers often begin in epithelial cells of the skin, or the tissue that covers the surface of internal organs and glands—including prostate, breast, lung, and colorectal cancers.
“Siglecs are family of receptors on immune cells that usually recognize cell surface host molecules called sialoglycans,” says Dr. Nissi Varki, a professor of pathology at the University of California, San Diego School of Medicine. “SIGLEC12 is a gene that has developed two human-specific mutations during the course of evolution. The first SIGLEC12 mutation eliminates its binding to sialoglycans, and the second mutation has caused SIGLEC12 gene to be eliminated in the majority of humans and is only found in about 30 percent of normal humans. As it happens, carcinomas appear rarely in our closest living evolutionary relatives, the chimpanzees, which carry the non-mutated form of SIGLEC12.”
“At some point during human evolution, the SIGLEC12 gene—and more specifically, the Siglec-12 protein it produces as part of the immune system—suffered a mutation that eliminated its ability to distinguish between ‘self’ and invading microbes, so the body needed to get rid of it. But it’s not completely gone from the population—it appears that this dysfunctional form of the Siglec-12 protein went rogue and has now become a liability for the minority of people who still produce it,” explained Dr. Ajit Varki, a distinguished professor at the UC San Diego School of Medicine and Moores Cancer Center, and senior author of the study.
Ajit Varki, who is co-director of both the Glycobiology Research and Training Center and the Center for Academic Research and Training in Anthropogeny, led the study with Nissi Varki, who co-authored the study. By studying normal and cancerous tissue samples, the researchers discovered that the roughly 30 percent of people who still produce Siglec-12 proteins have more than twice the risk of developing an advanced cancer during their lifetimes, compared to those who can’t produce the proteins.
“The presence of the SIGLEC12 gene in individuals with late-stage carcinomas appears to be associated with a poorer prognosis in those individuals, [although] it does not appear to impact the risk of development of carcinomas,” Nissi Varki points out. “We did not find a correlation between SIGLEC12 genomic status and progression for early-stage cancers, but in colorectal cancer cohorts with advanced carcinoma, more than 80 percent of cancer patients had the functional form of the gene, correlated with poor outcome.”
“We also established a method to identify subjects who are Siglec-12 protein-positive vs. negative,” she adds. “Thus, advanced carcinomas appear much more likely to occur in individuals whose genomes have an intact SIGLEC12 gene, likely because the encoded Siglec-12 protein recruits oncogenic pathways.”
The researchers validated their findings in mice by introducing tumor cells engineered to produce Siglec-12. The resulting cancers grew much faster, and turned on many biological pathways known to be involved in advanced cancers, compared to control tumor cells without functioning Siglec-12. The team has also developed a simple urine test that can be used to detect the presence of the dysfunctional protein.
“The Siglec-12 protein was expressed on normal salivary epithelial cells, as well as in normal urinary tract epithelial cells shed into the urine. A simple dot blot method was thus established, using urine samples to identify individuals who were Siglec-12 expressors and non-expressors,” notes Nissi Varki. “Siglec-12 also appears to be of prognostic value, such that patients on the same therapy who were Siglec-12 non-expressors showed longer survival times, as compared to those patients who were Siglec-12 expressers.
“We also performed RNA-Seq analysis on Siglec-12 non-expressing prostate cancer cells and PC-3 cells transfected with Siglec-12 cDNA, identifying multiple cancer promoting changes that are highly enriched in the Siglec-12 expressing cells. Notably, we have also shown in the past that an anti-Siglec-12 antibody attached to a toxin can be taken up by this molecule, to kill cancer cells. Given the likelihood that the molecule has no critical function in normal humans, it is an excellent target for therapeutic consideration as well.”
More work is needed to fully understand SIGLEC12, and the study states that a knockdown in a Siglec-12-expressing cell line will be informative, especially for future studies that expand on these findings.
“These very unexpected findings have raised even more questions than answers, and there are some limitations to this initial study. First of all, we have not deciphered the signaling pathways of SHP2 downstream of Siglec-12 and gene expression analysis with SHP2 inhibitors,” Nissi Varki remarks. “Advanced carcinomas are much more likely to occur in individuals whose genomes have an intact SIGLEC12 gene, likely because the encoded Siglec-12 protein recruits Shp2-related oncogenic pathways.”
“Secondly, we don’t know if there is an alternate ligand for Siglec-12 in humans. Thirdly, although we have very clean RNA-seq data, the in-vivo relevance of the differentially expressed genes has not been conclusively shown. These are important aspects of the work which will be part of future studies. Given additional, less-common inactivating mutations in humans, more precise prognostic studies in future would require complete bi-allelic exome analysis,” she concludes.
