Few organisms are as well understood at the genetic level as Saccharomyces cerevisiae. Given that there are more yeast geneticists than yeast genes and exemplary resources for the community (largely a result of their size), this comes as no surprise. What is curious is the large number of yeast genes for which we’ve been unable to characterize. Of the ~6000 genes currently identified in the yeast genome, 1253 have no verified function (for the uninclined, this is roughly 21% of the yeast proteome). Egads! If we can’t figure this out in yeast, what hope do we have in non-model organisms?Lourdes Peña-Castillo and Timothy R. Hughes discuss this curious observation and its cause in their report in Genetics.
A recent paper “Targeted gene deletion in Candida parapsilosis demonstrates the role of secreted lipase in virulence”, from the Nosanchuk lab at Yeshiva University, shows the role of secreted lipases in virulence of this pathogen. C. parapsilosis is second only to the evolutionarily closely related commensal Candida albicans as worldwide cause of invasive candidiasis. This paper demonstrates a knockout system using selectable marker which confers resistance to the drug Nourseothricin. The authors sought to delete the adjacent and convergently-transcribed lipase genes CpLIP1 and CpLIP2 and characterize the phenotype of the lipase deficient mutants as blood-borne C. parapsilosis infections are in a lipid rich environment.
Through a series of experiments testing growth in rich media, media with olive-oil, and in infection models they showed that the importance of lipase activity. The knockout strain was unable to grow efficiently on YNB media+olive oil indicating that these two genes are the only ones capable of lipase activity. The murine infection experiments indicated that the knockout could be cleared in 4 days while the WT and reconstituted were cleared in 7. The authors acknowledge some limitations in the infection model in that it does not fully recapitulate an invasive candidiasis because mice were infected intravenously so the role of endothelial cell invasion was tested in vivo.
This is not the first paper on targeted gene knockouts in this fungus. A paper from earlier this summer, “Development of a gene knockout system in Candida parapsilosis reveals a conserved role for BCR1 in biofilm formation”, from Geraldine Butler’s group at University College group, who work on both evolutionary and pathogenesis questions in Candida species, developed a knockout system using the same drug marker. The Butler laboratory also showed that the C. parapsilosis MAT locus, part of the sexual reproduction machinery of fungi, has degraded, consistent with the observed asexuality of these species.
The improving genetic tools for targeted disruption of loci in additional species is permitting experiments that get at the heart of what makes some fungi pathogenic. With the genome sequence of many of the relatives of the pathogens we can systematically dissect what genetic differences have a role in virulence. It will be interesting to reconstruct whether the ancestor of many of these Candida spp always had the potential for virulence or if it co-evolved with its human or other mammalian commensal lifestyle.