Ctso, encoding cathepsin O, is a cysteine proteinase from the papain superfamily. The gene is composed of eight coding exons and seven introns, spanning more than 30 kb, and maps to chromosome 4q31-q32 [6]. Cathepsin O is likely involved in proteolytic processes, though the exact biological pathways it is intricately part of are still being explored.

Ctso has been implicated in multiple disease conditions. In Prader-Willi syndrome, Ctso along with other proteins showed differential expression in liver steatosis, with levels increasing from grade 1 to grade 3, potentially serving as a biomarker for this condition [1]. In obesity-related liver steatosis in adolescents, Ctso was identified as one of the circulating proteins that could be a biomarker for steatosis [3]. In breast cancer, a single-nucleotide polymorphism (SNP) near the Ctso gene was associated with a poor prognosis in hormone receptor-positive breast cancer patients receiving adjuvant tamoxifen therapy. Ctso was also found to reduce BRCA1 protein levels, and variant Ctso SNP genotypes were associated with resistance to tamoxifen, suggesting it could be a biomarker for drug selection in breast cancer treatment [4,5]. Additionally, Ctso was among the genes identified as a signature cancer-associated fibroblast-related gene in breast cancer, which may be used to assess prognosis and guide individualized treatment [7]. There was also an unadjusted low P-value indication of a positive correlation between Ctso and breast cancer in a Mendelian randomization study, though more research is needed to establish a causal relationship [2].

In conclusion, Ctso, as a cysteine proteinase-encoding gene, is associated with various disease states, particularly liver steatosis in syndromic and non-syndromic obesity, and breast cancer. Its role in these diseases, especially in breast cancer prognosis and drug response, shows the potential of Ctso-related research to provide new insights into disease mechanisms and treatment strategies.