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Severe acute respiratory syndrome (SARS) coronavirus‑2 (SARS‑CoV‑2), the causative viral agent for the ongoing COVID‑19 pandemic, enters its host cells primarily via the binding of the SARS‑CoV‑2 spike (S) proteins to the angiotensin‑converting enzyme 2 (ACE2). A number of other cell entry mediators have also been identified, including neuropilin‑1 (NRP1) and transmembrane protease serine 2 (TMPRSS2). More recently, it has been demonstrated that transmembrane protease serine 4 (TMPRSS4) along with TMPRSS2 activate the SARS‑CoV‑2 S proteins, and enhance the viral infection of human small intestinal enterocytes. To date, a systematic analysis of TMPRSS4 in health and disease is lacking. In the present study, using in silico tools, the gene expression and genetic alteration of TMPRSS4 were analysed across numerous tumours and compared to controls. The observations were also expanded to the level of the central nervous system (CNS). The findings revealed that TMPRSS4 was overexpressed in 11 types of cancer, including lung adenocarcinoma, lung squamous cell carcinoma, cervical squamous cell carcinoma, thyroid carcinoma, ovarian cancer, cancer of the rectum, pancreatic cancer, colon and stomach adenocarcinoma, uterine carcinosarcoma and uterine corpus endometrial carcinoma, whilst it was significantly downregulated in kidney carcinomas, acute myeloid leukaemia, skin cutaneous melanoma and testicular germ cell tumours. Finally, a high TMPRSS4 expression was documented in the olfactory tubercle, paraolfactory gyrus and frontal operculum, all brain regions which are associated with the sense of smell and taste. Collectively, these data suggest that TMPRSS4 may play a role in COVID‑19 symptomatology as another SARS‑CoV‑2 host cell entry mediator responsible for the tropism of this coronavirus both in the periphery and the CNS.

Severe acute respiratory syndrome (SARS) coronavirus‑2 (SARS‑CoV‑2) enters into human host cells via mechanisms facilitated mostly by angiotensin‑converting enzyme 2 (ACE2) and transmembrane protease serine 2 (TMPRSS2). New loss of smell (anosmia/hyposmia) is now recognized as a COVID‑19 related symptom, which may be caused by SARS‑CoV‑2 infection and damage of the olfactory receptor (OR) cells in the nasal neuro‑epithelium and/or central involvement of the olfactory bulb. ORs are also expressed peripherally (e.g., in tissues of the gastrointestinal and respiratory systems) and it is possible that their local functions could also be impaired by SARS‑CoV‑2 infection of these tissues. Using Gene Expression Profiling Interactive Analysis, The Cancer Genome Atlas, Genotype‑Tissue Expression, cBioPortal and Shiny Methylation Analysis Resource Tool, we highlight the expression of peripheral ORs in both healthy and malignant tissues, and describe their co‑expression with key mediators of SARS‑CoV‑2 infection, such as ACE2 and TMPRSS2, as well as cathepsin L (CTSL; another cellular protease mediating SARS‑CoV‑2 infection of host cells). A wide expression profile of peripheral ORs was noted, particularly in tissues such as the prostate, testis, thyroid, brain, liver, kidney and bladder, as well as tissues with known involvement in cardio‑metabolic disease (e.g., the adipose tissue, pancreas and heart). Among these, OR51E2, in particular, was significantly upregulated in prostate adenocarcinoma (PRAD) and co‑expressed primarily with TMPRSS2. Functional networks of this OR were further analysed using the GeneMANIA interactive tool, showing that OR51E2 interacts with a plethora of genes related to the prostate. Further in vitro and clinical studies are clearly required to elucidate the role of ORs, both at the olfactory level and the periphery, in the context of COVID‑19.