BJ-HCC-20, a potential novel cancer-testis antigen

Cancer/testis antigens: promising immunotherapy targets for digestive tract cancers

Digestive tract cancers, including esophageal, gastric, and colorectal cancers, are the major cause of death among cancer patients worldwide due to the heterogeneity of cancer cells, which limits the effectiveness of traditional treatment methods. Immunotherapy represents a promising treatment strategy for improving the prognosis of patients with digestive tract cancers. However, the clinical application of this approach is limited by the absence of optimal targets. Cancer/testis antigens are characterized by low or absent expression in normal tissues, but high expression in tumor tissues, making them an attractive target for antitumor immunotherapy. Recent preclinical trials have shown promising results for cancer/testis antigen-targeted immunotherapy in digestive cancer. However, practical problems and difficulties in clinical application remain. This review presents a comprehensive analysis of cancer/testis antigens in digestive tract cancers, covering their expression, function, and potential as an immunotherapy target. Additionally, the current state of cancer/testis antigens in digestive tract cancer immunotherapy is discussed, and we predict that these antigens hold great promise as an avenue for breakthroughs in the treatment of digestive tract cancers.

The therapeutic potential value of Cancer-testis antigens in immunotherapy of gastric cancer

Gastric cancer (GC) is the fourth most common cause of mortality and the fifth for incidence, globally. Diagnosis, early prognosis, and therapy remains challenging for this condition, and new tumor-associated antigens are required for its detection and immunotherapy. Cancer-testis antigens (CTAs) are a subfamily of tumor-associated antigens (TAAs) that have been identified as potential biomarkers and targets for cancer immunotherapy. The CTAs-restricted expression pattern in tumor cells and their potential immunogenicity identify them as attractive target candidates in CTA-based diagnosis or prognosis or immunotherapy. To date, numerous studies have reported the dysregulation of CTAs in GC. Several clinical trials have been done to assess CTA-based immunotherapeutic potential in the treatment of GC patients. NY-ESO-1, MAGE, and KK-LC-1 have been used in GC clinical trials. We review recent studies that have investigated the potential of the CTAs in GC regarding the expression, function, aggressive phenotype, prognosis, and immunological responses as well as their possible clinical significance as immunotherapeutic targets with a focus on challenges and future interventions.

Deficiency of cancer/testis antigen gene CT55 causes male infertility in humans and mice

The Cancer/Testis Antigen (CTA) genes comprise a group of genes whose expression under physiological conditions is restricted to the testis but is activated in many human cancers. Depending on the particular expression pattern, the CTA genes are speculated to play a role in spermatogenesis, but evidence is limited thus far. Here, we reported patients with a hemizygous nonsense mutation in cancer-testis antigen 55 (CT55) suffering from male infertility with extreme disruption in sperm production, morphology, and locomotion. Specifically, the insufficiency of sperm individualization, excessive residue of unnecessary cytoplasm, and defects in acrosome development were evident in the spermatozoa of the patients. Furthermore, mouse models with depletion of Ct55 showed accelerated infertility with age, mimicking the defects in sperm individualization, unnecessary cytoplasm removal, and meanwhile exhibiting the disrupted cumulus-oocyte complex penetration. Mechanistically, our functional experiments uncovered CT55 as a new autophagic manipulator to regulate spermatogenesis via selectively interacting with LAMP2 and GABARAP (which are key regulators in the autophagy process) and further fine-tuning their expression. Therefore, our findings revealed CT55 as a novel CTA gene involved in spermatogenesis due to its unprecedented autophagy activity.

Cancer/Testis Antigen 55 is required for cancer cell proliferation and mitochondrial DNA maintenance

Cancer/Testis Antigens (CTAs) represent a group of proteins whose expression under physiological conditions is restricted to testis but activated in many human cancers. Also, it was observed that co-expression of multiple CTAs worsens the patient prognosis. Five CTAs were reported acting in mitochondria and we recently reported 147 transcripts encoded by 67 CTAs encoding for proteins potentially targeted to mitochondria. Among them, we identified the two isoforms encoded by CT55 for whom the function is poorly understood. First, we found that patients with tumors expressing wild-type CT55 are associated with poor survival. Moreover, CT55 silencing decreases dramatically cell proliferation. Second, to investigate the role of CT55 on mitochondria, we first show that CT55 is localized to both mitochondria and endoplasmic reticulum (ER) due to the presence of an ambiguous N-terminal targeting signal. Then, we show that CT55 silencing decreases mtDNA copy number and delays mtDNA recovery after an acute depletion. Moreover, demethylation of CT55 promotor increases its expression, which in turn increases mtDNA copy number. Finally, we measured the mtDNA copy number in NCI-60 cell lines and screened for genes whose expression is strongly correlated to mtDNA amount. We identified CT55 as the second highest correlated hit. Also, we show that compared to siRNA scrambled control (siCtrl) treatment, CT55 specific siRNA (siCT55) treatment down-regulates aerobic respiration, indicating that CT55 sustains mitochondrial respiration. Altogether, these data show for first time that CT55 acts on mtDNA copy number, modulates mitochondrial activity to sustain cancer cell proliferation.

Cancer testis antigens and genomic instability: More than immunology

Cancer testis antigens or genes (CTA, CTG) are predominantly expressed in adult testes while silenced in most or all somatic tissues with sporadic expression in many human cancers. Concerted misexpression of numerous CTA/CTGs is rarely observed. This finding argues against the germ cell theory of cancer. A surprising number of CTA/CTGs are involved in meiotic chromosome metabolism and specifically in meiotic recombination. Recent discoveries with a group of CTGs established that their misexpression in somatic cells results in genomic instability by interfering with homologous recombination (HR), a DNA repair pathway for complex DNA damage such as DNA double-stranded breaks, interstrand crosslinks, and single-stranded DNA gaps. HR-deficient tumors have specific vulnerabilities and show synthetic lethality with inhibition of polyADP-ribose polymerase, opening the possibility that expression of CTA/CTGs that result in an HR-defect could be used as an additional biomarker for HR status. Here, we review the repertoire of CTA/CTGs focusing on a cohort that functions in meiotic chromosome metabolism by interrogating relevant cancer databases and discussing recent discoveries.

Cancer testis antigen 55 deficiency attenuates colitis-associated colorectal cancer by inhibiting NF-κB signaling

Colitis-associated cancer (CAC), a prototype of inflammation-associated cancer, is one of the most common gastrointestinal tumors. As a potential cancer testis antigen (CT antigen), cancer testis antigen 55 (CT55) is expressed in different tumors and normal testes. However, its role in CAC remains unknown. Here, we identified CT55 as a new potent promoter of CAC. We discovered that Ct55 deficiency alleviated inflammatory responses, decreased cell proliferation and colitis-associated tumorigenesis in an azoxymethane/dextran sulfate sodium (AOM/DSS) mouse model. Mechanistically, CT55 acts as an accelerator of tumor necrosis factor (TNF)-α-induced nuclear factor-κB (NF-κB) signaling. Upon stimulation with TNF-α, CT55 interacts with the IκB kinase (IKK) complex, which increases the phosphorylation of IKKα/β and activates IKK–p65 signaling, while knockout of CT55 blocks IKK–p65 signaling. Notably, inhibition of IKK abolished the positive effect of CT55 on NF-κB activation. Collectively, our findings strongly indicate that CT55 deficiency suppresses the development of CAC and that the CT55-TNF-α-induced NF-κB axis may represent a promising target for CAC therapy.

CXorf48 is a potential therapeutic target for achieving treatment-free remission in CML patients

Although the introduction of tyrosine kinase inhibitors (TKIs) has improved overall survival of patients with chronic myeloid leukemia (CML), about half of the patients eventually relapse after cessation of TKIs. In contrast, the remainder of the patients maintain molecular remission without TKIs, indicating that the patients' immune system could control proliferation of TKI-resistant leukemic stem cells (LSCs). However, the precise mechanism of immunity against CML-LSCs is not fully understood. We have identified a novel immune target, CXorf48, expressed in LSCs of CML patients. Cytotoxic T cells (CTLs) induced by the epitope peptide derived from CXorf48 recognized CD34⁺CD38^- cells obtained from the bone marrow of CML patients. We detected CXorf48-specific CTLs in the peripheral blood mononuclear cells from CML patients who have discontinued imatinib after maintaining complete molecular remission for more than 2 years. Significantly, the relapse rate of CXorf48-specific CTL-negative patients was 63.6%, compared to 0% in CXorf48-specific CTL-positive patients. These results indicate that CXorf48 could be a promising therapeutic target of LSCs for immunotherapy to obtain durable treatment-free remission in CML patients.

Novel BRCA2-interacting protein BJ-HCC-20A inhibits the induction of apoptosis in response to DNA damage

The major hereditary breast cancer susceptibility gene BRCA2 is associated with familial breast and ovarian cancer. BRCA2 plays a role in DNA repair, transcription, cell cycle regulation, maintenance of genomic stability in response to DNA damage, centrosome regulation, and cytokinesis. To further understand the function of BRCA2, we used a yeast two-hybrid method and identified a novel BRCA2-interacting protein, BJ-HCC-20A, which is reported to be a potential cancer-testis antigen. We confirmed the interaction between endogenous BJ-HCC-20A and BRCA2 in mammalian cells, and showed that BJ-HCC-20A interacts with a portion of the highly conserved region of BRCA2 in various mammals, and M phase-specific phosphorylation of the binding region of BRCA2 modulates BJ-HCC-20A binding. Overexpression of BJ-HCC-20A increases cell growth, and downregulation of endogenous BJ-HCC-20A expression using small interfering RNA suppresses cell growth and leads to the induction of apoptosis. Importantly, the BJ-HCC-20A mRNA level is downregulated by adriamycin (ADR)-induced DNA damage and depletion of BJ-HCC-20A expression by small interfering RNA promotes the reduction of BRCA2 expression and enhances cell apoptosis in response to DNA damage. Additionally, the recovery of BJ-HCC-20A expression in ADR-induced DNA damage inhibits ADR-induced apoptosis. The data suggest that BJ-HCC-20A promotes cell growth and may regulate the induction of cell apoptosis in response to DNA damage in cooperation with BRCA2 in an M phase-dependent manner. Therefore, we speculate that targeting BJ-HCC-20A may aid in the treatment of breast tumors.

Developing effective tumor vaccines: basis, challenges and perspectives

A remarkable advance in tumor immunology during the last decade is the elucidation of the antigenic basis of tumor recognition and destruction. A variety of tumor antigens have been identified using several strategies including conventional experiments and newly developed bioinformatics. Among these antigens, cancer/testis antigen (CT antigen) is considered to be the most promising target for immunotherapy by vaccination. Successful immunotherapy of tumors requires understanding of the natural relationship between the immune system and tumor in the status of differentiation, invasion and maturation. Continued progress in development of effective cancer vaccines depends on the identification of appropriate target antigens, the establishment of optimal immunization strategies without harmful autoimmune responses and the ability of manipulating tumor microenvironment to circumvent immune suppression and to augment the anti-tumor immune response.