To the best of our knowledge, this is the first report of a somatic KMT2C c.5605 T > C mutation associated with AORT. AORT is a very rare tumor with a poor prognosis and fewer than 100 cases have been reported in the literature [7]. The most common symptom is painless scrotal mass and 19% of patients present with symptoms related to metastasis, such as hip or back pain. As a result of the atypical symptoms and aggressive features of AORT, many patients have developed distant metastases prior to diagnosis and 3- and 5-year disease-free survival rates are only 49% and 13%, respectively. Patients with a tumor diameter < 5 cm have better survival rates than patients with larger tumors [8, 9].

As regards the current case, the chief complaint of "left testis swelling for 16 years” is unlikely to be compatible with this aggressive AORT tumor. Pathomorphological examination and negative expression of β-hCG, CD30, CD117 and AFP completely excluded germ cell tumors, such as teratoma, and we guess that this patient has had a benign disease for many years, such as hydrocele of tunica vaginalis, that also exhibits testis swelling, but has not sought medical help for reasons such as privacy or embarrassment. Regrettably, the opportunity to confirm the cause of this chief complaint no longer exists following the patient’s death.

The pathological and immunohistochemical features of AORT have not been established due to the extremely rare incidence of the disease. Most cases show a mixed morphology of papillary, glandular and solid tumors. AE1/AE3 is expressed by 96% of AORT patients, epithelial membrane antigen (EMA) by 84%, CK 7 by 83% and vimentin by 81%. Malignant mesothelioma (MM) shares overlapping morphology and immunohistochemical markers (such as WT-1, CR, CK 5/6 and paired box 8 [PAX8]) with AORT and represents the most significant differential diagnosis [7]. However, unlike MM, AORT is mainly located on the testicular hilum and is continuous with the normal rete testis epithelium (Fig. 1d). In addition, AORT has more slit-like branched tubules and greater nuclear pleomorphism than MM, in which these features are rare. Electron microscopy can be used as a final technology to distinguish the two diseases since AORT has fewer short, blunt microvilli than MM [10, 11].

The diagnosis of AORT is generally based on the modified criteria first described by Nochomovitz and Orenstein [12] in 1984: (1) the tumor is located in the testicular hilum; (2) the microscopic morphology differs from that of other testicular or paratesticular tumors; (3) there is no histologically similar primary tumor outside the scrotum and (4) other possible tumors, such as MM, should be excluded by immunohistochemical analyses. Other tumors, such as MM, germ cell tumors and sex cord-stromal tumors, were ruled out in the present case as far as possible, based on pathomorphological and immunohistochemical analyses. The tumor was finally diagnosed as AORT.

Orchiectomy currently represents the primary treatment modality for AORT, but retroperitoneal lymph node dissection remains under consideration. In a retrospective study of 78 patients conducted by Khaleel I. Al-Obaidy [7], patients who underwent retroperitoneal lymph-node dissection survived slightly longer than those who did not, but a statistically significant difference was not achieved (p = 0.2). In addition, no statistically significant differences in survival between patients who received adjuvant chemotherapy and those who did not were found (p = 0.6), although Shunsuke Owa et al. [13] reported that a patient with metastatic AORT did not progress for 7 months and survived for 20 months after 7 cycles of platinum-based combined chemotherapy. Most previous reports indicate poor responses to traditional chemotherapy by metastatic cases [14]. Consistent with these findings, the current patient developed multiple metastases to the lung and thoracic vertebra during the two cycles of platinum-based combined chemotherapy and showed resistance to second- and third-line chemotherapy regimens.

The mechanisms whereby AORT achieves resistance to chemotherapy have not previously been described. NGS of the current patient’s tumor tissue demonstrated a somatic c.5605 T > C mutation in KMT2C gene exon 36, with an abundance of 49.27%. The association of KMT2C with cancer was first discovered through tumor gene-sequencing studies which revealed KMT2C deletions as the most common chromosomal abnormality in acute myeloid leukemia. Thus, a tumor suppressor role was suspected for KMT2C [15, 16]. The TCGA-BRCA study has also shown KMT2C to be one of the most frequently mutated genes in ER-positive breast cancer and patients with low KMT2C expression had longer overall survival (OS) [17].

In addition, mutations in KMT2C are also involved in the genetic mechanism of primary resistance to chemotherapy and may be a potential chemosensitivity biomarker in childhood acute myeloid leukemia [18]. Breast cancer patients with somatic KMT2C mutations had worse OS and disease-free survival (DFS) than those with wild-type KMT2C. However, patients with low KMT2C expression had much longer DFS than those with high expression if they received doxorubicin, cisplatin, and carboplatin chemotherapy [19]. Soo-Jeong Cho et al. demonstrated that KMT2C knockout promoted the epithelial-to-mesenchymal transition (EMT) and induced chemoresistance in diffuse-type gastric adenocarcinomas and KMT2C re-expression reversed chemotherapy resistance via promotion of DNA damage and apoptosis [20]. However, the c.5605 T > C mutation is located in a sequence encoding an intrinsically disordered peptide region and its effect on KMT2C function is unclear. We speculate that the c.5605 T > C mutation in KMT2C gene exon 36 may result in inactivation of the protein product and contribute to primary chemoresistance in the current patient.

Recent studies have suggested a role for KMT2C in DNA damage repair (DDR). Functional deficiency of the KMT2C protein predicted a good response to Poly-(ADP)-ribose polymerase 1/2 (PARP1/2) inhibitor therapy. Rampias et al. [21] indicated that bladder cancer cells with low levels of KMT2C expression have heightened sensitivity to PARP1/2 inhibitor therapy. Chang et al. further proved that KMT2C promoted DDR through direct recruitment to DNA damage sites and that KMT2C mutations sensitized non-small- cell lung cancer (NSCLC) to PARP1/2 inhibitors by disrupting homologous recombination (HR)-mediated DNA repair [22]. Hence, we look forward to more evidence supporting the administration of PARP1/2 inhibitors in cases of tumors with KMT2C mutations.

In summary, the current case demonstrates a potential correlation between primary multi-drug resistance AORT and KMT2C mutation. Further studies are needed to prove this association and explore the efficacy of PARP1/2 inhibitors for tumors with KMT2C mutations.