Tumors with different histological types are rarely seen pathologically in the same organ and are even rarer in RTRs. We report for the first time an RTR with a KRAS-mutated PRNRP and an FGFR3 and KDM6A mutated UC in the ipsilateral kidney. Between 5–6% of multiple ipsilateral renal tumors develop a contralateral metachronous recurrence, with the risk five times that of patients with a sporadic single tumor [15, 16]. Post-transplant tumor management depends on the type and severity of the tumor, with different tumors having different invasiveness, recurrence rates, and prognosis. Treatment options therefore vary, especially when planning for nephron-sparing surgery and active surveillance for some renal masses. For multiple renal masses, close attention should be paid to the different histological subtypes of each mass, and the prognosis of each mass should be assessed separately. Therefore, it is necessary to further understand the type, prognosis, and molecular characteristics of urological tumors in RTRs, in order to customize an appropriate therapeutic regimen for each case.
In 2019, Al-Obaidy and his colleagues  reported 18 cases of papillary renal neoplasm with unique morphological characteristics and named it “PRNRP”. Because PRNRP is rare and only recently defined, it is currently poorly studied and has not been reported in RT patients. This renal tumor exhibits inert biological behavior with no recurrence, metastasis, or tumor-related death, making it important to distinguish PRNRP from other renal cell carcinomas with papillary architecture and eosinophilic cytoplasm. The main morphological characteristics of PRNRP are usually observed as a well-circumscribed and encapsulated lesion with branching papillae of slender fibrovascular axes or tubular papillary structures; a surface that is covered with a monolayer of cubic or columnar cells; abundant eosinophilic cytoplasm; rounded nuclei arranged in the cavity surface away from the basement membrane, “reverse polarity ” characteristics with inconspicuous nucleoli with a low WHO/ISUP nuclear grade; rarely mitotic; and accompanied by hemorrhage and cystic degeneration, but no psammoma. Minor morphological features include edematous and hyalinized papillae, filled with transparent to eosinophilic liquid in which phagocytes can be observed; eosinophilic hobnail cells; intracytoplasmic vacuoles; a peritumoral lymphoid cuff or a small amount of lymphocyte infiltration; foamy histiocyte aggregation; intracellular hemosiderin; and mast cell infiltration in the stroma. These morphologies are typically focal and only present in a small subset of cases [12, 17]. A recent study by Yang  found that among 11 cases of PRNRP, 5 cases had multifocal or patchy necrosis, 6 cases had a small focal invasion of renal parenchyma or a pseudo capsule, and 1 case had renal capsule breakthrough with neural invasion. Although the morphology of these patients showed an invasive growth pattern, all the patients were alive without metastasis or recurrence at the end of the follow-up period. In our case, we also found a small focal invasion of the renal parenchyma. Immunophenotypically, tumor cells usually express EMA, CK7, GATA3, and L1CAM, while CD117 and CAIX are negative. EMA is a recognized molecule that displays cell polarity, with its apical membrane showing enhanced immunostaining and exhibiting polar reversal. However, our experience suggests that immunostaining of MUC1 is better at exhibiting polar reversal than EMA. GATA3 and L1CAM always exhibit diffuse and strong expression, with a recent study of PRNRP showing that GATA3 and L1CAM demonstrated more heterogeneous staining in a pattern of varied intensity (weak to strong) and extent (20% to 100% of the tumor cells) . In our case, diffuse and strong expression of GATA3 was observed. Because our laboratory does not perform L1CAM, as an alternative we performed immunostaining for E-cadherin and Ksp-cadherin and found it to be expressed diffusely in the basolateral membrane, while 34bE12 was diffusely and strongly positive. These findings were consistent with a recent report that positive expressions of E-cadherin and 34bE12 are found in 87.5% (14/16) and 75% (12/16) of PRNRP cases, respectively [13, 20]. The co-expression of GATA3 and 34bE12 is relatively rare in renal cell tumors but is often seen in tumors of distal tubule or collecting duct origin, such as a collecting duct carcinoma [21, 22]and clear cell papillary RCC [23, 24]. The co-expression of GATA3 and 34bE12 in PRNRP may also point to its distal tubule or collecting duct origin. Proximal renal tubular markers such as vimentin, CD10, CD15, and AMACR may be positive in PRNRP, but are usually weak and focal. PRNRP has a high frequency of KRAS missense mutations, with KRAS mutations found in 57-93% of PRNRPs at a total frequency of 85%, with the most common KRAS mutation being p.G12V (54%) [12, 17, 25]. KRAS is therefore one of the important tumor pathogenic genes. Codons 12/13 in exon 2 of the KRAS gene lead to continuous activation of the EGFR signaling pathway that accelerates tumor cell proliferation. PCR in our case detected seven mutation hotspots at codon 12 and codon 13 in exon 2 of the KRAS gene (G12A, G12C, G12D, G12R, G12S, G12V, G13D), as well as mutation hotspots in exon 3 and exon 4 (Q61L, A146X). Our test results showed a G12D mutation in exon 2. In previous studies, 4 hotspot mutations were found in codon 12 in exon 2 (G12C, G12D, G12V, G12R), with an incidence of 80% to 90% [12, 17, 19, 26]. Other studies found that in the absence of the KRAS mutation, other somatic mutations detected by NGS in PRNRP included BRCA2, BRIP1, RAD50, TP53, and BRAF [26,27,28]. In cases of overlapping histology, immunohistochemical staining and KRAS mutational analysis can help make the correct diagnosis.
UC usually (90–95%) occurs in the bladder, but rarely (5–10%) occurs in the renal pelvis/ureter (i.e., UTUC). UC is more common in RTRs than in the general population. Genomic alterations may differ between Western and Chinese UC patients, especially differences in the genetic background and exposure to aristolochic acid in Chinese herbal medicines . In addition, fewer drugs are developed and approved in China for advanced UC, with UTUC patients often suffering from chronic kidney disease, which makes them unsuitable for platinum treatment. It is therefore important to understand and develop therapy for the genomic characteristics of Chinese UTUC patients. In the present case, NGS was performed and detected the S249C mutation in exon 7 of FGFR3, a Q271Ter mutation in exon 10, and an A782Lfs mutation in exon 17 of KDM6A. In contrast to our test findings, Lai et al.  reported that there was no FGFR3 mutation in new UTUC patients after RT, which indicated that the mutation responsible for UTUC in patients after RT may be more complex. The FGFR3 gene encodes fibroblast growth factor receptor 3 and belongs to the family of tyrosine kinase receptors (FGFR1-4). The combination of FGFR3 protein with its cognate ligand fibroblast-growth factor (FGF), leads to receptor dimerization, which subsequently regulates cell proliferation, differentiation, migration, and apoptosis [31, 32]. FGFR3 exists as two isoforms, FGFR3b and FGFR3c. FGFR3b is the predominant form in epithelial cells and derived tumors. NIH-3T3 cells transfected with a mutated form of FGFR3b—FGFR3b-S249C induce cells to transform into spindle cells that have a higher proliferation rate and are tumorigenic in nude mice . Activating point mutations in FGFR3 are observed in up to 70% of bladder cancers, with the S249C mutation being the most common point mutation, accounting for 69% of all mutations . That study also found a relationship between FGFR3 mutations and the stage and grade of UC, with the frequency of FGFR3 mutations decreasing with increasing tumor stage and grade . A recent study also showed that mutations in FGFR3 correlated strongly with the T-cell-depleted phenotype of UTUC. FGFR3 may remodel the immune environment of a UTUC by upregulating interferon response genes to reverse its T-cell-depleted phenotype. The authors of that study also proposed to use of FGFR3 inhibitors in combination with PD-1/PD-L1 inhibitors as a targeted therapeutic strategy to regulate the T-cell-depleted phenotype of UTUC . Erdafitinib is a pan-FGFR tyrosine kinase inhibitor and the first FDA-approved targeted therapy for metastatic urothelial carcinoma (mUC) with FGFR2/3 alterations . One case of advanced UC and mUC after RT has been reported with significant tumor regression and stable graft function after the administration of a combination of PD-1/PD-L1 inhibitors, chemotherapeutics, and immunosuppressants [38, 39]. KDM6A (lysine-specific demethylase 6A) encodes a chromatin-modifying enzyme that mediates transcriptional coactivation by functioning as a dimethylation and trimethylation histone H3 lysine 27 (H3K27) demethylase. Part of this effect is achieved by antagonizing histone lysine N-methyltransferase EZH2, while KDM6A-inactivating mutations may confer sensitivity to EZH2 inhibitors . Compared with normal urothelial samples, the expression level of KDM6A in UTUC specimens was shown to be significantly reduced, while low KDM6A expression was associated significantly with higher tumor grade, shorter cancer-specific and disease-free survival time, suggesting that low expression and downregulation of KDM6A may accelerate the progression of UTUC . Conversely, there is evidence that overexpression and upregulation of KDM6A are associated with poor prognosis in breast cancer and clear cell RCC [42,43,44]. Other studies also reported that mutations in FGFR3 and KDM6A were more common in low-grade UTUC [45,46,47] and were significantly associated with the risk of UTUC recurrence . It is necessary to further investigate the function, pathogenic mechanism, and mutation status of FGFR3 and KDM6A. However, there is currently no consensus on the optimal therapeutic management of UC after transplantation. Physicians are cautious about using immunotherapy, given the transplantation rejection and the safety and efficacy of the new therapeutic regimen, such as checkpoint inhibition therapy or FGFR3 inhibitors. When a death occurs due to tumor progression, immunotherapy is not provided as first-line treatment for patients, but as a “remedial” therapy, which may have important value in clinical management strategies.
The potential mechanisms of cancer development after RT are complex, with the major mechanisms that may be involved including the use of immunosuppressive agents, decreased immune surveillance, and an oncogenic viral infection . The application of immunosuppressants after RT puts the recipient in a state of immunosuppression for a long time. At this time, the cellular immune function of the recipient is severely suppressed, resulting in weakened or damaged immune surveillance function of the body, unable to remove cancerous cells in time. This leads to Viruses, including cancer-causing viruses, increasing the chances of infection and greatly increasing the incidence of tumors. According to animal experiments by Rovira et al , CsA down-regulates T-bet on the surface of CD8 + T cells, resulting in a decline in the immune surveillance of CD8 + T cells on tumor cells, thereby leading to tumor growth. Calcineurin inhibitors (CNIs), such as cyclosporin, raise transforming growth factor (TGF-β) levels which may promote tumor growth . Jiyad et al  systematically analyzed that MMF’s carcinogenic effects. After RT, two types of tumors with distinct shapes occur in the autologous kidney, and the mechanism of occurrence is still unclear. Three inferences were put forward to explain its mechanism: (1) The two tumors may develop adjacently due to the presence of common oncogenic factors in this region, such as immunosuppressants, malignant transformation and changes in the microenvironment, etc. (2) A tumor stimulates the adjacent tissues of the tumor by secreting potential carcinogens, and induces the occurrence of adjacent tumors. (3) Gene mutation is also a potential cause. Network-based analysis of muscle-invasive bladder cancer (MIBC) samples by Kamoun et al.  revealed six categories: luminal papillary (LumP), luminal unspecified, luminal unstable (LumU), stroma-rich, basal/squamous (Ba/Sq) and neuroendocrine-like (NE-like). All luminal subtypes overexpressed markers of urothelial differentiation (FOXA1, GATA3, and PPARG). A study found that the luminal subtype had a higher frequency of KRAS mutations . In this case, no KRAS mutation was detected in UC, but a KRAS mutation was detected in PRNRP. Interestingly, GATA3 expression was present in both UC and PRNRP. GATA3 is expressed in the epithelium of the bladder, ureters, renal pelvis, collecting ducts, distal tubules, and mesangial cells. GATA3 is a very sensitive and specific marker for urothelial carcinomas and their variants . Tong et al.  confirmed that the tissue source of PRNRP is closer to that of the distal tubule epithelium using bioinformatics cluster analysis. The finding by Saleeb et al.  of enrichment of the nuclear receptor transcription signaling pathway in PRNRP, and GATA3 is a component of this signaling pathway. Studies have shown that overexpression of GATA3 was found in KRAS-driven lung cancer cells and further promoted tumorigenesis through microRNA . Whether there are common gene mutations in the pathogenesis of UC and PRNRP, and whether there is an association between GATA3 and KRAS, needs further study.