Mediators of receptor tyrosine kinase activation in infantile fibrosarcoma: a Children's Oncology Group study

In silico mutagenesis-based designing of oncogenic SHP2 peptide to inhibit cancer progression

Cancer is among the top causes of death, accounting for an estimated 9.6 million deaths in 2018, it appeared that approximately 500,000 people die from cancer in the United States alone annually. The SHP2 plays a major role in regulation of cell growth, proliferation, and differentiation, and functional upregulation of this enzyme is linked to oncogenesis and developmental disorders. SHP2 activity has been linked to several cancer types for which no drugs are currently available. In our study, we aimed to design peptide inhibitors against the SHP2 mutant. The crystal structure of the human Src SH2-PQpYEEIPI peptide mutant was downloaded from the protein databank. We generated several peptides from the native wild peptide using an in silico mutagenesis method, which showed that changes (P302W, Y304F, E306Q, and Q303A) might boost the peptide's affinity for binding to SHP2. Furthermore, the dynamical stability and binding affinities of the mutated peptide were confirmed using Molecular dynamics simulation and Molecular Mechanics with Generalized Born and Surface Area Solvation free energy calculations. The proposed substitution greatly enhanced the binding affinity at the residue level, according to a study that decomposed energy into its component residues. Our proposed peptide may prevent the spread of cancer by inhibiting SHP2, according to our detailed analyses of binding affinities.

Infantile Fibrosarcoma and Other Spindle Cell Neoplasms of Infancy. A Review of Morphologically Overlapping yet Molecularly Distinctive Entities

BACKGROUND

Regardless of age at presentation, many soft tissue neoplasms have overlapping histopathologic and immunophenotypic features to serve as a diagnostic challenge.

CASE REPORT

We reported a case of a spindle cell neoplasm in an infant, which was initially considered a vascular anomaly clinically and an eventual biopsy revealed marked inflammation with a spindle cell component that was resolved as an infantile fibrosarcoma with an ETV6 break-apart.

CONCLUSION

The context of this case lead to a further consideration of various other spindle cell neoplasms arising predominantly in the soft tissues during the infancy period as defined by the first two years of age. Though sharing similar morphologic features, these tumors can be categorized into several molecular genetic groups, which have provided both diagnostic and pathogenetic insights as well as treatment options in some cases.

Novel BRAF gene fusions and activating point mutations in spindle cell sarcomas with histologic overlap with infantile fibrosarcoma

Infantile fibrosarcoma (IFS)/cellular congenital mesoblastic nephroma (cCMN) commonly harbors the classic ETV6-NTRK3 translocation. However, there are recent reports of mesenchymal tumors with IFS-like morphology harboring fusions of other receptor tyrosine kinases or downstream effectors, including NTRK1/2/3, MET, RET, and RAF1 fusions as well as one prior series with BRAF fusions. Discovery of these additional molecular drivers contributes to a more integrated diagnostic approach and presents important targets for therapy. Here we report the clinicopathologic and molecular features of 14 BRAF-altered tumors, of which 5 had BRAF point mutations and 10 harbored one or more BRAF fusions. Of the BRAF fusion-positive tumors, one harbored two BRAF fusions (FOXN3-BRAF, TRIP11-BRAF) and another harbored three unique alternative splice variants of EPB41L2-BRAF. Tumors occurred in ten males and four females, aged from birth to 32 years (median 6 months). Twelve were soft tissue based; two were visceral including one located in the kidney (cCMN). All neoplasms demonstrated ovoid to short spindle cells most frequently arranged haphazardly or in intersecting fascicles, often with collagenized stroma and a chronic inflammatory infiltrate. No specific immunophenotype was observed; expression of CD34, S100, and SMA was variable. To date, this is the largest cohort of BRAF-altered spindle cell neoplasms with IFS-like morphology, including not only seven novel BRAF fusion partners but also the first description of oncogenic BRAF point mutations in these tumors.

GAB1-ABL1 fusions in tumors that have histologic overlap with NTRK-rearranged spindle cell tumors

Fibroblastic spindle cell tumors are a heterogeneous group of rare soft tissue tumors that are increasingly recognized as associated with a variety of kinase gene fusions. We report two cases of GAB1-ABL1 fusions in spindle cell tumors that histologically overlap with neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell tumors. The first case occurred in a 76-year-old female who had a large deep-seated spindle cell tumor composed of monotonous ovoid to spindle cells in a background of thick stromal collagen bands with prominent hyalinized vessels and inconspicuous mitoses (<1/10 HPF). Immunohistochemical stains showed co-expression of S100 and CD34. A GAB1-ABL1 fusion was detected by whole transcriptome RNA sequencing. The patient had a partial response to imatinib. The second case was previously described as a solitary fibrous tumor, occurring in a 9-year-old female with a cellular spindle cell tumor with patchy CD34 immunoexpression but no expression of S100. Upon clinicopathologic re-review, including anchored multiplex next-generation sequencing, a GAB1-ABL1 fusion was identified. In summary, we report the first two cases of spindle cell tumors with variable expression of CD34 and/or S100, driven by GAB1-ABL1 gene fusions with histologic overlap with NTRK-rearranged spindle cell tumors, suggesting that ABL-fusions may also be oncogenic drivers within this spectrum of tumors. These cases highlight the evolving understanding of fibroblastic spindle cell tumor biology and the utility of sequencing in identifying a targetable alteration.

Silencing SAPCD2 Represses Proliferation and Lung Metastasis of Fibrosarcoma by Activating Hippo Signaling Pathway

The primary problem associated with fibrosarcoma is its high potential to metastasize to the lung. Aberrant expression of SAPCD2 has been widely reported to be implicated in the progression and metastasis in multiple cancer types. However, the clinical significance and biological roles of SAPCD2 in fibrosarcoma remain unknown. Here, we reported that SAPCD2 expression was markedly elevated in fibrosarcoma tissues, and its expression was differentially upregulated in fibrosarcoma cell lines compared with that in several primary fibroblast cell lines. Kaplan-Meier survival analysis revealed that SAPCD2 overexpression was significantly correlated with early progression and metastasis, and poor prognosis in fibrosarcoma patients. Our results further showed that silencing SAPCD2 inhibited the proliferation and increased the apoptosis of fibrosarcoma cells in vitro. Importantly, silencing SAPCD2 repressed lung metastasis of fibrosarcoma cells in vivo. Mechanistic investigation further demonstrated that silencing SAPCD2 inhibited the proliferation and lung metastasis of fibrosarcoma cells by activating the Hippo signaling pathway, as evidenced by the finding that constitutively active YAP1, YAP1-S127A, significantly reversed the inhibitory effect of SAPCD2 downregulation on the colony formation and anchorage-independent growth capabilities of fibrosarcoma cells, as well as the stimulatory effect on the apoptotic ratio of fibrosarcoma cells. In conclusion, SAPCD2 promotes the proliferation and lung metastasis of fibrosarcoma cells by regulating the activity of Hippo signaling, and this mechanism represents a potential therapeutic target for the treatment of lung metastatic fibrosarcoma.

Cervical Infantile Fibrosarcoma: a rare cause of paediatric parapharyngeal neck mass

Soft tissue tumors are not uncommon in childhood and comprise entities that range from common to very rare malignancies. Infantile fibrosarcoma (IFS) is a rare pediatric malignancy mainly seen in the first two years of life. The data about the incidence of infantile fibrosarcoma occurring in the neck in the Indian subcontinent is scarce. To the best of our knowledge, only one case of infant cervical IFS has been reported previously in the Indian subcontinent. We present another case of an eight-year-old male patient with a rapidly growing mass on the left side of the neck. He was successfully treated with a combined modality of surgery and chemotherapy with a good outcome. Among the soft tissue tumors of childhood, IFS is a rare entity. It has a good prognosis and lesser chance of distant metastasis as compared to adult fibrosarcoma. Though surgical excision is the mainstay of treatment, chemotherapy also has a significant role in the treatment of primary tumor and metastasis. We discuss the stated case to bring to the notice this uncommon cause, which can be considered as a differential diagnosis of upper cervical swellings. A better understanding of this entity would help in early diagnosis and aggressive treatment, reducing the overall morbidity and mortality.

NTRK3 overexpression in undifferentiated sarcomas with YWHAE and BCOR genetic alterations

The BCOR family of tumors includes a number of undifferentiated sarcomas, occurring in various age groups and anatomic sites, characterized by a spindle and round cell phenotype and diffuse immunoreactivity for BCOR. Prior RNA sequencing data revealed that NTRK3 was a top-upregulated gene in BCOR-CCNB3 sarcomas. In this study, we investigate a large cohort of tumors harboring BCOR/YWHAE genetic alterations for NTRK3 upregulation at both the mRNA and protein levels, compared with other sarcoma types. Pan-Trk immunohistochemistry was assessed for intensity and extent. A correlation between NTRK3 expression and the type of BCOR alteration and BCOR immunoreactivity was also performed. Most soft tissue undifferentiated round cell sarcomas with YWHAE or BCOR rearrangements or BCOR internal tandem duplications (ITD) showed NTRK3, but not NTRK1 or NTRK2, upregulation by RNA sequencing data analysis. Cytoplasmic pan-Trk immunoreactivity was also observed in most soft tissue round cell sarcomas with YWHAE rearrangements (100%), BCOR ITD (80%), and BCOR-CCNB3 fusions (67%), as well as clear cell sarcomas of kidney (75%), another BCOR family tumor, and ossifying fibromyxoid tumors with ZC3H7B-BCOR fusion (100%), with variable staining intensity and extent. Pan-Trk staining was also seen in solitary fibrous tumors (100%) and less frequently in synovial sarcoma and Ewing sarcoma, but rarely in other sarcomas tested. Tumors harboring rare fusion variants of BCOR, such as BCOR-CHD9, a novel fusion identified by targeted RNA sequencing, and KMT2D-BCOR, were also positive for pan-Trk staining and NTRK3 overexpression. In conclusion, NTRK3 upregulation resulting in pan-Trk overexpression is common in the BCOR family of tumors as well as in subsets of BCOR-expressing sarcomas through alternative mechanisms. The therapeutic implication of this finding awaits further investigation.

Expression of upstream and downstream targets of mTOR pathway in seven cases of secretory carcinoma of salivary gland origin

PURPOSE

To investigate the expression of upstream and downstream targets of mTOR signalling pathway in the secretory carcinoma of salivary gland origin (SCsg).

METHODS

Seven cases of secretory carcinoma diagnosed by a combination of immunohistochemistry and/or molecular testing were retrieved from our pathology files. For comparison purposes, 27 other salivary carcinomas were selected. Immunohistochemical staining was performed against phospho-Akt, PTEN, phospho-mTOR, phospho-4E-BP, eIF4E and phospho-S6 ribosomal protein.

RESULTS

With the exception of Akt, all the other proteins were present at some level in the SCsg and in other salivary carcinomas. PTEN was diffusely expressed in 57.1% of SCsg, but only in 14.8% of other salivary carcinomas. mTOR is expressed in more than half of the cases both for SCsg and other salivary tumour types. Most cases of SCsg showed negative expression for S6 ribosomal protein (71.4%) and 4E-BP1 (57.1%). For both groups evaluated, eIF4E was the most expressed protein.

CONCLUSION

SCsg shows different expression patterns for the mTOR signalling molecules, but only eIF4E was highly expressed. This may suggest alternative signalling pathways other than Akt and mTOR in this group of tumours.

Spontaneous Regression in a Patient With Infantile Fibrosarcoma

Infantile fibrosarcoma usually presents as a rapidly growing mass on the extremities or trunk. We describe spontaneous regression in a 5-month-old female infant with biopsy proven, molecularly confirmed, right leg infantile fibrosarcoma currently at 26 months of age with no signs of local recurrence. Previously reported cases of spontaneous regression are reviewed, suggesting a benign clinical course in some cases. Although evidence for spontaneous regression is anecdotal in this rare tumor type, physicians should weigh the risks and benefits of surgery and chemotherapy against watchful waiting.

Recurrent EML4-NTRK3 fusions in infantile fibrosarcoma and congenital mesoblastic nephroma suggest a revised testing strategy

Infantile fibrosarcoma and congenital mesoblastic nephroma are tumors of infancy traditionally associated with the ETV6-NTRK3 gene fusion. However, a number of case reports have identified variant fusions in these tumors. In order to assess the frequency of variant NTRK3 fusions, and in particular whether the recently identified EML4-NTRK3 fusion is recurrent, 63 archival cases of infantile fibrosarcoma, congenital mesoblastic nephroma, mammary analog secretory carcinoma and secretory breast carcinoma (tumor types that are known to carry recurrent ETV6-NTRK3 fusions) were tested with NTRK3 break-apart FISH, EML4-NTRK3 dual fusion FISH, and targeted RNA sequencing. The EML4-NTRK3 fusion was identified in two cases of infantile fibrosarcoma (one of which was previously described), and in one case of congenital mesoblastic nephroma, demonstrating that the EML4-NTRK3 fusion is a recurrent genetic event in these related tumors. The growing spectrum of gene fusions associated with infantile fibrosarcoma and congenital mesoblastic nephroma along with the recent availability of targeted therapies directed toward inhibition of NTRK signaling argue for alternate testing strategies beyond ETV6 break-apart FISH. The use of either NTRK3 FISH or next-generation sequencing will expand the number of cases in which an oncogenic fusion is identified and facilitate optimal diagnosis and treatment for patients.

Recurrent BRAF Gene Fusions in a Subset of Pediatric Spindle Cell Sarcomas: Expanding the Genetic Spectrum of Tumors With Overlapping Features With Infantile Fibrosarcoma

Infantile fibrosarcomas (IFS) represent a distinct group of soft tissue tumors occurring in patients under 2 years of age and most commonly involving the extremities. Most IFS show recurrent ETV6-NTRK3 gene fusions, sensitivity to chemotherapy, and an overall favorable clinical outcome. However, outside these well-defined pathologic features, no studies have investigated IFS lacking ETV6-NTRK3 fusions, or tumors with the morphology resembling IFS in older children. This study was triggered by the identification of a novel SEPT7-BRAF fusion in an unclassified retroperitoneal spindle cell sarcoma in a 16-year-old female by targeted RNA sequencing. Fluorescence in situ hybridization screening of 9 additional tumors with similar phenotype and lacking ETV6-NTRK3 identified 4 additional cases with BRAF gene rearrangements in the pelvic cavity (n=2), paraspinal region (n=1), and thigh (n=1) of young children (0 to 3 y old). Histologically, 4 cases including the index case shared a fascicular growth of packed monomorphic spindle cells, with uniform nuclei and fine chromatin, and a dilated branching vasculature; while the remaining case was composed of compact cellular sheets of short spindle to ovoid cells. In addition, a minor small blue round cell component was present in 1 case. Mitotic activity ranged from 1 to 9/10 high power fields. Immunohistochemical stains were nonspecific, with only focal smooth muscle actin staining demonstrated in 3 cases tested. Of the remaining 5 BRAF negative cases, further RNA sequencing identified 1 case with EML4-NTRK3 in an 1-year-old boy with a foot IFS, and a second case with TPM3-NTRK1 fusion in a 7-week-old infant with a retroperitoneal lesion. Our findings of recurrent BRAF gene rearrangements in tumors showing morphologic overlap with IFS expand the genetic spectrum of fusion-positive spindle cell sarcomas, to include unusual presentations, such as older children and adolescents and predilection for axial location, thereby opening new opportunities for kinase-targeted therapeutic intervention.

Primitive Spindle Cell Neoplasm of Ileum with Extensive Heterotopic Cartilage, Presenting as Acute Abdomen in a 6-Day-Old Neonate

Neonatal intestinal masses with spindle cell morphology have broad differential diagnoses and require a multidisciplinary approach to make the final diagnosis. Spindle cell masses with heterotopic cartilage in the gastrointestinal tract are very rare, and, to our knowledge, have not previously been reported in the neonate. Here we present a case of intestinal primitive spindle cell neoplasm with extensive heterotopic cartilage that manifested initially as acute abdomen in a 6-day-old term infant. Plain radiography demonstrated pneumoperitoneum, prompting diagnostic laparotomy that identified a perforated mass involving the midileum. Histologic and immunohistochemical examination demonstrated an infiltrative spindle cell tumor most compatible with infantile fibrosarcoma (IFS) by a process of exclusion, with nodules of mature heterotopic cartilage. Additional staging studies did not reveal any evidence of residual or metastatic disease. Recognition of this rare variant of IFS will aid in differentiation from other neonatal intestinal mesenchymal tumors.

JAK-STAT signaling in cancer: From cytokines to non-coding genome

In the past decades, studies of the Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) signaling have uncovered highly conserved programs linking cytokine signaling to the regulation of essential cellular mechanisms such as proliferation, invasion, survival, inflammation and immunity. Inhibitors of the JAK/STAT pathway are used for treatment of autoimmune diseases, such as rheumatoid arthritis or psoriasis. Aberrant JAK/STAT signaling has been identified to contribute to cancer progression and metastatic development. Targeting of JAK/STAT pathway is currently one of the most promising therapeutic strategies in prostate cancer (PCa), hematopoietic malignancies and sarcomas. Notably, newly identified regulators of JAK/STAT signaling, the non-coding RNAs transcripts and their role as important targets and potential clinical biomarkers are highlighted in this review. In addition to the established role of the JAK/STAT signaling pathway in traditional cytokine signaling the non-coding RNAs add yet another layer of hidden regulation and function. Understanding the crosstalk of non-coding RNA with JAK/STAT signaling in cancer is of critical importance and may result in better patient stratification not only in terms of prognosis but also in the context of therapy.

Conservative strategy in infantile fibrosarcoma is possible: The European paediatric Soft tissue sarcoma Study Group experience

BACKGROUND

Infantile fibrosarcoma (IFS) is a very rare disease occurring in young infants characterised by a high local aggressiveness but overall with a favourable survival. To try to reduce the total burden of therapy, the European pediatric Soft tissue sarcoma Study Group has developed conservative therapeutic recommendations according to initial resectability.

MATERIAL AND METHODS

Between 2005 and 2012, children with localised IFS were prospectively registered. Initial surgery was suggested only if possible without mutilation. Patients with initial complete (IRS-group I/R0) or microscopic incomplete (group II/R1) resection had no further therapy. Patients with initial inoperable tumour (group III/R2) received first-line vincristine-actinomycin-D chemotherapy (VA). Delayed conservative surgery was planned after tumour reduction. Aggressive local therapy (mutilating surgery or external radiotherapy) was discouraged.

RESULTS

A total of 50 infants (median age 1.4 months), were included in the study. ETV6-NTRK3 transcript was present in 87.2% of patients where investigation was performed. According to initial surgery, 11 patients were classified as group I, 8 as group II and 31 as group III. VA chemotherapy was first delivered to 25 children with IRS-III/R2 and one with IRS-II/R1 disease. Response rate to VA was 68.0%. Mutilating surgery was only performed in three cases. After a median follow-up of 4.7 years (range 1.9-9.0), 3-year event-free survival and overall survival were respectively 84.0% (95% confidence interval [CI] 70.5-91.7) and 94.0% (95% CI 82.5-98.0).

CONCLUSIONS

Conservative therapy is possible in IFS as only three children required mutilating surgery, and alkylating or anthracycline based chemotherapy was avoided in 71.0% of patients needing chemotherapy. VA regimen should be first line therapy in order to reduce long term effects.

Intestinal congenital/infantile fibrosarcoma: a new clinico-pathological entity?

Congenital/infantile fibrosarcoma (IFS) is a relatively rare form of fibrosarcoma diagnosed at birth or during early years of life and that differs from its adult counterpart because of a more favorable behavior. IFS is also known as cellular congenital mesoblastic nephroma, when it affects the kidney and is often but not always characterized by the ETV6-NTRK3 fusion transcript. We report herein the first series of an exceptional tumor of the small intestine occurring in newborns. The four patients shared a stereotyped clinico-pathological presentation with early and acute onset, intestinal perforation, and an infiltration by a highly cellular spindle cell tumor within the dilated intestinal wall exhibiting pathologic features typical of IFS. Molecular studies for the ETV6-NTRK3 translocation were negative in the three cases tested. Patients were treated by surgical wide resection alone and are alive and well (follow-up: 36 months-25 years). Thus, this new clinico-pathological entity, even with lack of documented evidence of the ETV6-NTRK3 translocation, should be included in the differential diagnosis of congenital bowel perforation or obstruction and may represent an intestinal counterpart of IFS.

Clear cell sarcoma of the kidney demonstrates an embryonic signature indicative of a primitive nephrogenic origin

Clear cell sarcoma of the kidney (CCSK) is a tumor affecting children with a median age of 3 years at diagnosis. The cell of origin of CCSK is unknown and data on the molecular changes giving rise to CCSK is scarce. This has hindered the identification of positive diagnostic markers and development of molecularly targeted treatment protocols for CCSK. We have characterized a panel of CCSK to gain information regarding its molecular profile and possible origin. High-resolution genomic analysis with single nucleotide polymorphism array of 37 tumors did not reveal any clues to the mechanisms behind tumor development as remarkably few genetic imbalances were found. Gene expression analysis revealed a highly characteristic gene signature, enriched for pathways involved in embryonic development, including kidney formation. The presence of markers for two different developmental lineages in the embryonic kidney was therefore investigated in the tumor cells. FOXD1 which identifies cells giving rise to stromal elements, and CITED1, a marker for cells primed for nephrogenic epithelial differentiation, were both highly expressed in CCSK. In addition, the early embryonic marker OSR1 was expressed at higher levels in CCSK than in Wilms tumor, normal fetal kidney or adult kidney. As this marker discriminates the intermediate mesoderm from other mesodermal structures, our study could suggest that CCSK arises from a mesodermal cell type that retains the capacity to initiate differentiation towards both nephrons and stroma, but remains locked in a primitive state.

Cytogenetic and single nucleotide polymorphism array findings in soft tissue tumors in infants

Soft tissue tumors in children under one year of age (infants) are rare. The etiology is usually unknown, with external factors or congenital birth defects and hereditary syndromes being recognized in only a small proportion of the cases. We ascertained the cytogenetic findings in 16 infants from whom tumor tissue had been obtained during a 25-year period. In eight of them, single nucleotide polymorphism (SNP) array analyses could also be performed. No constitutional chromosome aberrations were detected, and assessment of clinical files did not reveal any congenital or later anatomical defects. Three tumors--one infantile fibrosarcoma, one embryonal rhabdomyosarcoma, and one angiomatoid fibrous histiocytoma (AFH)--had abnormal karyotypes. As the AFH had an exchange between chromosome arms 12p and 15q, additional fluorescence in situ hybridization and reverse transcription-polymerase chain reaction analyses were performed, unexpectedly revealing an ETV6/NTRK3 fusion. Three of the eight tumors, including the AFH with an abnormal karyotype, analyzed by SNP array showed aberrations (loss of heterozygosity or imbalances). The present series suggests that the addition of array-based technologies is valuable for detecting underlying pathogenetic mechanisms.

Chaperones as thermodynamic sensors of drug-target interactions reveal kinase inhibitor specificities in living cells

The interaction between the HSP90 chaperone and its client kinases is sensitive to the conformational status of the kinase, and stabilization of the kinase fold by small molecules strongly decreases chaperone interaction. Here we exploit this observation and assay small-molecule binding to kinases in living cells, using chaperones as 'thermodynamic sensors'. The method allows determination of target specificities of both ATP-competitive and allosteric inhibitors in the kinases' native cellular context in high throughput. We profile target specificities of 30 diverse kinase inhibitors against >300 kinases. Demonstrating the value of the assay, we identify ETV6-NTRK3 as a target of the FDA-approved drug crizotinib (Xalkori). Crizotinib inhibits proliferation of ETV6-NTRK3-dependent tumor cells with nanomolar potency and induces the regression of established tumor xenografts in mice. Finally, we show that our approach is applicable to other chaperone and target classes by assaying HSP70/steroid hormone receptor and CDC37/kinase interactions, suggesting that chaperone interactions will have broad application in detecting drug-target interactions in vivo.

Clinically relevant subsets identified by gene expression patterns support a revised ontogenic model of Wilms tumor: a Children's Oncology Group Study

Wilms tumors (WT) have provided broad insights into the interface between development and tumorigenesis. Further understanding is confounded by their genetic, histologic, and clinical heterogeneity, the basis of which remains largely unknown. We evaluated 224 WT for global gene expression patterns; WT1, CTNNB1, and WTX mutation; and 11p15 copy number and methylation patterns. Five subsets were identified showing distinct differences in their pathologic and clinical features: these findings were validated in 100 additional WT. The gene expression pattern of each subset was compared with published gene expression profiles during normal renal development. A novel subset of epithelial WT in infants lacked WT1, CTNNB1, and WTX mutations and nephrogenic rests and displayed a gene expression pattern of the postinduction nephron, and none recurred. Three subsets were characterized by a low expression of WT1 and intralobar nephrogenic rests. These differed in their frequency of WT1 and CTNNB1 mutations, in their age, in their relapse rate, and in their expression similarities with the intermediate mesoderm versus the metanephric mesenchyme. The largest subset was characterized by biallelic methylation of the imprint control region 1, a gene expression profile of the metanephric mesenchyme, and both interlunar and perilobar nephrogenic rests. These data provide a biologic explanation for the clinical and pathologic heterogeneity seen within WT and enable the future development of subset-specific therapeutic strategies. Further, these data support a revision of the current model of WT ontogeny, which allows for an interplay between the type of initiating event and the developmental stage in which it occurs.