Adult Wilms Tumor

Abstract

The genetics of nephroblastoma (Wilms tumor) occurring in adults is largely unknown, as studies have largely been limited to isolated case reports. We, therefore, studied 14 adult Wilms tumors for genetic alterations, using expanded targeted sequencing on 11 cases. The patients ranged from 17 to 46 years of age (mean and median, 31 y), and there were 8 males and 6 females. Five Wilms tumors harbored BRAF V600E mutations. All of these had better-differentiated areas identical to metanephric adenoma, as has previously been described. In 3 such cases, microdissection studies revealed that the BRAF V600E mutation was present in both the metanephric adenoma and Wilms tumor areas; however, additional genetic alterations (including TERT promoter mutations in 2 cases, ASLX1/ATR mutations in 1 other case) were limited to the Wilms tumor component. These findings suggest that the Wilms tumor developed from the metanephric adenoma. Other adult Wilms tumors harbored genetic alterations previously reported in the more common pediatric Wilms tumors, including WT1 mutations (2 cases), ASLX1 mutations (3 additional cases), NSD2 mutation (1 additional case), and 11p loss (3 cases). In summary, a significant subset of adult Wilms tumors (specifically those of epithelial type with differentiated areas) harbor targetable BRAF V600E mutations and appear to arise from metanephric adenomas as a consequence of additional acquired genetic alterations. Other adult Wilms tumors often harbor genetic alterations found in their more common pediatric counterparts, suggesting at least some similarities in their pathogenesis.

Approximately 3% of Wilms tumor (nephroblastoma) occur in adults,¹ and these have long been problematic for oncologists. For many years, the general consensus has been that adult Wilms tumors are more aggressive than their pediatric counterparts.^2–6 However, several factors could account for this difference. First, adult Wilms may not be treated as expeditiously and appropriately as pediatric cases. There is frequently a delay in diagnosis of this typically pediatric tumor in adults, where it comprises <1% of all renal tumors.⁷ Moreover, adults with Wilms tumor are often not treated on standardized protocols developed by the National Wilms Tumor Study Group (now COG) or the Société International d’Oncologie Pédiatrique (SIOP). Along these lines, treatment of adult Wilms tumors according to standardized pediatric protocols has been shown to improve outcomes.^7–10 Second, adult Wilms tumors have a greater tendency to present at an advanced stage relative to pediatric cases, perhaps reflecting the delay in seeking medical attention. Third, unlike pediatric Wilms tumors, adult Wilms tumors have not been shown to arise in association with nephrogenic rests or be shown to be associated with developmental conditions associated with Wilms tumor, potentially reflecting differing biology. Fourth, other recently described more aggressive lesions (such as renal Ewing sarcoma^11,12 or synovial sarcoma¹³) have historically been misdiagnosed as adult Wilms tumor, contributing to the observed poor outcome.

The genetics of adult Wilms tumors has remained largely unstudied, limited to isolated case reports frequently assessing only a few genetic alterations.¹⁴ Along these lines, in a recent study, our group found that a subset of Wilms tumors with areas morphologically resembling metanephric adenoma harbored BRAF V600E mutations that are typical of metanephric adenoma¹⁵ and not previously found in Wilms tumor.¹⁶ One pediatric patient with metastatic disease had a complete response to combined targeted RAF/MEK inhibitors dabrafenib and trametinib.^16,17 While these cases were identified based on morphologic features, 2 of the 4 cases occurred in adults, raising the possibility that BRAF V600E mutations may be more frequent in adult Wilms tumors. In the current study, we perform a more comprehensive genetic analysis of 14 adult Wilms tumors, including expanded targeted sequencing of 11 of these cases.

MATERIALS AND METHODS

Institutional Review Board Approval

This study was approved by the Institutional Review Boards at our respective institutions.

Cases

We identified all cases of nephroblastoma (Wilms tumor) in patients at or above the age of 17 years in the consultation and teaching files of 2 authors (P.A., J.I.E.) at Johns Hopkins Hospital and the institutional files of another hospital (Memorial Sloan-Kettering Cancer Center). We acknowledge that patients aged 17 to 20 could be considered older adolescents, but they fall far outside the typical age range for Wilms tumor (ages 3 to 6 y), and this age group has been included in prior adult Wilms tumor cohorts.^3–6 Cases included in our prior study¹⁶ were excluded. None of the cases in this study have previously been published. Cases were reviewed by one author (P.A.) to verify the diagnosis of Wilms tumor, and cases were included in the study if additional formalin-fixed paraffin-embedded (FFPE) material was available for genetic analysis.

Molecular Techniques

DNA and RNA sequencing were performed by the Genomics and Molecular Pathology Core at UT Southwestern Medical Center on all cases except cases 5, 6, and 12. Briefly, tumor hematoxylin and eosin slides were examined and marked by 2 pathologists (P.A. and D.N.P.) for subsequent macrodissection, nucleic acid isolation, and molecular testing. Areas enriched with tumor were then scraped from adjacent 5-μm thick FFPE sections. Adjacent normal tissue was separately isolated and processed for select cases when available. Extraction and purification were performed using Qiagen Allprep kits (Qiagen, Germantown, MD). Libraries were prepared using KAPA Hyperplus kits (Roche Sequencing and Life Science Kapa Biosystems, Wilmington, MA) with genomic regions of interest captured by custom DNA probes covering all exons of over 1425 cancer-related genes. The libraries were sequenced to an average unique read depth of > ×600 (95% of exons at or above ×100) using Sequencing by Synthesis (SBS) paired-end cluster generation on the Illumina NextSeq. 550 platform (Illumina Inc., San Diego, CA). Sequence reads were aligned to reference genome GRCh38, and subsequent analyses were performed using custom germline, somatic, and mRNA bioinformatics pipelines run on the UTSW BioHigh Performance Computer cluster and optimized for detection of single nucleotide variants, indels, and known gene fusions. Minor allele frequency limit of detection for the assay: single nucleotide variants—5%; indels and known gene fusions—10%. Variants in exons with <×100 coverage are not reported.

Somatic variants were identified on the basis of their variant allele frequencies and relative absence in matched normal tissue (if available) and/or presence at frequencies below 0.5% in databases of germline variants (dbSNP, gnomAD). Variants were classified according to the Association for Molecular Pathology/American Society for Clinical Oncology/College of American Pathologists guidelines,¹⁸ and all variant calls were inspected using Integrated Genomics Viewer, version 2.3.4 (IGV; Broad Institute, MIT Harvard, Cambridge, MA) before reporting.

Copy number variants were detected using CNVKit and an internally derived panel of normal FFPE tissue samples.

Adequate tissue for expanded targeted sequencing was not available for cases 5, 6, and 12. For these cases, BRAF mutation status alone was assessed by using pyrosequencing to detect mutations at codons 600 and 601 or AmpliSeq Cancer Hotspot Panel (v2) (Life Technologies, Carlsbad, CA) to detect mutations within exons 11 and 15 as described previously.^19,20

RESULTS

Cases

The clinicopathologic and genetic features of the cases are summarized in Table 1. The patients ranged from 17 to 46 years of age (mean and median, 31 y), and there were 8 males and 6 females. Tumors were generally large—sizes ranged from 2.5 to 23 cm (mean, 11.8 cm; median, 11 cm)—and often presented at an advanced stage. Of 13 cases with staging information, 6 were stage 1, 2 were stage 2, 3 were stage 3, and 2 were stage 4.

TABLE 1.

Adult Wilms Tumor Cases in This Study

Case No.	Age/Sex	Size/Stage	Histology	IHC	Genetic Alterations of Interest
1	38/F	17 cm/stage 1	Epithelial-predominant triphasic with MA-like areas, rim of calcified MA; hyaline calcified nodule	WT1+, EMA−, CK7−	MA and WT: BRAF V600E, NOTCH1 G842del, STYK1 T100fs WT only: TERT c.-124G- > A, LTBP N1187fs, WHSC1(NSD2) C846Y, 1q gain (MDM4), 17q loss (BRCA1)
2	30/F	3 cm/stage 1	Epithelial predominant with MA-like areas; hyaline calcified nodule	WT1+	MA and WT areas: BRAF V600E WT only: TERT c.-124G- > A, CREB1 R284*
3	46/M	6 cm/stage 2	Epithelial predominant with MA-like areas	WT1+ CK7−	MA and WT areas: BRAF V600E WT only: ATR c.2078+1G> A, ASXL1 G660fs
4	44/F	11 cm/stage 1	Epithelial predominant with MA-like areas; hyaline calcified nodule	WT1+, PAX8+; CK7−	BRAF V600E, CTCF T400fs, EZH2 R313Q, 1q gain (MDM4), 12p/q gain (KRAS, GLI1, CDK4, MDM2)
5	40/M	2.5 cm/stage 1	Epithelial predominant with MA-like areas; hyaline calcified nodule	WT1+	BRAF V600E+
6	35/F	NA	Epithelial	WT1+, PAX8+; EMA−	BRAF −
7	32/M	11 cm/stage 3	Epithelial predominant, not anaplastic	PAX8+ WT1−	TP53 R175H, BCOR N1459S, BCORL1 R1266*, ASXL1 c.1086–2A > T, KRAS G12V, ERBB3 S1094*, 4p loss (WHSC1/NSD2), 11p LOH (WT1), 12p gain (KRAS), 22p/q loss (BRD1, CHEK2, SMARCB1)
8	35/M	23 cm/stage 3	Blastemal predominant, triphasic	WT1+	BRCA1 R1720W, TBX3 E255fs, ASXL1 R596fs, MELK Q67*, 2p/q gain (ASXLX2, ERBB4, GLI2, MYCN), 3q gain (TERC, TP63), 7p/q gain (BRAF, MET), 8p/q gain (MYC, TERF1, WHSC1L1), 12p/q gain (KRAS, GLI1, CDK4, MDM2, TBX3), 16p/q gain (TERF2), 20p/q gain (ASXL1)
9	17/M	19 cm/stage 1	Epithelial predominant, serpentine, and nodular blastema	WT1+	4p loss (WHSC1/NSD2); 11p loss (WT1); 8p loss (WHSC1L1); 3p loss (BAP1); 16p/q loss (CTCF, FANCA); 1p loss (MTOR); 19p gain (BRD4), 19q loss (POLD1)
10	20/F	17 cm/stage 4 (lung metastasis)	Blastemal predominant, nodular	WT1+	1q loss, 9q loss (FANCC, PTCH1, XPA), 10p/q loss (RET), 11p loss (CENPU)
11	19/M	14 cm/stage 2	Epithelial-blastemal	WT1+	ATM R248*, WHSC1(NSD2) E1099K, KAT6B W606fs, BIRC6 L2121F
12	28/F	Stage 4 (brain metastasis)	Epithelial predominant	WT1+	BRAF −
13	23/M	10 cm/stage 3	Triphasic with skeletal muscle but prominent diffuse blastemal pattern	WT1+, EMA−	WT1 H235fs, ASXL1 G645fs, CCDC88C L47fs, CHD2 L1018R, KMTD2 L389fs, 1q gain (MDM4), 2p/q gain (ASXL2, ERBB4, GLI2, MYCN), 12p/q gain (KRAS, GLI1, CDK4, MDM2)
14	29/M	8.5 cm/stage 1	Blastemal-epithelial	WT1+	WT1 D436Y, SPOP R354C, KMT2A T2230I

Bolded genes are of special interest.

^*Stop codon.

F indicates female; fs, frameshift; IHC, immunohistochemistry; M, male; MA, metanephric adenoma; NA, not applicable.

Histologic Features

Among the 14 cases, 9 were epithelial predominant, 3 were blastemal predominant, and 2 were an even mixture of epithelial and blastemal elements. There were no stromal predominant neoplasms. By immunohistochemistry, 13 of the 14 Wilms tumors were immunoreactive for WT1 protein. The background kidney demonstrated no specific abnormities. Nephrogenic rests were not identified in the surrounding renal parenchyma in any case.

Genetics

Overall, 5 Wilms tumors harbored BRAF V600E mutations. Other adult Wilms tumors harbored genetic alterations previously reported in the more common pediatric Wilms tumors, including WT1 mutations (2 cases), ASLX1 mutations (3 additional cases not including one of the BRAF V600E-mutated cases), NSD2 mutation (1 additional case not including one of the BRAF V600E-mutated cases), and 11p loss (3 cases) (Fig. 1). Comprehensive data is presented in Supplementary Table 1 (Supplemental Digital Content 1, http://links.lww.com/PAS/B319).^21–41

FIGURE 1.

Next-generation sequencing of adult Wilms tumors. A, Epithelial-predominant Wilms tumor with metanephric adenoma (MA)-like areas (cases 1 to 4) consistently harbored BRAF V600E mutations in both the MA and Wilms tumor components (cases 1 to 3) with additional genomic alterations involving Wnt and/or TGFβ signaling, telomere maintenance, DNA damage response, apoptosis, or histone modification/chromatin remodeling/DNA methylation in the Wilms tumor areas. The remaining adult Wilms tumor cases without identifiable MA-like areas (cases 7 to 11, 13 to 14) demonstrated genetic alterations in similar pathways as well as receptor-mediated signaling associated with receptor tyrosine kinases. B, Large chromosomal alterations were detected in most but not all cases of adult Wilms tumor. Both chromosomal gains, particularly involving 1q and 12p/q, and losses were noted. †Matched normal not sequenced. ‡Also detected in MA-like area. Note: WHSC1 is also known as NSD2. CN indicates copy number; LOH, loss of heterozygosity; MA, metanephric adenoma.

Correlation of Histologic Features With Genetics

As previously found by our group,¹⁶ the BRAF V600E-mutated Wilms tumors all had distinctive histology—in addition to epithelial-predominant Wilms tumor areas featuring mitotically active columnar-shaped cells with primitive (finely dispersed but hyperchromatic) chromatin, there was a mitotically inactive differentiated tubular component associated with calcifications that is typical of metanephric adenoma (Figs. 2–4). Four of the 5 BRAF V600E-mutated cases had large hyalinized nodules associated with calcification adjacent to the metanephric adenoma component. In 3 of these cases, the Wilms tumor area formed a discrete expansile nodule, such that it could be dissected apart from the metanephric adenoma areas and the 2 components sequenced separately. In each case, both components harbored the BRAF V600E mutation. However, the Wilms tumor component from each case harbored additional genetic alterations not found in the metanephric adenoma component. There were additional genomic alterations involving Wnt and/or TGFβ signaling, telomere maintenance, DNA damage response, apoptosis, or histone modification/chromatin remodeling/DNA methylation. Of note, in cases 1 and 2, the Wilms tumor component had an additional TERT promoter mutation. In case 3, ATR and ASXL1 mutations were identified only in the Wilms tumor areas. Analysis of telomere lengths in the 2 neoplasms with TERT mutations revealed short telomeres in both the metanephric adenoma and Wilms tumor components (Supplemental Fig. 1, Supplemental Digital Content 2, http://links.lww.com/PAS/B320). No difference between the 2 components could be discerned.

FIGURE 2.

Case 1: Wilms tumor arising from metanephric adenoma. This whole-mount slide section demonstrates a nodule of epithelial-predominant Wilms tumor (right) arising in the background of a metanephric adenoma (bottom) associated with extensive sclerosis (left) (A). At intermediate power, the transition between the differentiated metanephric adenoma (left) and the epithelial Wilms tumor (right) is sharp (B). At higher magnification, one can appreciate the mitotically inactive metanephric adenoma cells (left) and the mitotically active columnar-shaped epithelial Wilms tumor cells (right) (C). The metanephric adenoma areas have the typical solid and focally papillary architecture of metanephric adenoma, set in the hyalinized stroma (D). At high power, the metanephric adenoma cells have minimal cytoplasm and show no mitotic activity (E). In contrast, the Wilms tumor areas have foci of stroma in addition to the epithelial areas and are associated with necrosis (lower left) (F). At high power, the columnar epithelial cells demonstrate mitotic activity (G). In this case, while both areas harbored BRAF V600E mutations, the Wilms tumor area demonstrated TERT promoter and WHSC1(NSD2) mutations (the latter 2 not found in the metanephric adenoma areas).

FIGURE 4.

Case 3: Wilms tumor arising from metanephric adenoma. This whole-mount section shows a primitive neoplasm which abuts the renal sinus of the native kidney at the left (A). One can appreciate the nodule of epithelial Wilms tumor (white star) separated by a capsule from the adjacent metanephric adenoma area (black star) (A). The metanephric adenoma area abuts the cortical type parenchyma adjacent to the renal pelvic urothelium and demonstrates the typical un-encapsulated border and intermingling with native nephrons (B). At higher power, the primitive metanephric adenoma cells are mitotically inactive and composed of small tubules which touch the adjacent non-neoplastic renal tubules (C). The sclerotic metanephric adenoma area (left) transitions into the nodule of epithelial-predominant Wilms tumor (right) (D). High magnification of the epithelial Wilms tumor area demonstrates tubules formed by cells with primitive columnar nuclei showing mitotic activity (E). In this case, while both areas harbored BRAF V600E mutations, the epithelial Wilms tumor demonstrated ASXL1 and ATR mutations (which were not found in the metanephric adenoma area).

The remaining adult Wilms tumors demonstrated the typical morphology seen in their pediatric counterparts (Fig. 5). They harbored genomic alterations involving Wnt and/or TGFβ signaling, telomere maintenance, DNA damage response, apoptosis, histone modification/chromatin remodeling/DNA methylation, as well as receptor-mediated signaling associated with receptor tyrosine kinases. In case 13, a triphasic neoplasm with a prominent diffuse blastemal component, the diffuse blastemal component demonstrated striking angioinvasion within the renal parenchyma and a positive renal vein margin. This neoplasm harbored a WT1 mutation. Cases 9 and 10 had prominent nodular blastemal patterns typically seen in pediatric Wilms tumor, and both demonstrated loss of portions of chromosome 11p. The TP53-mutated Wilms tumor (case 7) was epithelial predominant but did not demonstrate anaplasia. The BRCA1-mutated Wilms tumor (case 8) demonstrated the typical triphasic morphology of the Wilms tumor.

FIGURE 5.

Additional adult Wilms tumors associated with mutations in genes commonly altered in pediatric Wilms tumor. Case 13, Adult Wilms tumor with WT1 mutation. This neoplasm had a predominant blastemal pattern and permeated native nephrons (A). Focal areas of well-formed tubules were present in addition to spindled stroma (B). Case 14, Adult Wilms tumor associated with WT1 mutation. This neoplasm was predominantly composed of blastemal cells and primitive tubules (C). The neoplasm demonstrated diffuse nuclear labeling for WT1 (D). Case 7, Adult Wilms tumor associated with TP53, BCOR, BCORL1, ASXL1, and KRAS mutations and 11p loss of heterozygosity. This favorable histology neoplasm was predominantly blastemal and epithelial and separated from the kidney by a fibrous capsule (E). The neoplasm demonstrates the typical primitive round cell blastemal and tubular patterns of Wilms tumor (F). Case 8, Adult Wilms tumor associated with a BRCA1 mutation. The neoplasm demonstrates triphasic morphology with blastemal, stromal, and epithelial elements (G). At high power, the primitive cytology of the 3 elements is typical of Wilms tumor (H).

DISCUSSION

Accumulated data now show that adult Wilms tumors differ in many respects from their pediatric counterparts. First, adult Wilms tumors are rarely associated with developmental syndromes such as the WAGR (Wilms, aniridia, genitourinary malformations, and intellectual disability), Denys-Drash (Wilms tumor, glomerular disease, gonadal dysgenesis), or Beckwith Wiedemann (overgrowth and predisposition to neoplasia) syndromes. One adult Wilms tumor patient has been reported to have a germline mutation in WT1, but no clear evidence of a syndrome was reported.⁴² One other case was reported in a patient with cryptorchidism and hypospadias, but genetic testing was not performed.⁴³ In contrast, ~15% of pediatric Wilms tumors are syndromic. Second, unlike pediatric Wilms tumors, adult Wilms tumors have not been shown to arise from nephrogenic rests, the established precursor of Wilms tumors.^{1,3,5,6,9,10,44} While most studies of adult Wilms tumors have not mentioned nephrogenic rests specifically, there have been no cases reported of adult Wilms tumor arising in association with nephrogenic rests, which is consistent with our findings in this study. This also fits data on bilateral Wilms tumors, which are consistently associated with nephrogenic rests. Only one in over 200 adult Wilms tumors reported in large studies has been bilateral, while ~6% of pediatric Wilms tumors are bilateral (Supplemental Table 2, Supplemental Digital Content 3, http://links.lww.com/PAS/B321). This difference has been corroborated by the SEER database and is statistically significant.⁴ Third, adult Wilms tumors have been associated with diminished prognosis relative to pediatric cases, in large part due to a more frequent presentation at an advanced stage, delay in therapy, and use of nonstandardized therapeutic protocols. Along these lines, the prognosis for Wilms tumor in adults is improved when cases are treated according to pediatric protocols.

The most common genetic alterations/dysregulated pathways in pediatric Wilms tumor include IGF2 overexpression, WNT pathway alterations (including WT1 and CTNNB1 mutations), and microRNA processor mutations. Whether the genomics of adult Wilms tumor is distinct from its more common pediatric counterpart is not currently known. Studies have generally been limited to a few cases assessing only a few genetic alterations. One study did not find β-catenin mutations in 4 adult Wilms tumors, whereas β-catenin mutations were found in 15% of pediatric Wilms tumors.⁴⁵ A single nucleotide polymorphism array of a single Wilms tumor in a 71-year-old female showed greater genomic complexity than typically seen in a pediatric Wilms tumor.⁴⁶ Hence, the purpose of our study was to address this issue.

We found that the majority of adult Wilms tumor cases harbored genetic alterations which are well-known to be associated with pediatric Wilms tumors.^47,48 These included mutations in WT1 and loss of heterozygosity at chromosome 11p, which are among the more common changes found in pediatric Wilms tumors. We also found recurrent mutations in genes less commonly associated with pediatric Wilms tumor, such as the ASXL transcriptional regulator one (ASXL1) gene.^49,50 The latter is one of the more commonly mutated genes in clonal hematopoiesis, myeloid malignancies, and small cell lung carcinoma and is a known epigenetic regulator which binds to BAP1. Mutations in ASXL1 give rise to the Bohring-Opitz syndrome, a rare developmental condition associated with facial abnormalities, unusual posture, intellectual disability, and an association with Wilms tumor.^51,52 We additionally identified in 2 cases mutations in the NSD2 gene (also widely known as WHSC1), which has previously been shown to be deleted or amplified in <2% of pediatric Wilms tumors but not mutated.^47,48 The NSD2 gene encodes the NSD2 histone lysine methyltransferase, a regulator of development which methylates the H3K36 residue of histones to promote an open chromatin state that favors gene transcription. NSD2 mutations are found in 5% to 10% of several common adult cancers such as colonic or gastric adenocarcinoma, urothelial carcinoma, and melanoma, but far less commonly in pediatric solid tumors.^53,54 Mutations in the transcriptional corepressors BCOR and BCORL1, which we found in one of our adult cases, have also been found in pediatric Wilms tumor.⁵⁵ Hence, a significant proportion of adult Wilms tumors have molecular alterations similar to their pediatric counterparts, suggesting that they may be driven by similar molecular pathways. One limitation of our study is that we used targeted sequencing of 1425 cancer-related genes rather than whole-genome sequencing, so not all genetic alterations would be discovered. However, our platform is better suited to FFPE tissues of variable age, in large part due to the depth of sequencing.

We also identified a high proportion of BRAF V600E mutations in our adult Wilms tumor cohort (5/14, or 36%). These neoplasms had a morphology similar to that reported in our original study describing BRAF V600E-mutated Wilms tumors – the neoplasms contained areas morphologically identical to differentiated metanephric adenoma (which characteristically harbors BRAF V600E mutations) in addition to more proliferative, overtly malignant areas which are morphologically identical to Wilms tumor.¹⁶ Of note, all 5 neoplasms with BRAF V600E mutations in this cohort presented with localized disease (stages 1 or 2), while only 2 of the 8 other cases did so. We previously reported the BRAF V600E mutation in 4 metanephric adenoma-epithelial Wilms tumor overlap lesions, 2 of which were pediatric cases and 2 of which were adult cases. The skewing toward adult cases in this small morphologically selected cohort suggested the possibility that BRAF V600E-mutated Wilms tumors might be more prevalent in the adult population, and our current study appears to support this hypothesis. One confounding factor is that most of our cases were derived from our consultation files, so it is likely that the adult Wilms tumor cases we studied were skewed towards those with unusual morphology (such as having metanephric adenoma-like areas). In fact, 2 of the 5 adult BRAF-mutated cases were sent to us due to morphologic overlap with cases in our prior study.¹⁶ Removing these cases from consideration, the frequency of BRAF-mutated Wilms tumors in adults in our study would be 3 of 12 or 25%. As we and others have previously described, identification of BRAF V600E mutations in Wilms tumors has therapeutic significance, as these tumors may be treated with highly effective BRAF/MEK inhibitor therapy. In addition to the child we reported with metastatic disease who had a durable complete response, another recent report described a 51-year-old male with relapsed metastatic adult Wilms tumor harboring a BRAF V600E mutation who had a prolonged and dramatic response to the BRAF inhibitor dabrafenib.⁵⁶ While the histology was not illustrated, the tumor was described as predominantly epithelial and, in fact, was initially misdiagnosed as papillary renal cell carcinoma (RCC), suggesting that it was a lesion similar to those we described herein and previously.¹⁶

Interestingly, one prior genetic study analyzing the genomics of adult Wilms tumor identified isochromosome 17q as a recurrent finding in 3 of 5 cases.⁵⁷ Since the BRAF gene maps to this chromosome, one could hypothesize that these neoplasms harbored BRAF V600E mutations which were amplified by the isochromosome 17q. Unfortunately, BRAF sequencing was not performed on these neoplasms, and the illustrated morphology does not address whether these lesions harbored a differentiated metanephric adenoma-like component.

Given the known association of metanephric adenoma-like areas with Wilms tumor and their common underlying BRAF V600E mutation, 2 hypotheses could explain the findings. The first possibility is that epithelial-predominant BRAF V600E-mutated Wilms tumors focally differentiate to resemble metanephric adenoma. This pathway has always seemed most likely, given that preoperative treatment is known to induce some epithelial Wilms tumors to differentiate and resemble metanephric adenoma⁵⁸ and treated differentiated stromal Wilms tumors can mimic metanephric stromal tumors.^59,60 Another related precedent is that chemotherapy can induce another embryonal neoplasm of childhood, neuroblastoma, to differentiate and resemble ganglioneuroma. The second possibility, for which there has previously been no support, is that a BRAF V600E-mutated metanephric adenoma could undergo malignant transformation to become a Wilms tumor. While there is no clinical precedent for this, as metanephric adenomas are clinically benign, it should be noted that almost all metanephric adenomas are completely excised by partial or complete nephrectomy, so their untreated natural history is in fact not known. Our study provides molecular evidence which supports the hypotheses that BRAF V600E-mutated metanephric adenomas may progress to Wilms tumors, as the Wilms tumors areas of all 3 neoplasms in which microdissection could be performed harbored not only BRAF V600E mutations but also additional genomic alterations not found in the metanephric adenoma component. These findings are compatible with those reported by Chami et al,⁶¹ who found the same BRAF V600E mutation in the metanephric adenoma and papillary renal cell components of a composite metanephric adenofibroma-papillary RCC case. It is more logical to conceptualize this case as the transformation of a component of an underlying BRAF V600E differentiated composite metanephric neoplasm into a malignancy (papillary RCC) than to think of it as differentiation of papillary RCC (which is characterized by multiple distinctive genetic alterations) into metanephric adenoma. The molecular findings in our study are also supported by morphology, as the expansile nodules of Wilms tumors as illustrated in Figures 2–4 are more consistent with Wilms tumors arising from metanephric adenoma than a Wilms tumor demonstrating maturation, in which foci of maturation would be predicted to be more randomly dispersed.

Interestingly, 2 of our combined Wilms tumor-metanephric adenoma cases had TERT promoter mutations limited to the Wilms tumor component. High telomerase RNA expression has been associated with poor prognoses in Wilms tumor,⁶² and TERT promoter mutations are thought to synergize with BRAF alterations in thyroid cancer to promote an aggressive phenotype.⁶³ Correlating with these findings, we did find shortening of telomeres in the Wilms tumors associated with these secondary TERT promoter mutations as well as in the metanephric adenoma components. This is consistent with a proposed model in which TERT promoter mutations do not prevent bulk telomere length shortening in neoplasms but instead restore the length of critically shortened telomeres allowing these cells to survive, thereby promoting genetic instability.⁶⁴ The short telomere length of the metanephric adenoma component also matches the observation that most precancerous lesions have short telomeres but are telomerase negative.^65–67

While the concept of progression of metanephric adenoma to Wilms tumor proposed in our study is a novel one, we note that Chan et al⁶⁸ recently reported a single BRAF V600E-mutated metanephric adenoma associated with sarcomatous transformation. The sarcoma lacked morphologic features of the Wilms tumor, and both the metanephric adenoma and sarcomatous components had BRAF, EIF1AX, and TERT promoter mutations. However, the sarcomatous component contained a deep deletion of CDKN2A (which encodes for p16) and MYC amplification, while the metanephric adenoma component lacked these alterations. We did not identify loss of p16 protein immunoreactivity in the Wilms tumor components of our composite neoplasms (not shown). However, taken together with our study, these findings suggest that metanephric adenomas have some potential for malignant transformation and should be excised if possible and that if not, there should be active surveillance of such lesions. This risk would seem to be low since small foci of Wilms tumor are not commonly seen in large metanephric adenomas. Finally, we note that recent single-cell sequencing-based analyses of Wilms tumor have revealed a distinct cellular origin with transcriptional profiles overlapping with aberrant nephrogenesis; whether adult Wilms tumor display such evolutionary phenotypes is likely to be a subject of future studies.⁶⁹

Finally, given the absence of association with nephrogenic rests and association with metanephric adenoma, one could question whether BRAF-mutated Wilms tumors should be considered Wilms tumors at all or, instead, a separate entity such as “malignant metanephric adenoma.” We believe that these cases should be considered adult Wilms tumors, given their morphologic identity to pediatric epithelial-predominant Wilms tumors, the presence of blastemal, and stroma in selected cases (as illustrated in Figs. 2F–G of this manuscript and figure 7 of reference 16), and shared WT1 immunoreactivity. It is well-known that Wilms tumors arise by diverse genetic pathways, such as IGF2 overexpression or WT1 mutation/CTNNB1 mutation.⁵⁵ In our view, BRAF mutation represents another pathway which leads to the phenotype of Wilms tumor.

In conclusion, adult Wilms tumors harbor some of the same genetic alterations found in their more common pediatric counterparts. A higher percentage harbor BRAF V600E mutations and appear to arise from metanephric adenomas.

FIGURE 3.

Case 2: Wilms tumor arising from metanephric adenoma. In the whole-mount section, the nodule of epithelial-predominant Wilms tumor is surrounded by a rim of adjacent metanephric adenoma, most prominent to the left (A). The transition of the calcified metanephric adenoma, which abuts the native kidney, and the epithelial Wilms tumor is abrupt and associated with a fibrous capsule (B). At high power, the metanephric adenoma demonstrates the typical small tubules associated with psammomatous calcifications which directly abut the native nephrons (C, D). At high power, the epithelial Wilms tumor demonstrates elongated tubules formed by nuclei with primitive columnar nuclei, associated with prominent mitotic activity (E). In this case, while both areas harbored BRAF V600E mutations, the Wilms tumor area demonstrated an additional TERT promoter mutation not found in the metanephric adenoma area.