Renal cell carcinoma (RCC) accounts for 2% of all cancer cases worldwide, and majority are sporadic. The latest World Health Organization (WHO) classification of renal cell tumors (fifth edition, 2022) has molecularly defined renal tumor entities, which includes fumarate hydratase (FH)-deficient RCC. FH-deficient RCC is an aggressive carcinoma caused by pathogenic alterations in FH gene, seen in 15% of patients with hereditary leiomyomatosis and renal cell cancer syndrome (HLRCC) syndrome. These tumors occur more frequently at a younger age and present at an advanced stage, carrying a dismal prognosis. We report a series of 10 cases of FH-deficient RCC. The mean age was 49.8 years, and all cases presented in advanced stages (III and IV). Morphologically, the cases had varied architectural patterns with characteristic eosinophilic macronucleoli and perinucleolar halo. On immunohistochemistry (IHC), all showed diffuse nucleo-cytoplasmic expression of S-(2-succino)-cysteine (2-SC), with loss of FH in seven cases. FH-deficient RCCs are aggressive neoplasms and can be diagnosed using specific IHC markers (FH and 2-SC). These patients should undergo germline testing for FH gene mutation, genetic counseling, and surveillance of family members.

Fumarate hydratase-deficient renal cell carcinoma (FH-deficient RCC) is a rare aggressive type of renal cell carcinoma. The significant morphologic overlap with other types of renal neoplasia and the limited availability of FH and 2-succinylcholine immunostains for diagnostic use outside large referral centers have created numerous diagnostic pitfalls. As FH-deficient RCC can be associated with hereditary leiomyomatosis and renal cell cancer syndrome, the importance of an accurate diagnosis goes beyond the prognosis and treatment of an individual patient. We present 2 patients with FH-deficient RCC showing a peculiar pattern of GATA3 immunoexpression restricted to tumor nucleoli. If confirmed in further larger studies, this could provide an additional diagnostic clue for considering the FH-deficient RCC diagnosis, and given the frequent papillary morphology and possible hilar location can lead to the misdiagnosis as high-grade urothelial carcinoma, and is an important diagnostic pitfall to be aware of.

Hereditary leiomyomatosis and renal cell cancer (HLRCC) syndrome is a rare genetic disorder caused by a germline mutation in the fumarate hydratase (FH) gene. It is clinically characterized by cutaneous leiomyomas, uterine leiomyomas and renal cell cancer. A 31-year-old woman presented with severe abdominopelvic pain associated with severe menorrhagia which required a visit to the emergency department. Computed tomography (CT) showed a severe enlargement of the uterus with newly diagnosed fibroids. Magnetic resonance imaging (MRI) confirmed the finding of an enlarged uterus with mild left and moderate right hydronephrosis and hydroureter. The patient tried to manage the pain with oral over-the-counter medications and heat pads without significant relief. She was recommended to proceed with total abdominal hysterectomy and bilateral salpingectomy. She tolerated the procedure well and had an uneventful postoperative recovery. Pathology showed morphologic features, including the staghorn vessels, alveolar edema, eosinophilic cytoplasmic inclusions and prominent nucleoli which are characteristics for FH-deficient leiomyomas. Genetic testing was positive for a pathogenic variant in the FH gene associated with HLRCC. This case highlights the importance of proceeding with genetic testing in patients with personal and family history of leiomyomas and unusual pathology findings. Early identification of the syndrome can lead to appropriate screening for renal cell carcinoma.

Deficiency of fumarate hydratase (FH) protein expression in uterine corpus leiomyomas may be attributable to either germline or somatic mutations of the FH gene, the former being definitional for the hereditary leiomyomatosis and renal cell cancer syndrome. The authors assess whether, using previously reported FH-associated morphologic features, FH protein-deficient uterine corpus leiomyomas associated with a pathogenic germline mutations of the FH gene (group 1) are distinguishable from FH protein-deficient uterine corpus leiomyomas without such mutations (and whose FH protein loss is presumed to be attributable to somatic/epigenetic inactivation or other unknown phenomena: group 2). Groups 1 and 2 were compared regarding a variety of clinicopathologic features, including 7 core \"FH-associated\" tumoral morphologic features: staghorn vasculature; alveolar-type edema; bizarre nuclei; chain-like tumor nuclei; hyaline cytoplasmic globules; prominent nucleoli, intranuclear inclusions, and perinucleolar halos; and prominent eosinophilic/fibrillary cytoplasm. Among 2418 patients diagnosed with uterine corpus leiomyoma during the study period, FH-associated morphologic features were reported in 1.5% (37 patients), and FH immunohistochemistry was performed in 29 (1.19%). Fourteen (48.27%) of the 29 patients showed FH protein deficiency by immunohistochemistry. Twelve patients underwent germline testing, of which 8 (66.7%) were classified as group 1 and 4 (33.3%) as group 2. FH protein-deficient tumors were larger (10.44 vs 4.08 cm, P  =  0.01) and associated with younger patients (42.05 vs 47.97, P  =  0.004) than 370 randomly selected uterine leiomyoma controls. Groups 1 and 2 showed no significant differences in patient age and tumor size. In group 1 tumors, the FH-associated morphologic features were generally present diffusely; all group 1 tumors displayed ≥5 FH-associated features, whereas all group 2 tumors displayed <5 FH-associated features (means 6.5  ±  0.53 vs 3.5  ±  1.00, P < 0.001). Notably, eosinophilic/fibrillary cytoplasm and alveolar-type edema were each significantly more prevalent in group 1 tumors than group 2 tumors (P  =  0.018 for both). No single morphologic feature was found to be completely sensitive and specific in making the distinction between group 1 and 2 tumors. Our findings suggest that groups 1 and 2 are unlikely to be morphologically distinguishable by individual morphologic features. Whether there is a combination of features that can reliably make this distinction is unclear and will require additional studies with larger cohorts.