The histological and molecular spectrum of lipoblastoma: A case series with identification of three novel gene fusions by targeted RNA-sequencing

Lipoblastoma in one adult and 35 pediatric patients: Retrospective analysis of 36 cases

Lipoblastoma is a rare benign mesenchymal neoplasm that typically occurs at various sites in infants and children but may also occur in adults. Thus, differential diagnoses are often performed. To understand this tumor type, the present study described clinicopathological features, diagnosis and differential diagnosis of different morphological lipoblastomas. A single-institution retrospective review of 36 lipoblastoma cases diagnosed between 2015 and 2021 was performed. Formalin-fixed paraffin-embedded tissue was used for S-100, CD34, P16 and desmin immunohistochemistry analysis, along with rapid fluorescence in situ hybridization (FISH) detection with pleiomorphic adenoma gene 1 (PLAG1). The 36 cases included 14 females and 22 males [age range, 7 days to 33 years (median, 16.5 years); 28 patients were aged ≤3 years] and the tumors were located in the trunk (n=16), limbs (n=12), head and neck (n=6), and perineum (n=2). Histologically, lipoblastomas were divided into classic (n=15), lipoma-like (n=13) and myxoid (n=8) subtypes. They comprised lobules of mature adipose tissue of varying size and a fine capillary network surrounded by mucinous stroma. Single- or multivesicular lipoblasts positive for S-100 (29/36, 81%) were observed, with occasional mature adipocytes. Peripheral vessels and cytoplasm of primitive mesenchymal cells were diffusely positive for CD34 (36/36, 100%), whereas primitive mesenchymal cells and striated muscle tissue were positive for desmin (26/36, 72%). Most tumor cells were negative while only few were positive for P16 (8/36, 22%). FISH revealed PLAG1 breakage and rearrangement in 24/32 (75%) patients. In total, 28 patients were followed up post-operatively (range, 2-84 months; median, 41 months; 3 patients relapsed and 8 were lost to follow-up). In conclusion, diagnosis of a typical lipoblastoma is not difficult and PLAG1 breakage detection is key for the diagnosis.

Pediatric fibromyxoid tumor with PLAG1 fusion: An emerging entity with a novel intracranial location

Translocations involving PLAG1 occur in several tumors, most commonly pleomorphic adenoma and lipoblastoma. Recently, a distinctive soft tissue tumor with a PLAG1 fusion has been reported in the pediatric age group. These are low grade tumors with a fibroblastic or mixed fibroblastic and myxoid morphology but no other lines of differentiation. They are typically immunopositive for desmin and CD34. The partner genes for these tumors have included YWHAZ, EEF1A1, ZFHX4l, CHCHD7, and PCMTD1. We report another case of this fibromyxoid tumor with a PLAG1 fusion, this time with COL3A1 as the partner gene. The fusion placed expression of a full-length PLAG1 protein under the control of the constitutively active COL3A1 promoter. Overexpression of PLAG1 was confirmed by diffusely positive immunostaining for PLAG1. The most novel aspect of this tumor is the intracranial location. Opinion has been divided over whether these tumors are a specific entity, or related to lipoblastoma, since that tumor also typically occurs in soft tissue in the pediatric age group and shows many of the same gene fusions. However, lipoblastoma has never been reported in an intracranial location and, thus, our case provides compelling evidence that this fibromyxoid tumor is indeed a distinct entity.