磷酸化间质瘤（PMT）是一种罕见的肿瘤，主要通过分泌成纤维细胞生长因子23而导致低磷血症/骨软化症。我们之前在几乎一半PMT中发现了FN1::FGFR1/FGF1融合基因，而仅在未知融合基因的肿瘤中频繁表达KL（Klotho或α-Klotho）。在这里，我们对更大规模的PMT队列进行了KL表达和变异的研究。通过FN1分离荧光原位杂交（FISH）和重新评估先前的RNA测序数据，将之前被认为“融合阴性”的6个肿瘤（通过FN1::FGFR1融合和FGF1分离以及/或RNA测序的阴性结果进行定义）重新分类为融合阳性的PMT，其中一个含有新的FN1::ZACN融合基因。最终的融合阴性PMT队列包括32位患者的33个肿瘤，分布于骨骼（N=18）、软组织（10）、鼻窦道（4）和脑部。与先前的研究相结合，RNA测序、RNA原位杂交和免疫组织化学显示出基本一致的结果，并证明在28个融合阴性和10个融合阳性PMT中，有17个出现KL/α-Klotho过度表达，而没有一个出现KL/α-Klotho过低的。在这些融合阴性肿瘤中，有16个发生KL重排，包括一个放大的病例。其中有14个病例表现为KL/α-Klotho过度表达，而在11个KL/α-Klotho表达低、11个融合阳性PMT中没有发现KL重排。全基因组测序确认了2个FISH阳性病例，分别涉及KL上游区的易位和倒位，有待进一步探究这些重排可能导致KL上调的机制。甲基化DNA免疫沉淀和测序提示甲基化启动子在PMT中对KL调控没有主要作用。有趣的是，KL高/重排的病例似乎形成了一个临床病理学上同质的群体，倾向于骨骼/鼻窦部位，并且通常呈现贫基质、细胞孤立性纤维瘤样形态。重要的是，通过基因集富集分析，与非PMT间质肿瘤相比，无论KL状态如何，融合阴性PMT中的FGFR1信号通路都有上调，这可能证实了在治疗这一亚型PMT中抑制FGFR1的合理性。版权所有 © 2023 Elsevier Inc.

Phosphaturic mesenchymal tumors (PMT) are uncommon neoplasms that cause hypophosphatemia/osteomalacia mainly by secreting fibroblast growth factor 23. We previously identified FN1::FGFR1/FGF1 fusions in nearly half of PMTs, and frequent KL (Klotho or α-Klotho) overexpression only in those with no known fusion. Here, we studied a larger cohort of PMTs for KL expression and alterations. By FN1 break-apart FISH and reappraisal of previous RNA sequencing data, 6 tumors previously considered "fusion-negative" (defined by negative results of FISH for FN1::FGFR1 fusion and FGF1 break-apart and/or of RNA sequencing) were reclassified as fusion-positive PMTs, including one containing a novel FN1::ZACN fusion. The final cohort of fusion-negative PMTs included 33 tumors from 32 patients, which occurred in bone (N=18), soft tissue (10), sinonasal tract (4) and brain. In combination with previous work, RNA sequencing, RNA in situ hybridization, and immunohistochemistry showed largely concordant results and demonstrated KL/α-Klotho overexpression in 17 of the 28 fusion-negative and none of the 10 fusion-positive PMTs studied. Prompted by a patient in this cohort harboring germline KL upstream translocation with systemic α-Klotho overexpression and multifocal PMTs, FISH was performed and revealed KL rearrangement in 16 of the 33 fusion-negative PMTs (one also with amplification), including 14 of the 17 cases with KL/α-Klotho overexpression and none of the 11 KL/α-Klotho-low fusion-negative and 11 fusion-positive cases studied. Whole genomic sequencing confirmed in 2 FISH-positive cases translocation and inversion, respectively, involving KL upstream region, warranting further investigation into the mechanism whereby these rearrangements may lead to KL upregulation. Methylated DNA immunoprecipitation and sequencing suggested no major role of promoter methylation in KL regulation in PMT. Interestingly, KL-high/-rearranged cases seemed to form a clinicopathologically homogeneous group, showing a predilection for skeletal/sinonasal locations and typically matrix-poor, cellular solitary fibrous tumor-like morphology. Importantly, FGFR1 signaling pathways were upregulated in fusion-negative PMTs regardless of the KL status compared with non-PMT mesenchymal tumors by gene set enrichment analysis, perhaps justifying FGFR1 inhibition in treating this subset of PMTs.Copyright © 2023. Published by Elsevier Inc.