Patient information and medical history
A female which is 59 years old. In 2016, CT showed dense nodule shadow, high-density lock strip shadow and fuzzy patch shadow in the left lower lung, and focal thickening of the left pleura. Therefore, "left chest wall tumor resection + left lower lung tumor resection" was performed. Pathological report: low to moderate malignant fibrosarcoma(fibromyxoid type). In 2018, a peanut sized tumor was found on the left chest wall, which was hard without tenderness and untreated. CT Reexamination in 2020 showed multiple solid nodules in both lungs with clear boundary; The left chest wall is a kind of round mass shadow, which seems to have a pedicle connected with the rear muscle and is unevenly strengthened. It increased like an egg within one year. The CT re-examination in 2021 showed that there were multiple solid nodules in both lungs and tumors on the left chest wall, and the tumors on the left chest wall were significantly larger than before. No abnormality was found in other examinations.
Gross morphology and pathological diagnosis
The tumor is complete, the boundary is clear, the size is 6x5x4cm, the section is gray and white, and the local part is yellow and white(Fig. 1). Although the tumor generally seems to have a clear boundary, it infiltrates into adjacent tissues under microscope. At low magnification, we can see that the tumor is composed of alternating collagen like and mucinous areas. There is migration or transition between the two areas, and we can also see a relatively clear boundary. At high magnification, the tumor cells are consistent in morphology, spindle or short spindle, and star shaped in the myxoid area, similar to fibroblasts. The nucleus is round or oval, deeply stained, and the chromatin is evenly distributed. The mitotic nucleus is not obvious; The cytoplasm was lightly stained and the cell boundary was unclear. The tumor cells are arranged in bundles, linear arrangement or disorderly distribution. The blood vessels in tumor cells are relatively rare, mostly arched, curved or arc-shaped. In myxoid areas, sometimes branching capillary networks similar to myxoid liposarcoma can be seen. In the local area, the density of tumor cells increased significantly, mixed with collagen fibers, and small focal sclerosing epithelioid fibrosarcoma appeared(Fig. 2).
Immunohistochemical results showed that SMA, Desmin, CD34, STAT6, S100, SOX10, HMB45 and Melan A were negative, EMA was weakly positive, MUC4 was diffuse and strongly positive, and Ki-67 index was low(3%). FISH results showed that DDIT3 gene mutation was not found, but FUS gene was break mutation(Fig. 3).
Next-generation sequencing technology
In order to find out whether there are mutations in other genes in low-grade fibromyxoid sarcoma except FUS-CREB3L2 fusion, FUS-CREB3L1 and EWSR1- CREB3L1 fusion [4, 15, 18, 19]. Subsequently, we conducted next-generation sequencing experiment, which showed that there were mutation in four gene MET, EGFR, KMT2B and RET(Table 1). It is noteworthy that KMT2B mutation in Low-grade fibromyxoid sarcoma has not been reported.
KMT2B belongs to a member of the histone lysine N-methyltransferase 2 family. KMT2, also known as mixed lineage leukemia(MLL), is divided into KMT2A, KMT2B, KMT2C, KMT2D, KMT2F, KMT2G, etc. of which the two have high homology(KMT2A and KMT2B, KMT2C and KMT2D, KMT2F and KMT2G). The first four are the most common gene changes in tumor types, among which the mutation rate is higher in melanoma, Endometrial carcinoma and lung cancer. The mutation frequencies of single genes in KMT2 family are KMT2D(18%), KMT2C(15%), KMT2A(9%)and KMT2B(8%).
KMT2B, also known as mixed lineage leukemia 2(MLL2), OMIM 606834, is located on chromosome 19q13 12. It is composed of 2715 amino acids and expressed in most tissues(24), which has a similar gene structure to MLL1(KMT2A)located on chromosome 11q23. The structural components of the gene include the catalytically active C-terminal SET domain, a CXXC domain, an AT hook and several plant homeotic domains(PHD)in the N-terminal region[27, 29]. The SET domain binds to methyltransferase cofactor S-adenosylmethionine and the N-terminal tail of histone H3 catalyzing the methylation reaction, The CXXC domain composed of two zinc ions and four cysteine residues(Cys4),also known as the Zinc-finger(ZF)-CXXC domain, recognizes and binds to non-methylated CpG DNA, being critical for the association of MLL2 to chromatin,CXXC recognizes CpG islandsof most promoters, and PDH is next to zinc finger(ZF)-CXXC, including PDH1-PDH4, PDH possess a Cys4-His-Cys3 motif, coordinated by two zinc ions and mediating binding to methylated histone H3, Although all MLL families contain PDH, PDH3 of MLL2 show different specificity, which mainly binds to H3K4me3 tails.The C-terminal SEF domain of KMT2B forms a complex with WRAD(WDR5, RbBP5, ASH2L, ASH2L), host cell factors 1/2(HCF 1/2)and Menin[30, 35–37], the complex is responsible for the binding of PDH3 and H3K4me3, regulating bivalent developmental genes as well as stem cell and germinal cell Differentiation gene sets.In addition, MLL2 plays a key role in embryonic development, the deletion of MLL2 is related to early growth retardation, neural tube defects and apoptosis leading to embryonic death, it is also involved in the control of autonomic movement and the pathogenesis of dystonia in early children[38–40]. At the same time, MLL2 also has cancer promoting effects, including in colorectal cancer, gastric cancer, glioblastoma , etc.
Epidermal growth factor receptor(EGFR, also known as ErbB1 or HER-1)is a transmembrane glycoprotein consisting of an extracellular ligand-binding domain and a cytoplasmic tyrosine kinase domain. EGFR belongs to a family of four related receptor tyrosine kinases that play key mediators in cell signaling pathways such as proliferation, apoptosis, angiogenesis and metastasis. It has been reported that FUS contains two targeted EGFR phosphorylation sites, mainly Y6 and Y304 in the FUS, suggesting that FUS can be phosphorylated by EGFR, which promoting FUS phosphorylation and inducing nuclear translocation through activated EGFR, so that FUS can mediate collagen production and creates a collagenous background. Therefore, EGFR-mediated FUS phosphorylation regulates FUS nuclear translocation and promotes transcription of fibrotic collagen genes .
Table 1 The result of Next-generation sequencing.