Cellular/intramuscular myxoma and grade I myxofibrosarcoma are characterized by distinct genetic alterations and specific composition of their extracellular matrix

Abstract

Cellular myxoma and grade I myxofibrosarcoma are mesenchymal tumours that are characterized by their abundant myxoid extracellular matrix (ECM). Despite their histological overlap, they differ clinically. Diagnosis is therefore difficult though important. We investigated their (cyto) genetics and ECM. GNAS1-activating mutations have been described in intramuscular myxoma, and lead to downstream activation of cFos. KRAS and TP53 mutations are commonly involved in sarcomagenesis whereby KRAS subsequently activates c-Fos. A well-documented series of intramuscular myxoma (three typical cases and seven cases of the more challenging cellular variant) and grade I myxofibrosarcoma (n = 10) cases were karyotyped, analyzed for GNAS1, KRAS and TP53 mutations and downstream activation of c-Fos mRNA and protein expression. ECM was studied by liquid chromatography mass spectrometry and expression of proteins identified was validated by immunohistochemistry and qPCR. Grade I myxofibrosarcoma showed variable, non-specific cyto-genetic aberrations in 83,5% of cases (n = 6) whereas karyotypes of intramuscular myxoma were all normal (n = 7). GNAS1-activating mutations were exclusively found in 50% of intramuscular myxoma. Both tumour types showed over-expression of c-Fos mRNA and protein. No mutations in KRAS codon 12/13 or in TP53 were detected. Liquid chromatography mass spectrometry revealed structural proteins (collagen types I, VI, XII, XIV and decorin) in grade I myxofibrosarcoma lacking in intramuscular myxoma. This was confirmed by immunohistochemistry and qPCR. Intramuscular/cellular myxoma and grade I myxofibrosarcoma show different molecular genetic aberrations and different composition of their ECM that probably contribute to their diverse clinical behaviour. GNAS1 mutation analysis can be helpful to distinguish intramuscular myxoma from grade I myxofibrosarcoma in selected cases.

Figure 1

Overlapping histology of intramuscular myxoma, cellular myxoma and grade I myxofibrosarcoma and their immunohistochemical expression for c-Fos, decorin, collagen I, collagen VI and CD44. (A) Low-power view of intramuscular myxoma showing a hypocellular and hypovascular tumour. Its cellular variant is both more cellular and more vascular (B) but lacks the cytonuclear atypia and characteristic curvilinear vascular pattern of grade I myxofibrosarcoma (C). Low-power view of intramuscular myxoma (D) showing moderate cytoplasmic and nuclear expression for c-Fos in the majority (> 75%) of tumour cells. (E) Low-power view of intramuscular myxoma was completely negative for decorin, whereas grade I myxofibrosarcoma showed diffuse fibrillary staining for decorin in the ECM (F). Lack of collagen I expression in the ECM of intramuscular myxoma (G) and moderate staining for collagen VI in the ECM of grade I myxofibrosarcoma (H). A majority of tumour cells of grade I myxofibrosarcoma showed strong membranous staining for CD44 in the majority of tumour cells (I).

Figure 2

Box-plots showing qPCR results of structural ECM proteins. Abbreviations: IM, intramuscular myxoma; MFS, grade I myxofibrosar-coma. Intramuscular myx-oma showed significantly lower mRNA expression for decorin (P= 0.000), collagen I-A1 (P= 0.003), collagen VI-A1 (P= 0.023) and collagen XIV-A1 (P= 0.001).