MET-overexpressing myxofibrosarcoma frequently exhibit polysomy of chromosome 7 but not MET amplification, especially in high-grade cases: clinical and pathological review of 30 myxofibrosarcoma cases

A total of 30 patients were identified and retrieved from several pathological centers: Xijing Hospital (12 cases, including 1 epithelioid subtype), Xihua Hospital (1 case, epithelioid subtype), Hubei Provincial Cancer Hospital (11 cases), Affiliated Foshan Hospital, Sun Yet-sen University (6 cases). The 30 patients included 16 males and 14 females with a mean age of 56 years (range: 21–87 years) (Table 1). All tumors were graded according to the FNCLCC: 6 cases were classified as grade 1 (Fig. 1a), and 12 cases as grade 2 (Fig. 1b) and the left 12 cases as grade 3 (Fig. 1c). Two cases of grade 3 were epithelioid MFS, showing an infiltrative multinodular growth pattern with alternation of hypercellular and hypocellular myxoid areas. Tumor cells had epithelioid morphology with abundant eosinophilic cytoplasm and oval or plump spindle nuclei, scattered in the myxoid background (Fig. 1d). Here, 15 of the 30 cases were followed up for 7 to 42 months. All the slides were re-examined by two pathologists to ensure correct diagnosis. The specimen collection and study procedures were approved by the Ethics Committee of Xijing Hospital.

MET reactivity was predominantly seen in the cytoplasm and membrane. Immunostaining was evaluated on the basis of the staining intensity (negative, weak, moderate, strong) and the percentage of cells stained, according to the previous studies of MET IHC scoring criteria [11, 14] as follows: 0 (no staining or < 50% of tumor cells with any intensity), 1+ (≥ 50% of tumor cells with weak or higher staining but < 50% with moderate or higher intensity), 2+ (≥ 50% of tumor cells with moderate or higher staining but < 50% with strong intensity), 3+ (≥ 50% of tumor cells stained with strong intensity), MET overexpression was defined as a score of 2+ or 3+. IHC scores were independently evaluated by two pathologists who were blinded to all the cases.

FISH was performed on formalin-fixed, paraffin-embedded tissue sections in accordance with the manufacturer’s instructions. Unstained 4 μm sections were placed on electrostatically charged slides and then evaluated using MET (7q31) probe and centromere 7q (CEP7q) dual-color probe (LBP, Guangzhou, China). The hybridized slides were reviewed on an Olympus IX-50 microscope (Olympus, Tokyo, Japan) at 100× magnification with oil immersion using a DAPI/Green/Red triple band pass filter set. The tissues were scored by evaluating a minimum of 40 tumor nuclei per sample. The amplification of MET was defined as 2 ≦ MET gene (red)/CEP7q (green) per cell on average. We defined polysomy of chromosome 7 as three or more signals on average of CEP7 (green) per cell [9, 13]. To avoid false-positive results originating from the nuclear truncation that occurs in a subset of cells in paraffin-embedded samples, we excluded overlapping cells that did not clearly have separate nuclei from the current analysis.

For immunohistochemistry, MET overexpression was observed in 14 cases (including 1 case of epithelioid variant) of 30 cases (46.7%, Table 1), which consisted of 6 males and 8 females with a median age of 61 years (range, 38–87 years). Moreover, the positivity varied with tumor grade, and the positive cases were 1, 3 and 10 cases for grade 1 to 3, respectively (Fig. 2a, b), one of which was high grade epithelioid myxofibrosarcoma (Fig. 2c). Other MET-negative cases were of lower grade (Fig. 2d).

We further examined MET gene status using FISH with the probe MET/CEP7. There were also 11 cases with 3 signals on average of MET and more than 3 signals (4.6 signals on average) of CEP7. However, the MET/CEP7 was about 0.65 on average. This is suggested that chromosome 7 polysomy, rather than MET amplification, led to the overexpression of MET protein. Among the 11 cases, chromosome 7 polysomy was preferentially detected in cases with higher FNCLCC grade, for 2 of the 11 cases that were classified as grade 2 (Fig. 3a), and the other 8 cases that were classified as grade 3 (Fig. 3b), including one epithelioid variant (Fig. 3c). In contrast, most of the negative cases had lower FNCLCC grade (Fig. 3d).

As shown in Table 2 and Figs. 2 and 3, MET overexpression and chromosome 7 polysomy were statistically positively correlated with higher FNCLCC grade (P = 0.004, P = 0.021). In contrast, MET overexpression and chromosome 7 polysomy were almost absent from grade 1 tumors in our study. The 11 positive cases were also positively correlated with higher mitotic rate (P = 0.029), but it was not related to age, gender, or location.

Except for one censored case, 15 patients were followed up for survival. One patient died of metastasis seven months after surgery. There were 7 cases had local recurrence, of which one had metastasis to axillary (Table 1). Among the 15 follow up cases, there were 5 cases positive for MET overexpression and chromosome 7 polysomy, and this correlation was statistically significant (P = 0.010), which indicated that MFS patients with MET overexpression or chromosome 7 polysomy might have a high risk of local recurrence and metastasis. However, Log-rank test showed there to be no significant relationship between MET overexpression and chromosome 7 polysomy patients and the survival time of MFS patients after surgery (P = 0.317).