Tumors in this group account for about 50% of soft tissue sarcomas and characterized by pleo-morphic/spindle cell morphology, including Pleomorphic Malignant Fibrous Histiocytoma / undifferentiated (PMFH), pleomorphic liposarcoma (PLPS), leiomyosarcoma (LMS), pleomorphic rhabdomyosarcoma (PRMS) and malignant peripheral nerve sheath tumor (MPNST). Chromosomal breakpoints in these tumors are widely scattered, with no predilection of any of the recurrent breakpoints and losses to any of the morphological subtypes. However, high grade dedifferentiated liposarcoma in addition have distinctive genomic abnormalities (giant chromosome, ring chromosomes, amplification of 12q13-q21), similar to well-differentiated liposarcoma.
Pleomorphic Malignant Fibrous Histiocytoma/ Undiffrentiated sarcoma (PMFH)
PMFH has been considered to be the most frequent soft tissue sarcomas in adults. The cellular origin of PMFH has long been unknown, although different theories claimed that the tumor originates from histiocytes, fibroblasts, or primitive mesenchymal cells.
High-grade PMFH exhibits multiple complex chromosomal abnormalities with no specific aberrations, they often present cytogenetic signs of gene amplification, that is, homogeneously staining regions (hsr), double minute chromosomes (dmin), as well as add (19p) [20
]. These hsr composed of amplified DNA sequences from 12q13→q15 are commonly detected. Moreover, amplicons of the human homologue of MDM2, cyclin-dependent kinase 4 (CDK4), sarcoma amplified sequence (SAS), and high-mobility group protein IC (HMGIC) genes, which are mapped to this region, have been detected in PMFH. Both the MDM2 and CDK4 genes play a major role in permitting override of the G1-S cell cycle checkpoint in cell proliferation. Since the HMG proteins can bind to DNA and are involved in the organization of chromatin during DNA transcription, they have been referred to as architectural transcription factors. Ezrin (villin2), a protein that serves as an intermediate between the plasma membrane and the actin cytoskeleton, is considered as a marker of cancer progression and a potential target for cancer therapy. It plays a key role in cell morphology, adhesion, migration, and organization. Ezrin is over-expressed in many neoplasms including PMFH, and this over-expression was found to correlate with increased metastatic potential and reduced survival [40
Pleomorphic Liposarcoma (PLPS)
PLPS is the least common variant of liposarcoma, accounting for less than 5%. It usually occurs in elderly people (median age 55-65 years) predominantly in deep soft tissues (75% of cases) of the lower extremities, especially the thigh. PLSP characteristically shows high chromosome counts and complex structural arrangements. Deletion of 13q14.2-q14.3, targeting the RBI pathway, is observed in about 60% of PLPS [41
]. Despite their morphologic similarity, PLPS and high-grade dedifferentiated LPS show distinct chromosomal imbalances. Gains of 5p13-p15, 1p21, 1q21-q22, and 7q22 are more frequently observed in PLPS whereas high-level amplifications within chromosomal sub-region 12q13-q21 are observed in dedifferentiated LPS. MAD2, a gene involved in the RBI signaling pathway over-expressed by PLPS (and dedifferentiated LPS), might constitute a therapeutic target [42
LMS accounts for about 8–10% of adult soft tissue sarcomas. These malignant neoplasms show varying degrees of smooth muscle differentiation and can develop anywhere in the body. LMS usually show complex karyotypic alterations that differ generally from one tumor to another. Many LMSs show chromosomal imbalances or aberrations in the form of gains (chromosomes 1, 5, 6, 8, 15, 16, 17, 19, 20, 22, X), losses (chromosomes 1p, 2, 3, 4, 6q, 8, 9, 10p, 11p, 12q, 11q, 13, 16, 17p, 18 19, 22q), and amplifications (chromosomes 1, 5, 8, 12, 13, 17, 19, 20). Some gains and losses of chromosomal material are more frequently observed and tend to correlate with poor outcome, large tumor size, and metastatic dissemination (e.g. loss of 1p12-pter, 2p, 13q14-q21 (targeting the Rb pathway), 10q (targeting PTEN) , and 16q; gains of 17p, 8q, and 5p14 pter). Activation of the PI3K-AKT pathway through different mechanisms (e.g., activation of IGFR, inactivation of PTEN (a negative regulator of the PI3K-AKT) also plays a crucial role in the development and maintenance of LMS. This activation leads to the concomitant activation of downstream effectors such as mTOR and its targets (β-catenin, pS6, p4E-BP1, etc.), as well as to MDM2 stabilization. Recent clinical trials showed that analogs of rapamycin, such as the mTOR inhibitor everolimus (RAD001), have some efficacy in patients with LMS [43
]. It has also been realized that the more differentiated retroperitoneal LMSs tend to behave more aggressively and that this was mainly dependent upon amplification/overexpression of myocardin. Myocardin is a transcriptional cofactor of SRF that regulates smooth muscle differentiation. Inactivation of the myocardin pathway results in a significant reduction of smooth muscle differentiation, cell proliferation, and cell migration and was associated with less differentiated histology. These data suggest that myocardin might constitute a promising therapeutic target.
Pleomorphic Rhabdomyosarcoma (PRMS)
PRMS usually occurs in the extremities, especially the thigh, often in middle-aged men. It is an aggressive tumor, prone to recurrence and metastasis to the lungs. It shows nonspecific complex karyotypes. Numerical and unbalanced structural abnormalities are common with gains (chromosomes 1, 5, 8, 14, 18, 20, and 22) and loss (chromosomes 2, 5, 6, 10, 11, 13, 14, 15, 16, 17, 18, 19 and Y) of chromosomes, of which losses of chromosomes 2, 13, 14, 15, 16, and 19 are the most frequent. PRMS does not contain the t(2;13) or t(1;13) characteristic of alveolar rhabdomyosarcoma.
Malignant peripheral Nerve Sheath Tumor (MPNST)
MPNST occurs mostly in middle- to advanced-aged adults, without sex predilection. Fifty percent to 70% develop in a preexisting neurofibroma, especially in plexiform neurofibroma associated with NF1 and sporadic MPNSTs are less frequent. However there are no significant prognostic differences between NF1-associated and sporadic MPNSTs. Cytogenetically, MPNSTs display complex karyotypes and clonal chromosomal aberrations and loss of chromosome seem to be more frequent than gains. CGH studies demonstrate losses in chromosomes 17, 19p and 22q in NF1-associated neurofibromas, suggesting inactivation of tumor suppressor genes in the development, however, in MPNST gains are more frequent than losses, probably relating to proto-oncogene activation during MPNST progression. Losses of chromosomes (1p12-13, 1p21, 1p36, 3p21-pter, 9p13 -21, 9p22-24, 10, 10p11-15, 11p, 11q21-25, 13q14, 15p, 16/16q24, 17/17p, 17q11-12, 17q21-25, 22, 22p, 22q13, and 22q11-12) were the most frequent abnormalities observed. Gains mainly involved the regions 7p21-q36, 7p22, 7q, 8, 8q11-23, 1q25-44, and 5q13-35. Gain of 7p15-p21 was also found associated with poor prognosis and shortened survival [44
]. cDNA microarray analysis has found that six genes (keratin 18, survivin, tenascin C, adenosine deaminase, collagen type Vla3, and collagen type Vllal) were significantly upregulated in MPNST, whereas one gene, insulin-like growth factor binding protein 6, was downregulated in MPNST. MPNST is a highly aggressive sarcoma for which no effective treatment currently exists. PDGF-BB was identified as the most effective factor that induces MPNST cell invasion and proliferation. Expression of PDGF-BB receptor (PDGFR-β) mRNA was detected more frequently and its protein was expressed at higher levels in MPNST tissues than benign peripheral nerve sheath tumors (schwannomas and neurofibromas). The effectiveness of imatinib mesylate in vitro
suggests that targeting PDGFR-β may result in the establishment of novel treatments for MPNST [45
]. Survivin and tenascin C expression was validated on RT-PCR. Immunohistochemis-try confirmed upregulation of survivin in MPNST at the protein level.
In summary, genetic analysis of soft tissue sarcomas has proven to be invaluable for diagnostic and clinical management. In the future, clinical decisions will increasingly be based on a combination of histologic criteria and the molecular identification of genetic abnormalities that are indicative of biologic properties. Furthermore, elucidation of the functions of sarcoma-specific fusion proteins will continue to improve understanding of the oncogenic process and will lead to the identification of new therapeutic targets for the treatment of sarcomas.