Myxoid/Round Cell Liposarcoma P Åman, University of Gothenburg, Gothenburg, Sweden © 2013 Elsevier Inc. All rights reserved.
Glossary Basic-leucine zipper Proteins structure that binds other leucine zipper containing proteins. C/EBP (CAAT/enhancer binding factors) A family of transcription factors. FISH (fluorescence in situ hybridization) A method for detection of DNA sequences in chromosomes, chromatin, or RNA.
Definition Myxoid liposarcoma (MLS) is a malignant soft tissue tumor composed of mesenchymal tumor cells and a variable number of lipoblasts. Round cell liposarcoma (RCLS) is a morphologi cally distinct and more aggressive variant of MLS. In many cases, the two forms are mixed within the same tumor.
Characteristics MLS/RCLS is one of the most common types of liposarcoma and accounts for more than a third of all cases. The incidence increases with age and peaks at middle age. MLS/RCLS most often presents as a large painless mass. In �70% of the cases, the tumors are found in the large muscles of the thigh, but they may occur in deep seated locations in any part of the body. The tumors are multinodular with pale, myxoid tissue surfaces. MLS/RCLS tissue is composed of round to oval-shaped mesenchymal cells and a variable number of lipoblasts in a myxoid matrix containing a rich capillary network with a typi cal branched morphology. A subset of the tumors shows a hypercellular round cell morphology (RCLS) with less of the myxoid component. Hypercellular areas with an undifferentiated round cell mor phology ranges from 5% to 80% in the mixed variant to more than 80% in the pure round cell type.
Genetics MLS/RCLS tumor cells carry with few exceptions a t(12;16) chromosome translocation or, more rarely, a t(12;22). The translocations are not found in normal tissues of the patients, and there are no indications of heredity. The translocations result in fusions of FUS(akaTLS)on chromosome 16 or EWSR1on chromosome 22 with DDIT3 (aka CHOP) on chro mosome 12. The resulting fusion oncogenes typically consist of exons 1–5 or 1–7 of FUS, or exons 1–7 of EWSR1juxtaposed to exon 2 of DDIT3. Other, rare fusion variants are also reported. In some cases, alternative splicing produces two or more iso forms of the fusion transcripts and proteins within the same tumor. The variant break points are not associated with
Brenner’s Encyclopedia of Genetics, 2nd edition, Volume 4
RT-PCR Reverse transcription of RNA to DNA followed by a polymerase chain reaction for quantification or fragment/sequence analysis of RNA.
prognosis or tumor subtype. All fusions result in the formation of fusion proteins containing the N-terminal parts of FUS or EWS linked to the entire DDIT3 protein (Figure 1). The FUS DDIT3 and EWSR1-DDIT3 belongs to the FET family of fusion oncogenes in which all members have one of the three closely related genes FUS, EWSR1, or TAF15 as 5′ fusion partners and one of many different transcription factor encoding genes as 3′ partners (Figure 2). Each of the fusion oncogenes are, with exception for FUS-ERG, found in only one tumor entity. A mutation in PIK3CA is found in around 20% of MLS/ RCLS cases and is associated with a poor prognosis.
Etiology The mechanism causing the chromosome translocations in MLS/RCLS is not well understood. No specific sequence motifs are reported in or around the break-point regions and the translocations are probably the result of random events. The FUS-DDIT3 fusion gene has been shown to cause lipo sarcomas in transgenic mice and in mice inoculated with mesenchymal stem cells carrying FUS-DDIT3. FUS-DDIT3 also transforms NIH3T3 fibroblasts. Based on these observa tions, it is concluded that FUS-DDIT3 is a strong causing factor in development of MLS/RCLS. FUS-DDIT3 and the normal DDIT3 can induce an MLS/RCLS phenotype when expressed in human low-differentiated sar coma cells showing that the tumor morphology in MLS/ RCLS depends on the DDIT3 part of the fusion protein. The ‘instructive’ effect of FUS-DDIT3 may explain why this lipoblast-containing tumor occurs most often in muscle and rarely in adipose tissue. In addition to MLS/RCLS, DDIT3 is also overexpressed in well-differentiated liposarcomas and prob ably contributes to the liposarcoma phenotype in this tumor. DDIT3 encodes a DNA-binding transcription factor of basic-leucine zipper type (C/EBP family) that binds to DNA as a dimer. DDIT3 cannot form homodimers, but other C/EBP family members are considered as major dimer partners. The dimer forming function is retained in the FUS-DDIT3 fusion protein. FUS-DDIT3 may, therefore, interfere with C/EBP family transcription factors and their binding to transcription regulatory C/EBP sites. The DDIT3 protein has been implicated in adipocyte differentiation. C/EBPα is a critical factor for development of
Myxoid/Round Cell Liposarcoma
FUS-DDIT3 fusion protein DDIT3 DNA BIND L-ZIPP FUS-DDIT3 FUS RNP-1
Figure 1 The FUS-DDIT3 encoded fusion protein consists of N-terminal parts of the FUS protein juxtaposed to the entire DDIT3 protein. In the junction, 26 amino acids from normally nontranslated parts of DDIT3 are added to the fusion protein. L-ZIP, leucine zipper dimer-forming domain; RNP, RNA-binding domain.
adipose tissue and blocks proliferation as preadipocytes enter terminal differentiation. FUS-DDIT3 has been shown to interfere with this function by dimer formation with C/EBPα. The FUS-DDIT3 and EWSR1-DDIT3 encoded chimerical proteins probably act as abnormal transcription factors and several affected target genes such as IL6 and IL8 have been identified. Consistent with a role in transcriptional regulation, the fusion proteins have been found to localize predominantly to the nuclei and in a minority of the cells to multiple nuclear structures that also contain transcription- and splicingassociated factors. FUS, EWSR1, and TAF15 encode RNA-binding proteins with extensive similarities. Their proteins are found within protein complexes that participate in transcription, splicing, and mRNA transport. Some reports suggest functions in translational control and in stress response. FUS-deficient mice exhibit increased radiation sensitivity, deficient DNA
repair, and male sterility. These observations suggest that the FUS protein participate in DNA repair and homologous recombination. The natural functions of the N-terminal parts of FUS, EWSR1, and TAF15, which are present in the chimerical onco proteins, are not known. When expressed as fusion proteins with DNA-binding transcription factors, they have been shown to act as transactivating domains. Immunohistochemistry analysis of growth and cell cycle controlling factors in MLS/RCLS revealed a strong expression of cyclin D1, cyclin E1, CDK4, and CDK6 in the majority of the tumor cells. The tumor suppressor protein P16 (aka CDKN2a or INK4a) is also strongly expressed in most tumor cells.
Diagnostics The diagnosis of MLS/RCLS is based on microscopic examina tion of tissue specimens. MLS/RCLS tissue is characterized by presence of lipoblasts, myxoid matrix, and the typical plexi form capillary network. Fluorescence in situ hybridization (FISH) or Reverse transcription polymerase chain reaction (RT-PCR) detects the FUS-DDIT3 or EWSR1-DDIT3 fusion oncogenes, and tests for the fusion genes may be helpful when the morphology is unclear.
Treatment Surgery remains the main treatment of MLS/RCLS. However, recent studies have shown that MLS/RCLS tumors are sensitive to radiation therapy. Surgery is, therefore, often combined with pre- or postoperative radiotherapy. In cases where surgery is out of option, radiation and che motherapy is employed.
Acute myeloid leukemia
FLI1 ETV1 EWSR1
E1AF FEV Clear cell sarcoma
Desmoplastic small cell tumor
Askin-like cd99 negative sarcoma
Extraskeletal myxoid chondrosarcoma Acute lymphatic leukemia Acute myeloid leukemia
Figure 2 Some members of the FUS, EWSR, TAF15 group of fusion oncogenes. All fusion oncogenes of this group contain the 5′ parts of FUS, EWSR1, or TAF15 juxtaposed to the 5′ or central parts of transcription factor genes. With exception for FUS-ERG, which is found in myeloid leukemia and in Ewing sarcoma, the fusion oncogenes are specific for one tumor entity.
Myxoid/Round Cell Liposarcoma
Local recurrence is common in MLS but the pure MLS variant rarely metastasize. PIK3CA mutations and/or a histological RCLS component of more than 5% are unfavorable predictors. Overexpression of TP53 (p53) is also associated with a poor prognosis. Common sites for metastasis are retroperitoneum, subcutaneous soft tissues, bones, and lungs.
Crozat A, Åman P, Mandahl N, and Ron D (1993) Fusion of CHOP to a novel RNA-binding protein in myxoid liposarcoma. Nature 363(6430): 640–644. Engstrom K, Willen H, Kåbjorn-Gustafsson C, et al. (2006) The myxoid/round cell liposarcoma fusion oncogene FUS-DDIT3 and the normal DDIT3 induce a liposarcoma phenotype in transfected human fibrosarcoma cells. The American Journal of Pathology 168(5): 1642–1653. Fletcher CDM, Unni KK, and Mertens F (eds.) (2002) World Health Organization Classification of Tumours. Pathology and Genetics of Tumours of Soft Tissue and Bone. Lyon, France: IARC Press. Perez-Losada J, Pintado B, Gutierrez-Adan A, et al. (2000) The chimeric FUS/TLS-CHOP fusion protein specifically induces liposarcomas in transgenic mice. Oncogene 19 (20): 2413–2422.
See also: Mutation; Oncogenes; Transcription Factors; Translocation.