Chromosomal sublocalization of the 2;13 translocation breakpoint in alveolar rhabdomyosarcoma

Classification and pathology

Soft tissue tumors are a heterogeneous group of benign and malignant processes. Some are assumed to be reactive; others are clearly neoplastic. Because of their rarity, they frequently pose diagnostic problems for surgical pathologists. Accurate diagnosis of these tumors is enhanced by knowledge of the clinical features of the given lesions and, at times, by application of immunohistochemical and molecular techniques. In this article the lesions are described essentially in accordance with the World Health Organization classification.

Mechanisms of Sarcomagenesis

Sarcomas comprise a heterogeneous group of malignancies that are derived from mesenchymal cells, which under normal circumstances lead to the development of connective tissues such as bone, muscle, fat, and cartilage. During the past decade, insight has been gained regarding the aberrancies that occur during normal development that result in mesenchymal cells transforming into sarcomas. More recently, these insights have led to the development of successful therapies that target the specific mechanisms inherent to individual sarcomas. This overview discusses some of the aberrant molecular mechanisms shared in sarcomas and reviews several sarcoma subtypes in which the most advances have been made. Finally, the ways in which these advances in basic science are translating into and redefining clinical practice are highlighted.

Myogenin and MyoD1 expression in paediatric rhabdomyosarcomas

The diagnosis of paediatric solid tumours is often based on small tissue needle biopsies in which many different entities demonstrate a "small round cell tumour" phenotype and in which there may be insufficient tissue to allow the interpretation of diagnostic architectural features, which may be present in larger specimens. Therefore, the extensive use of a panel of immunohistochemical markers is part of the routine handling and investigation of such biopsies to reach a definite diagnosis. However, in some cases the morphological and routine immunohistochemical findings may be insufficient for a precise diagnosis or they may be difficult to interpret in the given clinical context. Although many paediatric tumours exhibit characteristic chromosomal translocations with resultant specific fusion transcripts, these require molecular methods for their detection, usually on fresh tissue samples, which may not always be available. As more immunohistochemical markers become available, more precise diagnosis on such small biopsies may be possible. This review examines the use of the immunohistochemical markers, MyoD1 and myogenin, in the diagnosis of paediatric rhabdomyosarcoma, including its subtypes.

Clinical relevance of molecular diagnosis in childhood rhabdomyosarcoma

Rhabdomyosarcoma may be divided into three subtypes--embryonal, alveolar, and undifferentiated sarcoma--which can be distinguished by molecular analysis. The authors applied reverse transcriptase-polymerase chain reaction analysis (RT-PCR) to analyze tumor samples from 14 children with rhabdomyosarcoma for the presence of the chimeric PAX3-FKHR transcript resulting from the translocation t(2;13)(q35,q14). Both fresh and paraffin-embedded tissues were used. In only nine specimens was the RNA intact for the analysis. The chimeric transcript was identified in seven samples: four alveolar type, one embryonal type, and two undifferentiated sarcoma. Histologic review was performed in the three samples with discordance between the molecular and histologic findings. A sample from a patient with a diagnosis of embryonal rhabdomyosarcoma on presentation and expression of PAX3-FKHR fusion transcript yielded a small focus of alveolar rhabdomyosarcoma and was reclassified as alveolar rhabdomyosarcoma. One of the samples from a patient with undifferentiated sarcoma was redefined as alveolar subtype; the diagnosis of the second undifferentiated sarcoma remained unchanged, in accordance with the histologic diagnosis. These findings further support the recommendation that molecular analysis be included in the diagnostic workup of childhood small round cell tumors to reach a more accurate diagnosis for tailoring of specific treatment.

Non-isotopic molecular cytogenetics in neuro-oncology

The molecular genetic analysis of brain tumours has been the focus of considerable interest for a number of years. However, these studies have been largely directed towards understanding the fundamental biological processes involved in tumorigenesis and the techniques which have been used require considerable molecular biological skills. Unfortunately, there has not been the impetus to correlate basic biological studies with clinical or neuropathological features. The development of non-isotopic molecular cytogenetic in situ hybridization (ISH) techniques which can be applied to archival tumour material provides an opportunity to address a wide range of neuropathological questions at a genetic level. Identification of specific chromosomes has been made possible by the isolation of probes which recognize the highly repeated sequences present in the centromeric regions of individual chromosomes. Libraries of human chromosome-specific painting probes are also available. A range of probes which bind to the whole or part of specific single copy genes are becoming available. These can be detected with either fluorochromes with different emission colours or with enzymatic detection systems in either interphase nuclei derived from fresh, fixed and embedded tumour samples, touch preparations or smears (so-called 'interphase cytogenetics') as well as conventional metaphase spreads. Comparative genomic hybridization can be used to scan the entire genome for deletions or amplifications without any pre-existing information about the likely locations of these abnormalities or the availability of any specific DNA probes. These techniques can be used to identify aneuploidy or structural alterations in individual chromosomes and are likely to yield important information about the location of genes important in the pathogenesis of brain tumours and may also provide the basis for the refinement of diagnostic or prognostic criteria of these neoplasms.

The PAX3-FKHR fusion protein created by the t(2;13) translocation in alveolar rhabdomyosarcomas is a more potent transcriptional activator than PAX3

Alveolar rhabdomyosarcomas are pediatric solid tumors with a hallmark cytogenetic abnormality: translocation of chromosomes 2 and 13 [t(2;13) (q35;q14)]. The genes on each chromosome involved in this translocation have been identified as the transcription factor-encoding genes PAX3 and FKHR. The NH2-terminal paired box and homeodomain DNA-binding domains of PAX3 are fused in frame to COOH-terminal regions of the chromosome 13-derived FKHR gene, a novel member of the forkhead DNA-binding domain family. To determine the role of the fusion protein in transcriptional regulation and oncogenesis, we identified the PAX3-FKHR fusion protein and characterized its function(s) as a transcription factor relative to wild-type PAX3. Antisera specific to PAX3 and FKHR were developed and used to examine PAX3 and PAX3-FKHR expression in tumor cell lines. Sequential immunoprecipitations with anti-PAX3 and anti-FKHR sera demonstrated expression of a 97-kDa PAX3-FKHR fusion protein in the t(2;13)-positive rhabdomyosarcoma Rh30 cell line and verified that a single polypeptide contains epitopes derived from each protein. The PAX3-FKHR protein was localized to the nucleus in Rh30 cells, as was wild-type PAX3, in t(2;13)-negative A673 cells. In gel shift assays using a canonical PAX binding site (e5 sequence), we found that DNA binding of PAX3-FKHR was significantly impaired relative to that of PAX3 despite the two proteins having identical PAX DNA-binding domains. However, the PAX3-FKHR fusion protein was a much more potent transcriptional activator than PAX3 as determined by transient cotransfection assays using e5-CAT reporter plasmids. The PAX3-FKHR protein may function as an oncogenic transcription factor by enhanced activation of normal PAX3 target genes.

Molecular biological aspects of soft tissue tumors

In the preceding, the reader has hopefully developed an appreciation of the major malignant tumors to be encountered in somatic soft tissues in children, adolescents, and young adults. In aggregate, this group of tumors accounts for about 20% of cancer in this age group. Importantly, they are curable tumors when nonmetastatic at presentation, but therapy appropriate to prognosis and tumor responsiveness is highly dependent on precise diagnosis. The historical morphologic methods alone will not suffice for this purpose, but the anticipated rapid advent of molecular genetic diagnostic and prognostic methods should. Useful, practical, and rapid genetic tests, available in the same time frame as the routine histopathologic evaluation of these tumors, are likely to forever change the diagnosis and management of these tumors, individually and as a group.

Pathologic classification of soft tissue sarcomas

A number of immunological markers and chromosomal abnormalities are described to aid in the diagnosis, prognosis and management of soft tissue sarcomas. Although the chromosomal abnormalities may be specific in some cases, they have only been reported in small series or single cases and their significance has not yet been established. None of the immunological markers described is either entirely specific or sensitive for a particular tumor type, and virtually none of these immunological reagents is currently approved for diagnostic use despite their widespread applications in pathology. They function in a supporting role to conventional tissue pathology, grading, and pathological and clinical staging of soft tissue sarcomas. Antibodies employed as an aid in diagnosis of soft tissue tumors should always be run in panels and never as single tests. New immunological markers and chromosomal abnormalities in soft tissue sarcomas should be carefully evaluated for their specificity and sensitivity and their usefulness in diagnosis and prognosis.

Rearrangement of the PAX3 paired box gene in the paediatric solid tumour alveolar rhabdomyosarcoma

We have determined that PAX3 (found previously to be mutated in Waardenburg syndrome) is the chromosome 2 locus rearranged by the t(2;13)(q35;q14) translocation of the paediatric solid tumour alveolar rhabdomyosarcoma. The rearrangement breakpoints occur within an intron downstream of the paired box and homeodomain-encoding regions. Upstream PAX3 sequences hybridize to a novel transcript in t(2;13)-containing lines. Cloning and characterization of this novel transcript indicate that the translocation juxtaposes the PAX3 DNA binding elements with chromosome 13 sequences, suggesting formation of a hybrid transcription factor. Therefore, PAX3 gene alterations are associated with two completely unrelated human diseases.

Assignment of the genes encoding human interleukin-8 receptor types 1 and 2 and an interleukin-8 receptor pseudogene to chromosome 2q35

Two human cDNA clones that encode different interleukin-8 (IL8) receptors have recently been isolated. The interleukin-8 receptor type 1 (IL8R1) binds IL8 only, whereas the interleukin-8 receptor type 2 (IL8R2) (previously designated IL8RA) also binds growth regulated gene (GRO), and neutrophil activating protein-2 (NAP-2) with high affinity. In the process of screening a genomic library with these cDNAs to obtain large clones for use in chromosomal localization studies, we isolated an interleukin-8 receptor pseudogene (IL8RP) that bears greatest similarity to IL8R2. Using Southern hybridization analysis of human x rodent somatic cell hybrid DNAs with cDNA probes for IL8R1 and IL8R2 and probes from the IL8RP locus, we assigned the three loci to chromosome 2; fluorescence in situ hybridization (FISH) to metaphase chromosome preparations using genomic clones from each locus refined this localization to chromosome 2, band q35, for all three. By virtue of their chromosomal location, IL8R1 and IL8R2 may be considered candidate genes for several human disorders in which the involved locus has been mapped to distal 2q or that are associated with structural abnormalities of this segment, including van der Woude syndrome and the neoplastic diseases rhabdomyosarcoma and uterine leiomyomata. In addition, because this region of chromosome 2q is homologous to proximal mouse chromosome 1 in the segment containing the Lsh-Ity-Bcg locus involved in mediating host resistance to infection with intracellular pathogens, examination for abnormalities of the murine homologues of the IL8R genes should be considered in mice affected by mutations of this locus.

Sublocalization of the chromosome 5 breakpoint of the 3;5 translocation in myelodysplastic syndromes and acute myeloid leukemia

A t(3;5)(q25.1;q34) reciprocal translocation identifies a subset of cases of myelodysplastic syndrome or acute myeloid leukemia (AML) that are characterized by increased numbers of megakaryocytes and severe trilineage dysplasia. As a first step in characterizing the t(3;5) breakpoints, we asked whether the translocation involves the CSFIR/PDGFRB locus at 5q33-q35. Pulsed-field gel electrophoretic analysis of a region extending 580 kb 5' to the PDGFRB gene and 120 kb 3' to the CSFIR gene did not reveal aberrant restriction fragments in leukemic cell DNA, confirming that the breakpoint does not occur in the vicinity of these genes. To sublocalize the breakpoint, we performed Southern blot hybridizations using DNA from human x hamster somatic cell hybrids containing the normal 3, the normal 5, the derivative 3, or the derivative 5 human chromosome. Using a series of polymorphic DNA probes from the long arm of chromosome 5, which have been linked by genetic recombination, we bracketed the breakpoint to within a region that spans approximately 13 centimorgans (sex average) and is flanked by the q34-qter markers cKK5.19 and L1200 (D5S62). This analysis places the chromosome 5 breakpoint of the t(3;5) considerably telomeric to the CSFIR/PDGFRB locus, confirming our studies with pulsed-field electrophoresis. Future efforts to identify the genes affected by the t(3;5) should focus on the 5q segment described in this study.