Elsevier

Human Pathology

Volume 43, Issue 2, February 2012, Pages 180-189
Human Pathology

Original contribution
The CIC-DUX4 fusion transcript is present in a subgroup of pediatric primitive round cell sarcomas

https://doi.org/10.1016/j.humpath.2011.04.023Get rights and content

Summary

Pediatric undifferentiated soft tissue sarcomas are a group of diagnostically challenging tumors. Recent studies have identified a subgroup of undifferentiated soft tissue sarcomas with primitive round to plump spindle cell morphology and a t(4;19)(q35;q13.1) translocation resulting in the expression of a CIC-DUX4 fusion transcript, including 2 tumors previously reported by our laboratory (Cancer Genet Cytogenet 2009;195:1). In the present study, reverse transcriptase polymerase chain reaction assays developed for both frozen and paraffin-based tissues were applied to a series of 19 pediatric undifferentiated soft tissue sarcomas using a combination of primer sets covering the CIC-DUX4 fusion transcript. Of the 19 undifferentiated soft tissue sarcomas, 16 had primitive round to plump spindle cell morphology, and 3 had pure spindle cell morphology. Three of the 16 undifferentiated soft tissue sarcomas with primitive round cell morphology were found to harbor the CIC-DUX4 fusion transcript by reverse transcriptase polymerase chain reaction. Automated DNA sequencing of the polymerase chain reaction products identified 2 distinct transcript variants. One CIC-DUX4–positive tumor showed membranous CD99 positivity, 2 showed focal S100 positivity, and 1 showed focal CD57 positivity. With the 2 previously reported cases, the total number of CIC-DUX4–positive primitive round cell sarcomas identified at our institution has been brought to 5 (28%) of 18. Given the consistent involvement of the CIC-DUX4 fusion in a subset of primitive round cell undifferentiated soft tissue sarcomas, these findings suggest a central role for the fusion transcript in such tumors. The current findings further define a novel genetic subset of pediatric primitive round cell sarcomas and provide an additional diagnostic tool for their characterization and diagnosis.

Introduction

Pediatric soft tissue sarcomas are a diverse group of tumors arising in the connective tissues of the body and are generally found to be quite aggressive [1], [2]. Collectively, these tumors account for up to 20% of all pediatric solid tumors [3]. Sarcomas have long been a challenging group of tumors to diagnose and classify because of the morphological similarities between established subgroups, as well as the relative rarity of these neoplasms [4]. However, advances in immunohistochemical and molecular analyses have allowed for the accurate demarcation of different sarcoma subtypes [5].

Sarcomas can be subclassified at both morphological and genomic levels. Morphologically, sarcomas are divided into rhabdomyosarcomas (RMSs) and nonrhabdomyomatous sarcomas [6]. Examples of the latter category include Ewing sarcoma (ES)/primitive neuroectodermal tumor, malignant peripheral nerve sheath tumor, and synovial sarcoma (SS) [6]. At the genomic level, sarcomas are divided into 2 major groups. The first group is composed of sarcomas with simple, near-diploid karyotypes that consistently harbor recurrent chromosomal rearrangements [7]. Members of this group include ESs, which are associated with rearrangements of the EWS gene; alveolar RMSs, which are associated with PAX-FKHR rearrangements; and SSs, which are associated with SYT-SSX rearrangements [8]. The second group is composed of tumors that have very complex karyotypes but in which no reproducible chromosomal aberrations have been identified. This group includes malignant peripheral nerve sheath tumor, embryonal RMS, and osteosarcoma [7].

Despite the advancement of diagnostic applications that allow for the identification of sarcoma subtypes, approximately 5% of sarcomas remain unclassifiable [9], [10], [11]. These tumors, termed undifferentiated soft tissue sarcomas (USTSs), show no specific lineage differentiation and no well-established histologic or immunohistochemical profiles [9], [12]. Furthermore, the karyotypes of these sarcomas are largely variable, with some tumors having a very complex genomic makeup and other tumors harboring a near-diploid genome [13]. As such, a diagnosis of USTS is largely considered one of exclusion [5]. Morphologically, most of these tumors have a primitive round to plump spindle cell phenotype; however, a small percentage of USTSs exhibit a pure spindled/myxoid phenotype that likely represents a different entity [5], [9]. Recent case reports have linked tumors of primitive round cell morphology with a translocation involving 4q35 and 19q13.1 [11], [14], [15], [16]. In 4 such tumors, molecular and cytogenetic analyses determined that the translocation between 19q13.1 and 4q35 resulted in the fusion of the CIC gene on chromosome 19 and the DUX4 gene on chromosome 4 [11], [16].

In this study, we detail specific reverse transcriptase polymerase chain reaction (RT-PCR) assays that can be used to detect the CIC-DUX4 fusion transcript in both frozen and paraffin-embedded tissues and describe an additional 3 cases of CIC-DUX4–positive pediatric USTSs with primitive round cell morphology.

Section snippets

Tumor specimens

An electronic search of the pathology database at the Hospital for Sick Children was performed to identify children diagnosed with USTS between 1987 and 2007. Primary tumors involving viscera and bone, and intradural tumors, were excluded. Pretreatment biopsy specimens were studied if available, and all tumors underwent extensive immunohistochemical and molecular genetic screening before being included in the series of USTS (see Somers et al [9] for methodology used). Of the 19 USTSs, 10

Clinicopathologic features of tumor samples

In total, 19 USTSs were included in the study. Table 2 summarizes the clinicopathologic features of the tumors; for 1 patient, the age and sex was not available. Some of the clinicopathologic features have been previously described [9], [13], [17] (see also “Materials and methods”). Briefly, 15 cases comprised sheets of primitive round to plump spindle cells; 1 case comprised nests of primitive round cells; and 3 cases comprised pure spindle cells within a myxoid, collagenous, or cellular

Discussion

Pediatric sarcomas can be subdivided into specific categories based on morphological, immunohistochemical, and molecular genetic features. Recently, several groups have identified a novel subcategory of sarcoma with primitive round cell morphology and recurrent translocations involving chromosomes 4q35 and 19q13 [11], [14], [15], [16] (Table 4). One such study determined that, in 2 cases, this rearrangement resulted in the fusion of the CIC gene on chromosome 19q13.1 and the DUX4 gene on

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    This work was funded by Canadian Institutes of Health Research (Ottawa, ON, Canada) grant number MOP86462 awarded to G. R. S.

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