Margareth Isaksson

Confirmation of the high frequency of the TMPRSS2/ERG fusion gene in prostate cancer.

In a recent study by Tomlins et al., (2005), it was shown that a large subset of prostate cancer harbors the fusion genes TMPRSS2/ERG and TMPRSS2/ ETV1, generated through cryptic interand intrachromosomal rearrangements. A gene fusion of the 50 untranslated region of TMPRSS2 to ERG or ETV1 was identified in 23 (79%) of 29 prostate cancer samples. As yet, however, there have not been any confirmatory reports. In the present study, total RNA was extracted from 18 prostate adenocarcinoma biopsies using the Trizol reagent (Invitrogen, Carlsbad, CA) and cDNA synthesis was conducted in a 20 ll reaction mixture which contained 2.5 lg of total RNA, 13 first strand buffer, 10 mM DTT, 1 mM of each dNTP, 20 U RNase inhibitor (RNA guard, Amersham, Biosciences, Piscataway, NJ), 500 pmol random hexamers, and 200 U M-MLV reverse transcriptase (Invitrogen). The reaction was carried out at 378C for 60 min, heated for 10 min at 658C, and then kept at 48C. PCR amplifications were performed using 1 ll cDNA as template in a final volume of 50 ll containing 13 PCR buffer, 0.2 mM of each dNTP, 1.25 mM MgCl2, 0.5 lM of each of the TMPRSS2-1F and ERG-541R primers (Table 1) for the amplification of the TMPRSS2/ERG transcript or TMPRSS2-1F and ETV1-580R for the TMPRSS2/ETV1 transcript, and 1U Platinum Taq DNA polymerase (Invitrogen). One microliter of the first PCR product was then used as template in a second PCR with primers TMPRSS2-20F and ERG-450R (for TMPRSS2/ERG) or TMPRSS220F and ETV1-502R (TMPRSS2/ETV1). The PCR was run on a PCT-200 DNA Engine (MJ Research, Waltham, MA), and both amplifications had an identical cycling profile: an initial denaturation at 948C for 5 min, followed by 30 cycles of 1 min at 948C, 1 min at 608C, and 1 min at 728C, and a final extension for 10 min at 728C. Amplified fragments were run on a 1.5% agarose gel, stained with ethidium bromide, purified using the QIAquick gel extraction kit (Qiagen, Hilden, Germany), and directly sequenced using the dideoxy procedure with an ABI Prism BigDye terminator v1.1 cycle sequencing ready reaction kit (Applied Biosystems, Foster City, CA) on the Applied Biosystems

The chimeric FUS/CREB3l2 gene is specific for low-grade fibromyxoid sarcoma.

Low‐grade fibromyxoid sarcoma (LGFMS) is a variant of fibrosarcoma that was recognized as a distinct tumor entity only quite recently. We previously described a translocation, t(7;16)(q33;p11), that resulted in a fusion of the FUS and CREB3L2 (also known as BBF2H7) genes in two soft tissue tumors that fulfilled morphologic criteria for LGFMS. To delineate the spectrum of tumors that may harbor the FUS/CREB3L2 gene, we selected 45 low‐grade spindle cell sarcomas for reverse transcriptase polymerase chain reaction (RT‐PCR) and/or fluorescence in situ hybridization (FISH) analyses; none of these tumors had originally been diagnosed as LGFMS. Furthermore, also included were two benign soft tissue tumors and nine high‐grade sarcomas with supernumerary ring chromosomes or 7q3 rearrangement and three tumors diagnosed as LGFMS prior to the genetic analysis. Of the 59 tumors analyzed, 12 were FUS/CREB3L2‐positive, all of which were diagnosed at histopathologic re‐examination as being LGFMS, of both the classical subtype and the subtype with giant collagen rosettes. The breakpoints in the fusion transcripts were always in exons 6 or 7 of FUS and exon 5 of CREB3L2. The results indicated that FUS/CREB3L2 is specifically associated with LGFMS and that RT‐PCR or FISH analysis may be useful for the differential diagnosis.

Fusion of the SEC31L1 and ALK genes in an inflammatory myofibroblastic tumor.

Inflammatory myofibroblastic tumor (IMT) is a neoplasm composed of myofibroblastic spindle cells and infiltrating inflammatory cells. Cytogenetic analyses have revealed that a subgroup of IMT, in particular among children and young adults, harbors clonal chromosomal rearrangements involving chromosome band 2p23. Further, molecular genetic studies have shown that these rearrangements target the ALK gene, serving as the 3′‐partner in fusion genes with various translocation partners. In the present study, we describe the finding of a novel SEC31L1/ALK fusion gene in an intraabdominal IMT of a young man. G‐band analysis revealed a translocation t(2;4)(p23;q21) and subsequent fluorescence in situ hybridization with locus‐specific probes strongly indicated disruption of the ALK locus on chromosome 2. Immunostaining with monoclonal mouse anti‐human CD246 ALK Protein showed diffuse cytoplasmic positivity. Using reverse primers for the ALK‐gene, we could, by 5′‐RACE methodology, amplify a single 1.2 kb fragment. Sequence analysis showed that the fragment was a hybrid cDNA product in which nt 3012 of SEC31L1 (NM_016211), located in band 4q21, was fused in‐frame to nt 4080 of ALK (NM_004304). RT‐PCR with two sets of primer pairs specific for SEC31L1 and ALK amplified two transcripts, which at sequencing corresponded to two types of chimeric SEC31L1/ALK transcripts. In the long, type I, transcript nt 3012 of SEC31L1 (NM_016211) was fused in‐frame to nt 4080 of ALK. In the short, type II, transcript nt 2670 of SEC31L1 was fused in‐frame to nt 4080 of ALK. Genomic PCR and subsequent sequencing showed that the breakpoints were located in intron 23 of SEC31L1 and intron 20 of ALK.

Molecular genetic characterization of the EWS/ATF1 fusion gene in clear cell sarcoma of tendons and aponeuroses.

Clear cell sarcoma (CCS) is a rare malignant soft tissue tumor particularly associated with tendons and aponeuroses. The cytogenetic hallmark is the translocation t(12;22)(q13;q12) resulting in a chimeric EWS/ATF1 gene in which the 3′‐terminal part of EWS at 22q is replaced by the 3′‐terminal part of ATF1 at 12q. To date, only 13 cases of CCS have been analyzed for fusion genes at the transcription level, and there is no information about the breakpoints at the genomic level. In the present study, we describe the molecular genetic characteristics of CCS from 10 patients. Karyotypes were obtained from 10 cases, 7 of which showed the characteristic t(12;22). As an initial step in the characterization of the EWS/ATF1 and ATF1/EWS chimeras, we constructed an exon/intron map of the ATF1 gene. The entire ATF1 gene spanned >40 kb and was composed of 7 exons. Intron 3, in which most of the genomic breakpoints occurred, was to a large extent (83%) composed of repetitive elements. RT‐PCR amplified EWS/ATF1 cDNA fragments in all patients and ATF1/EWS cDNA fragments in 6 of 10 patients. Four types of EWS/ATF1 chimeric transcript, designated types 1–4, were identified. The most frequent chimeric transcript (type 1) was an in‐frame fusion of exon 8 of EWS with exon 4 of ATF1. This was the only chimeric transcript in 5 patients but found together with other variants in 3 tumors. The type 2 transcript of EWS/ATF1, an in‐frame fusion of exon 7 of EWS with exon 5 of ATF1, was detected in 4 patients, as the only transcript in 1 case and together with other variants in 3 cases. An in‐frame fusion of exon 10 of EWS with exon 5 of ATF1 (type 3) was found in 1 patient as the only transcript, and an out‐of‐frame fusion of EWS exon 7 with ATF1 exon 7 (type 4) was detected in 1 patient together with type 1 and type 2 transcripts. Sequencing of the amplified ATF1/EWS cDNA fragments showed in 5 patients that ATF1 exon 3 was fused with EWS exon 10, resulting in an out‐of‐frame chimeric transcript. In 1 case, nt 428 of ATF1 (exon 4) was fused with EWS exon 8; at the junction, there was an insertion of 4 nucleotides, also resulting in an out‐of‐frame transcript. Genomic extra long PCR and sequence analysis mapped the genomic breakpoints to introns 7, 8 and 9 of EWS and intron 3 and exon 4 of ATF1. While a simple end‐to‐end fusion was observed in 2 cases, additional nucleotides were found at the junctions in 2 other cases. In addition, topoisomerase I consensus sequences were found close to the junctions, suggesting that this enzyme may participate in the genesis of the EWS/ATF1 fusion.

Clinical impact of molecular and cytogenetic findings in synovial sarcoma

Synovial sarcoma is an aggressive soft‐tissue tumor that accounts for up to 10% of soft‐tissue sarcomas. Cytogenetically, synovial sarcoma is characterized by the t(X;18)(p11;q11), found in more than 95% of the tumors. This translocation results in rearrangements of the SYT gene in 18q11 and one of the SSX1, SSX2, or SSX4 genes in Xp11, creating a SYT/SSX1, SYT/SSX2, or SYT/SSX4 chimeric gene. It has been shown that patients with SYT/SSX1 fusion genes have a shorter metastasis‐free survival than do patients with SYT/SSX2. Previous studies have also suggested that clonal evolution may be associated with disease progression. In the present study, RT‐PCR analysis showed that all 64 examined synovial sarcomas from 54 patients had SYT‐SSX chimeric genes. SYT/SSX1 was found in 40 tumors from 33 patients, SYT/SSX2 in 23 tumors from 20 patients, and SYT/SSX4 in one case. Two patients had variant SYT/SSX2 transcripts, with 57 bp and 141 bp inserts, respectively, between the known SYT and SSX2 sequences. Patients with tumors with SYT/SSX1 fusions had a higher risk of developing metastases compared to those with SYT/SSX2 fusions (P = 0.01). The reciprocal transcripts SSX1/SYT and SSX2/SYT were detected using nested PCR in 11 of the 40 samples with SYT/SSX1 and 5 of the 23 samples with SYT/SSX2, respectively. Among 20 blood samples, SYT/SSX1 and SYT/SSX2 were detected in one sample each. The t(X;18), or variants thereof, was found cytogenetically in all patients but three. Among 32 primary tumors, the t(X;18) or a variant translocation was the sole anomaly in 10. In contrast, of the seven metastatic lesions that were investigated prior to radiotherapy, only one had a t(X;18) as the sole anomaly; all other tumors displayed complex karyotypes. Cytogenetic complexity in primary tumors was, however, not associated with the development of metastases. Tumors with SYT/SSX2 less often (4/12 vs. 7/15) showed complex karyotypes than did tumors with SYT/SSX1, but the difference was not significant. Combining cytogenetic complexity and transcript data, we found that the subgroup of patients with tumors showing simple karyotypes and SYT/SSX2 fusion had the best clinical outcome (2/8 patients developed metastases), and those with tumors showing complex karyotypes together with SYT/SSX1 fusion the worst (6/7 patients developed metastases). This corresponded to 5‐year metastasis‐free survival rates of 0.58 and 0.0, respectively (P = 0.02).

Fusion of the BCR and the fibroblast growth factor receptor-1 (FGFR1) genes as a result of t(8;22)(p11;q11) in a myeloproliferative disorder: The first fusion gene involving BCR but not ABL

Constitutive activation of tyrosine kinases as a consequence of chromosomal translocations, forming fusion genes, plays an important role in the development of hematologic malignancies, in particular, myeloproliferative syndromes (MPSs). In this respect, the t(9;22)(q34;q11) that results in the BCR/ABL fusion gene in chronic myeloid leukemia is one of the best‐studied examples. The fibroblast growth factor receptor 1 (FGFR1) gene at 8p11 encodes a transmembrane receptor tyrosine kinase and is similarly activated by chromosomal translocations, in which three alternative genes—ZNF198 at 13q12, CEP110 at 9q34, and FOP at 6q27—become fused to the tyrosine kinase domain of FGFR1. These 8p11‐translocations are associated with characteristic morphologic and clinical features, referred to as “8p11 MPS.” In this study, we report the isolation and characterization of a novel fusion gene in a hematologic malignancy with a t(8;22)(p11;q11) and features suggestive of 8p11 MPS. We show that the breakpoints in the t(8;22) occur within introns 4 and 8 of the BCR and FGFR1 genes, respectively. On the mRNA level, the t(8;22) results in the fusion of BCR exons 1–4 in‐frame with the tyrosine kinase domain of FGFR1 as well as in the expression of a reciprocal FGFR1/BCR chimeric transcript. By analogy with data obtained from previously characterized fusion genes involving FGFR1 and BCR/ABL, it is likely that the oligomerization domain contributed by BCR is critical and that its dimerizing properties lead to aberrant FGFR1 signaling and neoplastic transformation.

Molecular genetic characterization of the EWS/CHN and RBP56/CHN fusion genes in extraskeletal myxoid chondrosarcoma.

Extraskeletal myxoid chondrosarcoma (EMC) is a soft‐tissue neoplasm cytogenetically characterized by the translocations t(9;22)(q22;q11–12) or t(9;17)(q22;q11), generating EWS/CHN or RBP56/CHN fusion genes, respectively. In the present study, 18 EMCs were studied both cytogenetically and at the molecular level. Chromosomal aberrations were detected in 16 samples: 13 with involvement of 9q22 and 22q11–12, and three with rearrangements of 9q22 and 17q11. Fifteen cases had an EWS/CHN fusion transcript and three had an RBP56/CHN transcript. The most frequent EWS/CHN transcript (type 1; 10 tumors), involved fusion of EWS exon 12 with CHN exon 3, and the second most common (type 5; two cases) was fusion of EWS exon 13 with CHN exon 3. In all tumors with RBP56/CHN fusion, exon 6 of RBP56 was fused to exon 3 of CHN. By genomic XL PCR and sequence analyses, the breakpoints from 14 cases were mapped in the EWS, RBP56, and CHN genes. In CHN, 12 breakpoints were found in intron 2 and only two in intron 1. In EWS, the breaks occurred in introns 7 (one break), 12 (eight breaks), and 13 (one break), and in RBP56 in intron 6. Repetitive elements such as Alu and LINE sequences seem to have limited, if any, importance in the genesis of EWS/CHN and RBP56/CHN chimeras. Furthermore, there were no chi, chi‐like, topoisomerase II, or translin consensus sequences in the introns harboring the translocation breakpoints, nor could the number of topo I sites in EWS, RBP56, and CHN introns explain the uneven distribution of the breakpoints among EWS or CHN introns. Additional genetic events, such as nucleotide insertions, homologies at the junction, deletions, duplications, and inversions, were found to accompany the translocations, indicating that the chromosomal translocations do not require sequence‐specific recombinases or extensive homology between the recombined sequences.

Fusion of the NUP98 gene and the homeobox gene HOXC13 in acute myeloid leukemia with t(11;12)(p15;q13).

The NUP98 gene at 11p15 is known to be fused to DDX10, HOXA9, HOXA11, HOXA13, HOXD11, HOXD13, LEDGF, NSD1, NSD3, PMX1, RAP1GDS1, and TOP1 in various hematologic malignancies. The common theme in all NUP98 chimeras is a transcript consisting of the 5′ part of NUP98 and the 3′ portion of the partner gene; however, apart from the frequent fusion to different homeobox genes, there is no apparent similarity among the other partners. We here report a de novo acute myeloid leukemia with a t(11;12)(p15;q13), resulting in a novel NUP98/HOXC13 fusion. Fluorescence in situ hybridization analyses, by the use of probes covering NUP98 and the HOXC gene cluster at 12q13, revealed a fusion signal at the der(11)t(11;12), indicating a NUP98/HOXC chimera, whereas no fusion was found on the der(12)t(11;12), suggesting that the translocation was accompanied by a deletion of the reciprocal fusion gene. Reverse transcription‐PCR and sequence analyses showed that exon 16 (nucleotide 2290) of NUP98 was fused in‐frame with exon 2 (nucleotide 852) of HOXC13. Neither the HOXC13/NUP98 transcript nor the normal HOXC13 was expressed. The present results, together with previous studies of NUP98/homeobox gene fusions, strongly indicate that NUP98/HOXC13 is of pathogenetic importance in t(11;12)‐positive acute myeloid leukemia.

A Novel Fusion Gene, SS18L1/SSX1, in Synovial Sarcoma

Synovial sarcoma is an aggressive soft tissue tumor that is characterized cytogenetically by the t(X;18)(p11;q11) translocation, resulting in fusion between the SS18 gene on chromosome 18 and one of the SSX genes on the X chromosome. The three fusion genes that have been detected thus far, SS18/SSX1, SS18/SSX2, and SS18/SSX4, account for more than 95% of the synovial sarcomas. Because SS18/SSX fusions do not seem to occur in other tumor types, and because synovial sarcomas may sometimes be difficult to distinguish from other spindle cell tumors, molecular genetic analysis has become established as an important diagnostic tool. Upon cytogenetic analysis of a soft‐tissue tumor that showed classic synovial sarcoma morphology, we detected two supernumerary marker chromosomes but no rearrangement of chromosomes X or 18. By fluorescence in situ hybridization, the marker chromosomes were shown to contain material from chromosomes X and 20, including the SSX gene cluster on the X chromosome and the SS18L1 gene, which shows strong homology with the SS18 gene, on chromosome 20. Further RT‐PCR analysis and sequencing of the amplified products revealed a novel SS18L1/SSX1 fusion transcript in which nucleotide 1216 (exon 10) of SS18L1 was fused in‐frame with nucleotide 422 (exon 6) of SSX1. Thus, the existence of genetic heterogeneity has to be taken into account when RT‐PCR is used for the diagnosis of synovial sarcoma.

Characteristic sequence motifs at the breakpoints of the hybrid genes FUS/CHOP , EWS/CHOP and FUS/ERG in myxoid liposarcoma and acute myeloid leukemia

We have sequenced the breakpoint regions in one acute myeloid leukemia (AML) with t(16;21)(p11;q22) resulting in the formation of a FUS/ERG hybrid gene and in four myxoid liposarcomas (MLS), three of which had the translocation t(12;16) (q13;p11) and a FUS/CHOP fusion gene and one with t(12;22;20)(q13;q12;q11) and an EWS/CHOP hybrid gene. The breakpoints were localized to intron 7 of FUS, intron 1 of CHOP, an intronic sequence of ERG and intron 7 of EWS. In two MLS cases with t(12;16) and in the AML, the breaks in intron 7 of FUS had occurred close to each other, a few nucleotides downstream from a TG dinucleotide repeat region. The break in the two MLS had occurred in the same ATGGTG hexamer and in the AML 40 nucleotides upstream from the hexamer. The third case of t(12;16) MLS had a break upstream and near a TC-dinucleotide repeat region and a sequence similar to the chi bacterial recombination element was found to flank the breakpoint. In the MLS with the EWS/CHOP hybrid gene, the break in intron 7 of EWS had occurred close to an Alu sequence. Similarly, in all 4 MLS, the breaks in intron 1 of CHOP were near an Alu sequence. No Alu or other repetitive sequences were found 250 bp upstream or downstream from the break in the ERG intron involved in the AML case. In the AML, the MLS with ESW/CHOP and in one MLS with FUS/CHOP there were one, two and six, respectively, nucleotide identity between the contributing germline sequences in the breakpoint. In the other two MLS cases, two and three extra nucleotides of unknown origin were inserted between the FUS and CHOP sequences. At the junction and/or in its close vicinity, identical oligomers, frequently containing a trinucleotide TGG, were found in both partner genes. Our data thus show that all four genes-FUS, EWS, CHOP and ERG-contain characteristic motifs in the breakpoint regions which may serve as specific recognition sites for DNA-binding proteins and have functional importance in the recombination events taking place between the chromosomes. Different sequence motifs may, however, play a role in each individual case.