Characterizing false-positive fluorescence in situ hybridization results by mate-pair sequencing in a patient with chronic myeloid leukemia and progression to myeloid blast crisis

False positive NUP98 fluorescence in situ hybridization rearrangements in B-acute lymphoblastic leukemia

Gene fusions involving NUP98 have been reported in several hematologic malignancies yet have been very rarely reported in B-acute lymphoblastic leukemia (B-ALL). Two cases of B-ALL for which chromosome banding analysis (CBA) and fluorescence in situ hybridization (FISH) suggested apparent NUP98 rearrangements were further investigated with next-generation sequencing-based methodologies to verify the findings obtained with traditional cytogenetic methodologies. In the first case, CBA revealed a hyperdiploid karyotype with multiple structural abnormalities including additional material of unknown origin at 11p15; subsequent break-apart probe (BAP) FISH for NUP98 demonstrated 2 intact fusion signals and a single separate 5'NUP98 signal. However, whole-genome sequencing found no evidence of a NUP98 gene fusion. The results obtained with conventional cytogenetic methodologies were in fact attributable to structural variants (SV) with breakpoints not within NUP98 but within the 5'NUP98 BAP probe-binding sequence. In the second case, CBA revealed several structural and numeric abnormalities including a complex translocation between chromosomes 11 (at 11p15.4) and 19 (at 19p13.3) and an insertion of unknown material at 11p15.4. BAP FISH demonstrated a typical FISH signal pattern consistent with an apparent NUP98 rearrangement. However, no evidence of a NUP98 fusion was found on RNA sequencing. In conclusion, the two cases thus presented with clinical false positive NUP98 rearrangements by FISH. In the clinical laboratory, SVs in the vicinity of genes involved in recurrent rearrangements in hematologic malignancies may result in misleading results with conventional chromosome methodologies. This may preclude an accurate definition of the genetic attributes of malignancies with ensuing impacts on risk stratification and management. Higher-resolution testing methodologies such as whole-genome sequencing and RNA sequencing may be helpful in resolving unexpected results with conventional chromosome methodologies and enhancing the accuracy of genetic characterization of hematological malignancies in the clinical laboratory.

Integrative cytogenetic and molecular studies unmask "chromosomal mimicry" in hematologic malignancies

ABSTRACT

The detection of structural variants (SVs) represents a critical component in the diagnostic evaluation and treatment of many hematologic malignancies. Although clinical SV testing mainly consists of traditional cytogenetic methodologies, technological innovations have led to alternative approaches with improved resolution. In this study, we sought to characterize the clinical impact of targeted RNA sequencing on the diagnosis of myeloid and immature lymphoid malignancies. Across a cohort (n = 380) of myeloid (87%) and immature lymphoid (13%) tumors, we compared SVs detected by chromosome banding analysis (CBA) and fusion events detected by anchored multiplex polymerase chain reaction (AMP)-targeted RNA sequencing. Variants detected by either assay were categorized using a 5-tier system: tier 1 (established clinical significance); tier 2 (possible clinical significance); tier 3 (unknown significance); tier 4 (known germ line variants), and tier 5 (no variants detected). The combined use of AMP and CBA improved the detection of clinically relevant (tier 1 or 2) findings in 10% of cases. Unexpectedly, in 1% (3/380) of the patients in our study, CBA appeared to detect a defining SV, for example, t(9;22)(q34;q11.2), that was not confirmed by AMP fusion studies. Subsequent evaluation by orthogonal approaches confirmed breakpoints on the expected chromosomes but did not involve the anticipated genes. Our study indicates that "chromosomal mimicry," a phenomenon in which chromosome morphology resembles a known SV but lacks the expected gene-level rearrangement, is an infrequent but recurrent finding with the potential to confound clinical management. Our study highlights the need for assays with gene-level resolution in the diagnostic evaluation of hematologic malignancies.

RNASeq Analysis for Accurate Identification of Fusion Partners in Tumor Specific Translocations Detected by Standard FISH Probes in Hematologic Malignancies

Background

Fluorescence labeled DNA probes and in situ hybridization methods had shorter turn round time for results revolutionized their clinical application. Signals obtained from these probes are highly specific, yet they can produce fusion signals not necessarily representing fusion of actual genes due to other genes included in the probe design. In this study we evaluated discordance between cytogenetic, FISH and RNAseq results in 3 different patients with hematologic malignancies and illustrated the need to perform next generation sequencing (NGS) or RNASeq to accurately interpret FISH results.

Methods

Bone marrow or peripheral blood karyotypes and FISH were performed to detect recurring translocations associated with hematologic malignancies in clinical samples routinely referred to our clinical cytogenetics laboratory. When required, NGS was performed on DNA and RNA libraries to detect somatic alterations and gene fusions in some of these specimens. Discordance in results between these methods is further evaluated.

Results

For a patient with plasma cell leukemia standard FGFR3 / IGH dual fusion FISH assay detected fusion that was interpreted as FGFR3-positive leukemia, whereas NGS/RNASeq detected NSD2::IGH. For a pediatric acute lymphoblastic leukemia patient, a genetic diagnosis of PDGFRB-positive ALL was rendered because the PDGFRB break-apart probe detected clonal rearrangement, whereas NGS detected MEF2D::CSF1R. A MYC-positive B-prolymphocytic leukemia was rendered for another patient with a cytogenetically identified t(8;14) and MYC::IGH by FISH, whereas NGS detected a novel PVT1::RCOR1 not previously reported.

Conclusions

These are 3 cases in a series of several other concordant results, nevertheless, elucidate limitations when interpreting FISH results in clinical applications, particularly when other genes are included in probe design. In addition, when the observed FISH signals are atypical, this study illustrates the necessity to perform complementary laboratory assays, such as NGS and/or RNASeq, to accurately identify fusion genes in tumorigenic translocations.

Feasibility of Optical Genome Mapping in Cytogenetic Diagnostics of Hematological Neoplasms: A New Way to Look at DNA

Optical genome mapping (OGM) is a new genome-wide technology that can reveal both structural genomic variations (SVs) and copy number variations (CNVs) in a single assay. OGM was initially employed to perform genome assembly and genome research, but it is now more widely used to study chromosome aberrations in genetic disorders and in human cancer. One of the most useful OGM applications is in hematological malignancies, where chromosomal rearrangements are frequent and conventional cytogenetic analysis alone is insufficient, necessitating further confirmation using ancillary techniques such as fluorescence in situ hybridization, chromosomal microarrays, or multiple ligation-dependent probe amplification. The first studies tested OGM efficiency and sensitivity for SV and CNV detection, comparing heterogeneous groups of lymphoid and myeloid hematological sample data with those obtained using standard cytogenetic diagnostic tests. Most of the work based on this innovative technology was focused on myelodysplastic syndromes (MDSs), acute myeloid leukemia (AML), and acute lymphoblastic leukemia (ALL), whereas little attention was paid to chronic lymphocytic leukemia (CLL) or multiple myeloma (MM), and none was paid to lymphomas. The studies showed that OGM can now be considered as a highly reliable method, concordant with standard cytogenetic techniques but able to detect novel clinically significant SVs, thus allowing better patient classification, prognostic stratification, and therapeutic choices in hematological malignancies.

Guiding the global evolution of cytogenetic testing for hematologic malignancies

Cytogenetics has long represented a critical component in the clinical evaluation of hematologic malignancies. Chromosome banding studies provide a simultaneous snapshot of genome-wide copy number and structural variation, which have been shown to drive tumorigenesis, define diseases, and guide treatment. Technological innovations in sequencing have ushered in our present-day clinical genomics era. With recent publications highlighting novel sequencing technologies as alternatives to conventional cytogenetic approaches, we, an international consortium of laboratory geneticists, pathologists, and oncologists, describe herein the advantages and limitations of both conventional chromosome banding and novel sequencing technologies and share our considerations on crucial next steps to implement these novel technologies in the global clinical setting for a more accurate cytogenetic evaluation, which may provide improved diagnosis and treatment management. Considering the clinical, logistic, technical, and financial implications, we provide points to consider for the global evolution of cytogenetic testing.

Utilizing next-generation sequencing to characterize a case of acute myeloid leukemia with t(4;12)(q12;p13) in the absence of ETV6/CHIC2 and ETV6/PDGFRA gene fusions

The t(4;12)(q12;p13) has been rarely reported in both myeloid/lymphoid neoplasms with eosinophilia (ETV6/PDGFRA gene fusion) and acute myeloid leukemia (AML) (ETV6/CHIC2 gene fusion). The ability to accurately characterize t(4;12) is critical as myeloid neoplasms with PDGFRA rearrangements may be amenable to tyrosine kinase inhibitor (TKI) therapy. Herein, we describe a 60-year-old male with newly diagnosed AML and t(4;12)(q12;p13) by conventional chromosome studies. While the ETV6 break-apart fluorescence in situ hybridization (FISH) probe set demonstrated a balanced ETV6 gene rearrangement, the FIP1L1/CHIC2/PDGFRA tri-color and PDGFRA break-apart FISH probe sets could not resolve the ETV6 gene fusion partner. Mate-pair sequencing (MPseq), a next-generation sequencing assay, was subsequently performed and identified an ETV6 gene rearrangement (at 12p13) that involved an intergenic chromosomal region at 4q12, located between the CHIC2 and PDGFRA gene regions. Having excluded involvement by the PDGFRA gene region, the patient will not be considered for TKI therapy at any point during his medical management. The accurate characterization of structural rearrangements by NGS-based technologies, as demonstrated in this case, highlights the clinical relevance and potential impact on patient medical management of modern cytogenetic techniques.