A Novel Acquired t(2;4)(q36.1;q24) with a Concurrent Submicroscopic del(4)(q23q24) in An Adult with Polycythemia Vera

Abstract

Background: Polycythemia vera (PV) is a clonal myeloid stem cell disease characterized by a growth-factor independent erythroid proliferation with an inherent tendency to transform into overt acute myeloid malignancy. Approximately 95% of the PV patients harbor the JAK2V617F mutation while less than 35% of the patients harbor cytogenetic abnormalities at the time of diagnosis. Methods and Results: Here we present a JAK2V617F positive PV patient where G-banding revealed an apparently balanced t(2;4)(q35;q21), which was confirmed by 24-color karyotyping. Oligonucleotide array-based Comparative Genomic Hybridization (aCGH) analysis revealed an interstitial 5.4 Mb large deletion at 4q23q24. Locus-specific fluorescent in situ hybridization (FISH) analyses confirmed the mono-allelic 4q deletion and that it was located on der(4)t(2;4). Additional locus-specific bacterial artificial chromosome (BAC) probes and mBanding refined the breakpoint on chromosome 2. With these methods the karyotype was revised to 46,XX,t(2;4)(q36.1;q24)[18]/46,XX[7]. Conclusions: This is the first report on a PV patient associated with an acquired novel t(2;4)(q36.1;q24) and a concurrent submicroscopic deletion del(4)(q23q24). The study also underscores the benefit of combined usage of FISH and oligo-based aCGH analysis in characterizing chromosomal abnormalities. The present findings provide additional clues to unravel important molecular pathways in PV to obtain the full spectrum of acquired chromosomal and genomic aberrations, which eventually may improve treatment options.

Keywords: chromosomal abnormality; oligonucleotide array-based Comparative Genomic Hybridization (aCGH); polycythemia vera; submicroscopic deletion del(4q).

Figure 1

Multi-color Fluorescence in Situ Hybridization (FISH). (A) 24-color karyotyping revealed the translocation t(2;4). White arrows indicate the derivative chromosomes. (B) mBanding analysis of chromosomes 2. The single-color gallery tool in ISIS software shows assigned false colors (FC) and individual color schemes of labeled chromosomes arranged in their capture sequence FITC (fluorescein isothiocyanate), SpO (spectrum orange), TR (Texas red), Cy5 (cyanine), DEAC (7-diethylaminocoumarin-3-carboxylic acid, succinimidyl ester). Upper row shows the normal chromosome 2, middle row shows the der(2)t(2;4) and lower row show the der(4)t(2;4) from the patient’s karyotype. The right-hand side shows a schematic representation of the localization of the different multicolor probes of XCyte 2 relative to the ideogram of chromosome 2 together with breakpoint marked by the arrow.

Figure 2

Oligo-based array Comparative Genomic Hybridization (oaCGH) analysis and FISH validation. (A) Whole genome view of the bone marrow sample showing a submicroscopic deletion at chromosome 4 indicated by the arrow. Horizontal blue lines indicate log₂ ratios +0.24 and +0.60 and red lines indicate log₂ ratios −0.24 and −1.0. The X-axis at the bottom indicates chromosomal position. (B) Chromosome view of chromosome 4 with deletion at 4q23-q24 indicated by an arrow and ideogram of chromosome 4 to the left. (C) A zoom view of the deleted region as indicated by red shade corresponding to the deletion’s maximal chromosomal position. The red and green bars indicate the position of FISH probes used for validation. (D) FISH using BAC (Bacterial Artificial Chromosomes) probes RP11-842N10 (red) and RP11-867L22 (green) at 4q23 and 4q24, respectively, and centromeric probe D4Z1 (aqua) confirms the interstitial mono-allelic deletion and translocation in nuclei and metaphases from the patient.

Figure 3

Break point mapping with FISH probes. Partial karyograms of normal chromosome 2, der(2)t(2;4), normal chromosome 4, and der(4)t(2;4) arranged from top to bottom showing FISH results after hybridization using respective dual color probe sets as indicated at the top. WCP indicates whole chromosome painting, and chromosome 4 is indicated by the centromeric D4Z1 probe (aqua).

Figure 4

Model of chromosomal rearrangement. (A) Schematic representation of the translocation between chromosomes 2 and 4 with breakage points indicated by arrows together with the deleted region followed by reciprocal joining. (B) Schematic representation of genes (light brown boxes) mapping in correspondence to the breakpoint regions. Upper panel shows the joined regions of 2q36.1 and 4q24 and the lower panel shows the joined regions 4q23 and 2q36.1. The deleted chromosomal fragment del(4)(q23q24) is omitted and the genes located in this region are listed in Table 2. The axis at the bottom of each panel indicates the chromosomal position of the involved regions. The resolution of the aCGH analysis is limited to the kilobase pair level and the density of the oligo probes differ according to chromosomal regions with the highest density at known cancer genes. Vertical red and green bars indicate the relative genomic position of deleted (red) and not deleted (green) oligonucleotide probes in the aCGH analysis. Vertical yellow bars indicate the position of BAC clone RP11-79C2 being involved in the translocation.