Cytogenetic and molecular study of the PRDX4 gene in a t(X;18)(p22;q23): a cautionary tale

Abstract

The PRDX4 gene located at Xp22 encodes for a member of the peroxiredoxin gene family. Genes within this family exhibit thioredoxin-dependent peroxidase activity and have been implicated in cellular functioning, including proliferation and differentiation. Recently, PRDX4 has been identified as a partner gene in a t(X;21) translocation in a patient with acute myeloid leukemia. To determine whether PRDX4 was involved in other translocations, leukemia cells from 15 patients with Xp22 abnormalities were screened for involvement of the gene using fluorescence in situ hybridization (FISH). One sample from a 41-year-old woman with acute lymphoblastic leukemia showed three signals when hybridized with the PRDX4 probe. Cytogenetic analysis of the sample had identified a t(X;18)(p22;q23). Assuming that the three signals indicated a break within the PRDX4 gene, we performed FISH experiments and successfully narrowed the breakpoint on chromosome 18 to a 50-kb region. Subsequent analysis using spectral karyotyping showed that the leukemic cells had undergone multiple rearrangements and that a third X chromosome was present, albeit rearranged. Additional FISH experiments revealed that the third PRDX4 signal was the result of a third copy of the gene. Analysis of the other rearrangements has helped to characterize the multiple abnormalities within the leukemic cells. The findings underscore the importance of using multiple techniques when analyzing complex chromosomal rearrangements in malignant cells.

Figure 1

Karyotype after second relapse, interpreted as 48,X, t(X;18)(p22;q23), add (1)(q44), t(4;11)(q21;q23), t(6;14)(q23;q21), der(7)(t7;7)(p15;q11), +13, −14, +17, +20 [60%]. The derivative chromosome 4 and 11 are indicated with black arrows, X and 18 with dashed-black arrows. The third chromosome 17 was shown to be a der(x) chromosome by spectral karyotyping.

Figure 2

Hybridization with BAC RP11-40P7, labeled with Spectrum^® orange. Signals were seen on the normal X, the derivative X and a chromosome that was initially interpreted as the derivative chromosome 18.

Figure 3

BAC RP11-40P7 spanning PRDX4 was labeled with Spectrum^® green, RP11- 432 G11 with Spectrum^® orange. Both normal chromosome 18 and X show one normal signal, respectively. The derivative X chromosome shows the expected fusion signal, while the derivative chromosome 18 shows only one red signal. The third green signal for PRDX4 appears to be on a then unidentified chromosome, indicated with a white arrow.

Figure 4

SKY Image and partial karyotype. A: DAPI stained metaphase cell, B: Spectral image, C: Classified image, D: Partial karyotype for chromosomes 1, 4, 11, 18 and X.

Figure 5

FISH using two probes flanking the PRDX4 gene. The centromeric probe RP13 314C10 is labeled with Spectrum^® green, the telomeric probe (RP11-911N20) with Spectrum^® orange. Both probes colocalize on all three X chromosomes indicating that PRDX4 is not split by the translocation.