The identification of chromosomal translocation, t(4;6)(q22;q15), in prostate cancer

Abstract

Our previous work identified a chromosomal translocation t(4;6) in prostate cancer cell lines and primary tumors. Using probes located on 4q22 and 6q15, the breakpoints identified in LNCaP cells, we performed fluorescence in situ hybridization analysis to detect this translocation in a large series of clinical localized prostate cancer samples treated conservatively. We found that t(4;6)(q22;q15) occurred in 78 of 667 cases (11.7%). The t(4;6)(q22;q15) was not independently associated with patient outcome. However, it occurs more frequently in high clinical T stage, high tumor volume specimens and in those with high baseline PSA (P=0.001, 0.001 and 0.01, respectively). The t(4;6)(q22;q15) occurred more frequently in samples with two or more TMPRSS2:ERG fusion genes caused by internal deletion than in samples without these genomic alterations, but this correlation is not statistically significant (P=0.0628). The potential role of this translocation in the development of human prostate cancer is discussed.

Figure 1

Maps showing the position of the BACs used as probes in FISH assays. (A) Probe set I includes three BACs, RP11-18N21, RP11-681L8 and RP11-240J11, corresponding to a 0.54Mb region of distal 4q22 breakpoint. (B) Probe set II includes three BAC clones, RP11-111J1, RP11-595C20 and RP1-214H13, corresponding to a 0.39Mb region of 6q14.3 (proximal to 6q15 breakpoint). (C) Probe set III includes three BACs, RP1-44N23, RP1-154G14 and RP11-104N3, corresponding to a 0.80Mb region of 6q15 deleted in LNCaP cells.

Figure 2

Schematic representation of FISH detection of t(4;6)(q22;q15). In the signal co-localisation study (A–B), probe set I and II were used and in the signal split apart analysis (C–E), probe set II and III were used. (A) In a normal cell, there are two pairs of normal chromosome 4 and 6 carrying hybridized red (probe set I) and green (probe set II) signals respectively. In an interphase nucleus, it shows two pairs of separated green and red signals. (B) In a cancer cell with t(4;6)(q22;q15), there are one of each normal chromosome 4 and 6 carrying hybridized red and green signals. The translocation brings probe set I on chromosome 4 and probe set II on chromosome 6 together. In an interphase nucleus, it shows one pair of separated red and green signals and one pair of co-localized red and green signals. (C) In a normal cell, one pair of chromosome 6 carry hybridized green (probe set II) and red (probe set III) signals. In an interphase nucleus, it shows two pairs of co-localized green and red signals. Bp1: centromeric breakpoint; bp2: telemetric breakpoint. (D) In a cancer cell with t(4;6)(q22;q15), one chromosome 6 carries hybridized green and red signals, but on the chromosome 6 with translocation, the signals are split apart. In an interphase nucleus, it shows one pair of co-localized green and red signals and another pair of separated red and green signals. (E) In the case if the telemeric part of 6q is lost or two breakpoints (bp1 and bp2) occurs due to the t(4;6) rearrangement, besides the normal chromosome 6 carrying hybridized red and green signals, only one green signal is left on the abnormal chromosome 6 fragment(s). In an interphase nucleus, it shows one pair of co-localized green and red signals and a single green signal.

Figure 3

Examples of FISH signals using the probe co-localization analysis as illustrated in Figure 2A and 2B. (A) A co-localization negative area in a normal control prostate sample where red signals were seen separated from the green signals. (B) In a prostate cancer sample, co-localization of red (4q22 probes) and green (6q14.3 probes) signals (arrow) were found in many cells.

Figure 4

Confirmation of the t(4;6) positive cells using split signal approach. (A) and (C) The co-localized signals generated by probes on 4q22 (probe set I, red) and 6q14.3 (probe set II, green) are indicated by yellow arrows. (B) and (D) The nuclei from A and B, respectively, were re-hybridized with probes on 6q14.3 (green) and 6q15 (probe set III, red). (B) The signals generated by probes on 6q14.3 and 6q15 showed split green and red signal (green and red arrows respectively), indicating translocation break point at 6q15 region as illustrated in Figure 2D. (D) A single green signal generated by the probe on 6q14.3 was detected (green arrow), whilst the red signal (probe on 6q15) was lost, indicating deletion at this region as illustrated in Figure 2E.

Figure 5

Kaplan-Meier analysis comparing prostate cancer patient outcomes with the t(4;6)(q22;q15) status. (A) and (B) Cause-specific survival and overall survival respectively.