Heterozygous submicroscopic inversions involving olfactory receptor-gene clusters mediate the recurrent t(4;8)(p16;p23) translocation

Abstract

The t(4;8)(p16;p23) translocation, in either the balanced form or the unbalanced form, has been reported several times. Taking into consideration the fact that this translocation may be undetected in routine cytogenetics, we find that it may be the most frequent translocation after t(11q;22q), which is the most common reciprocal translocation in humans. Case subjects with der(4) have the Wolf-Hirschhorn syndrome, whereas case subjects with der(8) show a milder spectrum of dysmorphic features. Two pairs of the many olfactory receptor (OR)-gene clusters are located close to each other, on both 4p16 and 8p23. Previously, we demonstrated that an inversion polymorphism of the OR region at 8p23 plays a crucial role in the generation of chromosomal imbalances through unusual meiotic exchanges. These findings prompted us to investigate whether OR-related inversion polymorphisms at 4p16 and 8p23 might also be involved in the origin of the t(4;8)(p16;p23) translocation. In seven case subjects (five of whom both represented de novo cases and were of maternal origin), including individuals with unbalanced and balanced translocations, we demonstrated that the breakpoints fell within the 4p and 8p OR-gene clusters. FISH experiments with appropriate bacterial-artificial-chromosome probes detected heterozygous submicroscopic inversions of both 4p and 8p regions in all the five mothers of the de novo case subjects. Heterozygous inversions on 4p16 and 8p23 were detected in 12.5% and 26% of control subjects, respectively, whereas 2.5% of them were scored as doubly heterozygous. These novel data emphasize the importance of segmental duplications and large-scale genomic polymorphisms in the evolution and pathology of the human genome.

Figure 1

OR contigs at 4p. BACs marked with an asterisk (*) are from the RP11 library. BACs shown in red were used in FISH experiments on metaphases and interphases from case subjects 1–7. REPD and REPP indicate the two duplicons containing the OR- and angiopoietin-gene clusters. Each of them is ∼400 kb. In both REPD and REPP, all the genes or pseudogenes have the 3′ oriented toward the 4p telomere. The distance between the two REPs is ∼6 Mb. Blue arrows on the BACs indicate the genes they contain; their orientation is that provided by the NCBI Entrez Genome View and the Celera Publication Site databases. The markers in the region are indicated above the continuous black line. The breakpoint of the der(4) in case subjects 1 and 2 fell in REPP; the breakpoint of the der(4) in case subjects 3–7 fell in the 4p REPD. Red and green circles indicate the BACs at the edge of the inverted region (fig. 4a). Red and green squares indicate BACs used, in FISH, to demonstrate that they were not inverted, thus demonstrating that the inverted region is included between the two REPs.

Figure 2

Metaphase FISH on case subject 7 (with the 46,XY,t(4;8)(p16;p23.1) translocation), showing the der(4) breakpoint. In the ideograms, the normal 4p is on the left, and the two derivative chromosomes are on the right; the red and orange squares indicate the REPD and the REPP, respectively. Red signals correspond to the distal 4p-REPD BAC RP11-324i10; these signals are present in several OR-gene cluster regions. a, Probe RP11-529e10 (green signals), which maps distal to the 4p REPD. This clone is translocated to the der(8). b, Probe RP11-323f5 (green signals), which is one of the more distal clones in the region included between the two 4p REPs. This clone remains on the der(4). These data demonstrate that the 4p breakpoint in case subject 7 is within the REPD (see also fig. 1).

Figure 3

Metaphase FISH in case subject 7 (with the 46,XY,t(4;8)(p16;p23.1) translocation), showing the der(8) breakpoint. In the ideograms, the normal 8p is on the left, and the two derivative chromosomes are on the right; the red and orange squares indicate the REPD and the REPP, respectively. Green signals correspond to GS-42i21, which maps within the 8p REPD; these signals are present in several OR-gene cluster regions. a, Probe GS-143g5 (red signals), which is distal but adjacent to the 8p REPD. This clone is transposed to the der(4). b, Probe GS-173o4 (red signals), which is the most distal clone within the region included between the two 8p REPs. This clone remains on the der(8). These data demonstrate that the 8p breakpoint is within the REPD (see also fig. 1 in Eichler 2001).

Figure 4

FISH in the mother of case subject 3 (with 46,XX). a, Normal and inverted 4p. At left, the ideograms show that clones RP11-323f5 (red signals) and RP11-448g15 (green signals), included between the two 4p REPs, are inverted in the chromosome 4 (arrowhead). In the square, a nucleus shows the same inversion; yellow signals refer to the control probe RP11-520m5, which is distal to the 4p REPD (see fig. 1), and red and green signals refer to RP11-323f5 and RP11-190l6, respectively (these two clones are at a distance of ∼2 Mb and thus are more appropriate for interphase FISH). b, Normal and inverted 8p. At right, the ideograms show that clones GS-173o4 (red signals) and GS-257o3 (green signals), included between the two 8p REPs, are inverted in one chromosome 8 (arrowhead). RP11-563o19 (yellow signals), at 8p12, was used as a control probe.