Frequent chromosome aberrations revealed by molecular cytogenetic studies in patients with aniridia

Abstract

Seventy-seven patients with aniridia, referred for cytogenetic analysis predominantly to assess Wilms tumor risk, were studied by fluorescence in situ hybridization (FISH), through use of a panel of cosmids encompassing the aniridia-associated PAX6 gene, the Wilms tumor predisposition gene WT1, and flanking markers, in distal chromosome 11p13. Thirty patients were found to be chromosomally abnormal. Cytogenetically visible interstitial deletions involving 11p13 were found in 13 patients, 11 of which included WT1. A further 13 patients had cryptic deletions detectable only by FISH, 3 of which included WT1. Six of these, with deletions <500 kb, share a similar proximal breakpoint within a cosmid containing the last 10 exons of PAX6 and part of the neighboring gene, ELP4. Two of these six patients were mosaic for the deletion. The remaining four had chromosomal rearrangements: an unbalanced translocation, t(11;13), with a deletion including the WAGR (Wilms' tumor, aniridia, genitourinary abnormalities, and mental retardation) region, and three balanced rearrangements with what appear to be position effect breakpoints 3' of PAX6: (a) a t(7;11) with the 11p13 breakpoint approximately 30 kb downstream of PAX6, (b) a dir ins(12;11) with a breakpoint >50 kb from PAX6, and (c) an inv(11)(p13q13) with a breakpoint >75 kb downstream of PAX6. The proportion and spectrum of chromosome anomalies in familial (4/14, or 28.5%) and sporadic (26/63, or 41%) cases are not significantly different. An unexpectedly high frequency of chromosomal rearrangements is associated with both sporadic and familial aniridia in this cohort.

Figure 1

A, Map showing the position of the four cosmids used in the original FISH deletion analysis. D11Z1 is the centromeric alphoid repeat probe used to identify the 11 homologues, and D11S317 is a cosmid A4160, which lies ∼400 kb telomeric of FO2121 (see fig. 2). B, Higher-resolution map showing the relative positions of the cosmid contig covering PAX6 and the ∼180-kb region telomeric of the gene.

Figure 2

Details of the FISH results in the chromosomally abnormal patients. Patient numbers in boldface italic type are those with a proximal breakpoint in A1280. Shaded areas define the approximate size of the deletion. Size (kb) is the deletion size estimated using known mapping and sequence data. m = mosaic for deletion; F = familial; +/+ = signal on both homologues; +/− = one homologue deleted; +/dim = one homologue with diminished signal; mv = cosmid signal moved; st = cosmid signal stays on der(11); split = cosmid signal split; NT = not tested for this probe; § = cytogenetic breakpoint 11p15 (FISH on distal breakpoint not performed); * = no stored material for additional FISH; ** = also had t(2;11)(q33;q25) de novo.

Figure 3

A, Result from use of cosmid A1280 on patient 2. The normal and abnormal homologue (indicated by the arrow) show a different size of hybridization signal, showing that only part of the genomic sequence contained in this cosmid binds to the deleted 11. The green signal represents the D11Z1 centromere probe. B, Result from use of cosmid FO2121 on metaphases from the mosaic patient 2. The metaphase on the left shows FO2121 signal on both 11 homologues, whereas the metaphase on the right has cosmid signal on only one of the 11 homologues. The green signal represents the D11Z1 centromere probe. C, FISH results in patient 64 with the contig of plasmid probes pExδ-11, pEE2250, and pS1P, showing signals on both homologues. D, FISH results in patient 64. In contrast to C, signal is only seen on the normal homologue.

Figure 4

Map showing the position of the plasmid and cosmid probes used for FISH