Analysis of the t(3;8) of hereditary renal cell carcinoma: a palindrome-mediated translocation

It has emerged that palindrome-mediated genomic instability generates DNA-based rearrangements. The presence of palindromic AT-rich repeats (PATRRs) at the translocation breakpoints suggested a palindrome-mediated mechanism in the generation of several recurrent constitutional rearrangements: the t(11;22), t(17;22), and t(8;22). To date, all reported PATRR-mediated translocations include the PATRR on chromosome 22 (PATRR22) as a translocation partner. Here, the constitutional rearrangement, t(3;8)(p14.2;q24.1), segregating with renal cell carcinoma in two families, is examined. The chromosome 8 breakpoint lies in PATRR8 in the first intron of the RNF139 (TRC8) gene, whereas the chromosome 3 breakpoint is located in an AT-rich palindromic sequence in intron 3 of the FHIT gene (PATRR3). Thus, the t(3;8) is the first PATRR-mediated, recurrent, constitutional translocation that does not involve PATRR22. Furthermore, we detect de novo translocations similar to the t(11;22) and t(8;22), involving PATRR3 in normal sperm. The breakpoint on chromosome 3 is in proximity to FRA3B, the most common fragile site in the human genome and a site of frequent deletions in tumor cells. However, the lack of involvement of PATRR3 sequence in numerous FRA3B-related deletions suggests that there are several different DNA sequence-based etiologies responsible for chromosome 3p14.2 genomic rearrangements.

A palindrome-mediated recurrent translocation with 3:1 meiotic nondisjunction: the t(8;22)(q24.13;q11.21)

Palindrome-mediated genomic instability has been associated with chromosomal translocations, including the recurrent t(11;22)(q23;q11). We report a syndrome characterized by extremity anomalies, mild dysmorphia, and intellectual impairment caused by 3:1 meiotic segregation of a previously unrecognized recurrent palindrome-mediated rearrangement, the t(8;22)(q24.13;q11.21). There are at least ten prior reports of this translocation, and nearly identical PATRR8 and PATRR22 breakpoints were validated in several of these published cases. PCR analysis of sperm DNA from healthy males indicates that the t(8;22) arises de novo during gametogenesis in some, but not all, individuals. Furthermore, demonstration that de novo PATRR8-to-PATRR11 translocations occur in sperm suggests that palindrome-mediated translocation is a universal mechanism producing chromosomal rearrangements.

Interrupted aortic arch type B in A patient with cat eye syndrome

We report a patient with cat eye syndrome and interrupted aortic arch type B, a typical finding in the 22q11.2 deletion syndrome. Chromosomal analysis and fluorescent in situ hybridization (FISH) showed a supernumerary bisatellited isodicentric marker chromosome derived from chromosome 22. The segment from 22pter to 22q11.2 in the supernumerary chromosome found in our patient does not overlap with the region deleted in patients with the 22q11.2 deletion syndrome. However, the finding of an interrupted aortic arch type B is unusual in CES, although it is a frequent heart defect in the 22q11 deletion syndrome.

Deletion 22q11.2: report of a complex meiotic mechanism of origin

We report on the case of a patient with a typical de novo 3 Mb 22q11.2 deletion. Haplotype reconstruction of the family, using polymorphic markers flanking the deleted region, demonstrated a complex mechanism of origin of the deletion, involving one intrachromosomal and two interchromosomal events.

A palindrome-driven complex rearrangement of 22q11.2 and 8q24.1 elucidated using novel technologies

Constitutional translocations at the same 22q11.21 low copy repeat B (LCR-B) breakpoint involved in the recurrent t(11;22) are relatively abundant. A novel 46,XY,t(8;22)(q24.13;q11.21) rearrangement was investigated to determine whether the recurrent LCR-B breakpoint is involved. Investigations demonstrated an inversion of the 3Mb region typically deleted in patients with the 22q11.2 deletion syndrome. The 22q11.21 inversion appears to be mediated by low copy repeats, and is presumed to have taken place prior to translocation with 8q24.13. Despite predictions based on inversions observed in other chromosomes harboring low copy repeats, this 22q11.2 inversion has not been observed previously. The current studies utilize novel laser microdissection and MLPA (multiplex ligation-dependent probe amplification) approaches, as adjuncts to FISH, to map the breakpoints of the complex rearrangements of 22q11.21 and 8q24.21. The t(8;22) occurs between the recurrent site on 22q11.21 and an AT-rich site at 8q24.13, making it the fifth different chromosomal locus characterized at the nucleotide level engaged in a translocation with the unstable recurrent breakpoint at 22q11.21. Like the others, this breakpoint occurs at the center of a palindromic sequence. This sequence appears capable of forming a perfect 145 bp stem-loop. Remarkably, this site appears to have been involved in a previously reported t(3;8) occurring between 8q24.13 and FRA3B on 3p14.2. Further, the fragile site-like nature of all of the breakpoint sites involved in translocations with the recurrent site on 22q11.21, suggests a mechanism based on delay of DNA replication in the initiation of these chromosomal rearrangements.

Low copy repeats mediate distal chromosome 22q11.2 deletions: sequence analysis predicts breakpoint mechanisms

Genomic disorders contribute significantly to genetic disease and, as detection methods improve, greater numbers are being defined. Paralogous low copy repeats (LCRs) mediate many of the chromosomal rearrangements that underlie these disorders, predisposing chromosomes to recombination errors. Deletions of proximal 22q11.2 comprise the most frequently occurring microdeletion syndrome, DiGeorge/Velocardiofacial syndrome (DGS/VCFS), in which most breakpoints have been localized to a 3 Mb region containing four large LCRs. Immediately distal to this region, there are another four related but smaller LCRs that have not been characterized extensively. We used paralog-specific primers and long-range PCR to clone, sequence, and examine the distal deletion breakpoints from two patients with de novo deletions mapping to these distal LCRs. Our results present definitive evidence of the direct involvement of LCRs in 22q11 deletions and map both breakpoints to the BCRL module, common to most 22q11 LCRs, suggesting a potential region for LCR-mediated rearrangement both in the distal LCRs and in the DGS interval. These are the first reported cases of distal 22q11 deletions in which breakpoints have been characterized at the nucleotide level within LCRs, confirming that distal 22q11 LCRs can and do mediate rearrangements leading to genomic disorders.

A unique case of der(11)t(11;22),-22 arising from 3:1 segregation of a maternal t(11;22) in a family with co-segregation of the translocation and breast cancer

OBJECTIVE

To report the first tertiary monosomy in a pregnancy loss to a female t(11;22) carrier.

METHODS

The patient was a 34-year-old G10P1 female known to have a balanced translocation t(11;22)(q23;q11.2). She had one female livebirth (a translocation carrier) and eight miscarriages. Five female relatives known to be translocation carriers had a history of breast cancer, three of them premenopausally. The patient herself had a malignant melanoma.

RESULTS

During the 10th pregnancy, ultrasound showed a viable embryo at 6 weeks of gestation, but loss of embryonic heartbeat by 7.5 weeks. Culture of the products of conception at 8 weeks of gestation showed the karyotype: 46,XY,+2,der(11)t(11;22)(q23;q11.2)mat,-22[4]/45,XY,der(11)t(11;22)(q23;q11.2)mat,-22[4], resulting from fertilization of the maternal 3:1 segregation product containing only the der(11) by a normal gamete. Subsequently, she became pregnant with a normal 46,XX fetus. FISH analysis indicated that the breakpoints on 11q and 22q in the patient were in the previously described region common to typical recurrent t(11;22). In addition, a nested-PCR-based approach showed that they were located within the same palindromic AT-rich sequence previously described.

CONCLUSION

This case demonstrates that the tertiary monosomy resulting from the 3:1 segregation is compatible with embryonic survival into the first trimester. It is also another example of apparent association of the constitutional translocation t(11;22) and breast cancer.

Aberrant interchromosomal exchanges are the predominant cause of the 22q11.2 deletion

Chromosome 22q11.2 deletions are found in almost 90% of patients with DiGeorge/velocardiofacial syndrome (DGS/VCFS). Large, chromosome-specific low copy repeats (LCRs), flanking and within the deletion interval, are presumed to lead to misalignment and aberrant recombination in meiosis resulting in this frequent microdeletion syndrome. We traced the grandparental origin of regions flanking de novo 3 Mb deletions in 20 informative three-generation families. Haplotype reconstruction showed an unexpectedly high number of proximal interchromosomal exchanges between homologs, occurring in 19/20 families. Instead, the normal chromosome 22 in these probands showed interchromosomal exchanges in 2/15 informative meioses, a rate consistent with the genetic distance. Meiotic exchanges, visualized as MLH1 foci, localize to the distal long arm of chromosome 22 in 75% of human spermatocytes tested, also reflecting the genetic map. Additionally, we found no effect of proband gender or parental age on the crossover frequency. Parental origin studies in 65 de novo 3 Mb deletions (including these 20 patients) demonstrated no bias. Unlike Williams syndrome, we found no chromosomal inversions flanked by LCRs in 22 sets of parents of 22q11 deleted patients, or in eight non-deleted patients with a DGS/VCFS phenotype using FISH. Our data are consistent with significant aberrant interchromosomal exchange events during meiosis I in the proximal region of the affected chromosome 22 as the likely etiology for the deletion. This type of exchange occurs more often than is described for deletions of chromosomes 7q11, 15q11, 17p11 and 17q11, implying a difference in the meiotic behavior of chromosome 22.

A molecular analysis of the phylogenetic affinities of Saccoglossus cambrensis Brambell & Cole (Hemichordata)

Traditional approaches to phylogeny reconstruction have not allowed precise resolution of the evolutionary relationships between the major deuterostome phyla (chordates, hemichordates, echinoderms). Here we report the use of a molecular approach to investigate deuterostome phylogeny. We have used a polymerase chain reaction-based strategy to amplify, clone and sequence parts of the genes coding for 18S ribosomal RNA from Saccoglossus cambrensis (Hemichordata), Arbacia sp. (Echinodermata) and, for comparison, Mytilus edulis (Mollusca). We report the results of phylogenetic reconstructions using these, and homologous sequences from other eukaryotes. The results of our analyses are consistent with the hypothesis that S. cambrensis and vertebrates share a common ancestor not shared by echinoderms.