Rearrangements involving the RET gene are common in radiation-associated papillary thyroid cancer (PTC). The RET/PTC1 type of rearrangement is an inversion of chromosome 10 mediated by illegitimate recombination between the RET and the H4 genes, which are 30 megabases apart. Here we ask whether despite the great linear distance between them, RET and H4 recombination might be promoted by their proximity in the nucleus. We used two-color fluorescence in situ hybridization and three-dimensional microscopy to map the positions of the RET and H4 loci within interphase nuclei. At least one pair of RET and H4 was juxtaposed in 35% of normal human thyroid cells and in 21% of peripheral blood lymphocytes, but only in 6% of normal mammary epithelial cells. Spatial contiguity of RET and H4 may provide a structural basis for generation of RET/PTC1 rearrangement by allowing a single radiation track to produce a double-strand break in each gene at the same site in the nucleus.

Rubinstein-Taybi syndrome (RTS) is a malformation syndrome characterised by facial abnormalities, broad thumbs, broad big toes, and mental retardation. In a subset of RTS patients, microdeletions, translocations, and inversions involving chromosome band 16p13.3 can be detected. We have previously shown that disruption of the human CREB binding protein (CREBBP or CBP) gene, either by these gross chromosomal rearrangements or by point mutations, leads to RTS. CBP is a large nuclear protein involved in transcription regulation, chromatin remodelling, and the integration of several diVerent signal transduction pathways. Here we report diagnostic analysis of CBP in 194 RTS patients, divided into several subsets. In one case the mother is also suspect of having RTS. Analyses of the entire CBP gene by the protein truncation test showed 4/37 truncating mutations. Two point mutations, one 11 bp deletion, and one mutation aVecting the splicing of the second exon were detected by subsequent sequencing. Screening the CBP gene for larger deletions, by using diVerent cosmid probes in FISH, showed 14/171 microdeletions. Using five cosmid probes that contain the entire gene, we found 8/89 microdeletions of which 4/8 were 5' or interstitial. This last subset of microdeletions would not have been detected using the commonly used 3' probe RT1, showing the necessity of using all five probes.

We report the third known case of mesenchymal hamartoma of the liver (MHL) with a balanced translocation involving a common breakpoint, 19q13.4. A common clonal chromosome abnormality appears to characterize an important subset of MHL, some of which may be low-grade neoplasms. We found no consistent karyotype abnormality in a post-treatment sample of embryonal sarcoma of the liver (ESL). Reports of coexistent MHL and ESL in two patients and detection of 19q abnormalities in two ESLs appear to support Stocker's hypothesis of a histogenetic link between these two rare liver lesions. More data are needed to clarify this relationship. It is possible that MHLs are etiologically heterogenous and may be developmental disorders, disruptions, or neoplasms.

Most reported microdeletions of the CREB-binding protein (CBP) gene in the Rubinstein-Taybi syndrome (RTS) were detected by fluorescence in situ hybridization (FISH) with a single cosmid probe specific to the 3' region of the gene. In order to test the hypothesis that the rate of microdeletion-positive cases would be greater if the entire gene was evaluated, we performed FISH on 66 patients with an established diagnosis of RTS, using a panel of five cosmids that span the CBP gene. Five of 66 patients had deletions by FISH (9%), consistent with those rates reported in various series that ranged between 3-25%. Among our cases, different deletions were observed; one was deleted for the 5' but not the 3' region of the CBP gene (case 055). Other deletions included a total CBP deletion extending from the 5' through the 3' region (case 017), a deletion of all but the 5' region (cases 006 and 060), and an interstitial deletion in the 3' region (case 028). Fine breakpoint mapping with additional cosmid and yeast artificial chromosome (YAC) constructs was performed on these patients. The findings of a partial 5' deletion and of interstitial deletions of the CBP gene add to the known spectrum of mutations of this gene in RTS and demonstrate the need for evaluation of the entire CBP gene region for deletions rather than only the 3' region in RTS patients. These results further suggest that the true rate of microdeletion across the CBP gene detectable by FISH has yet to be established firmly. No phenotypic differences between partial deletion, complete deletion, and nondeletion patients were observed, supporting a haploinsufficiency model for RSTS.

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