Dosage and Imprinting Effects in Abnormalities of Human Chromosome 15

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are distinct mental retardation disorders caused by paternal deficiency (PWS) or maternal deficiency (AS) of gene(s) in 15qll.2-ql3. We have constructed a 3.5 Mb yeast artificial chromosome (YAC) contig of the PWS/AS region and cosmid contigs of selected YACs at D15S13, SNRPN, S10, and S113. Cosmid clones have been used for fluorescence in situ hybridization (FISH) detection of deletions in PWS and AS patients. In addition, a total of 28 short tandem repeat polymorphisms (STRs) have been mapped to specific YACs in the contig, providing a highly informative set of markers for detection of deletion or uniparental disomy (UPD) in PWS and AS patients. Use of the 3 most informative markers in this region (S542, S128, and ASSCA-1) plus 3 markers distal on 15q (S123, S125, and S131) provide an efficient diagnostic strategy for UPD15.A combination of FISH and STR analysis has identified small deletions in one sporadic and one familial case of PWS (family O). Both deletions involve all or part of the SNRPN gene but do not extend telomeric to PAR-5 or PAR-1, two novel transcripts expressed exclusively from the paternal chromosome. However, expression of SNRPN, PAR-5, and PAR-1 is lost in both cases, implying the presence of an imprinting control region near SNRPN. The smallest deletion in family O is estimated at approximately 30-40 kb in size and involves a newly identified CpG island at the 5′ end of SNRPN which is methylated on the maternal chromosome. This small deletion in two PWS affected siblings was present in the father and the paternal grandmother, both of whom were phenotypically normal.

A physical map of the X chromosome of Drosophila melanogaster: cosmid contigs and sequence tagged sites.

A physical map of the euchromatic X chromosome of Drosophila melanogaster has been constructed by assembling contiguous arrays of cosmids that were selected by screening a library with DNA isolated from microamplified chromosomal divisions. This map, consisting of 893 cosmids, covers approximately 64% of the euchromatic part of the chromosome. In addition, 568 sequence tagged sites (STS), in aggregate representing 120 kb of sequenced DNA, were derived from selected cosmids. Most of these STSs, spaced at an average distance of approximately 35 kb along the euchromatic region of the chromosome, represent DNA tags that can be used as entry points to the fruitfly genome. Furthermore, 42 genes have been placed on the physical map, either through the hybridization of specific probes to the cosmids or through the fact that they were represented among the STSs. These provide a link between the physical and the genetic maps of D. melanogaster. Nine novel genes have been tentatively identified in Drosophila on the basis of matches between STS sequences and sequences from other species.

Molecular characterization of 16p deletions associated with inversion 16 defines the critical fusion for leukemogenesis

The inversion of chromosome 16 [inv(16)] in acute myeloid leukemia (AML) is associated with a p-arm deletion in a subset of patients. The inversion results in two fusion genes: 5′-CBFB/MYH11–3′ on 16p and 5′- MYH11/CBFB-3′ on 16q. We have studied cells from 42 patients with inv(16) (38 patients) or t(16;16) (four patients) to define the frequency and characteristics of the deletion further. Using fluorescence in situ hybridization (FISH) with probes from cosmids, cosmid contigs, and yeast artificial chromosomes (YACs), we found that six patients with inv(16) had a deletion of between 150 and 350 kb centromeric to the p-arm inversion breakpoint cluster region (p-ibc). This region was shown to contain the 5′ portion of the myosin heavy chain (MYH11) gene. YACs containing the p-ibc, which had been useful as FISH probes in the diagnosis of inv(16), detected the inversion in deletion as well as nondeletion patient cells. Thus, the deleted region identified in patients is entirely contained within the human genomic content of the YACs. Southern blot experiments using probes flanking the p-ibc indicated that the deletion removes segments within 10 kb centromeric of the p-ibc. Reverse transcription-polymerase chain reaction (RT-PCR) using primers from the 5′ region of CBFB and the 3′ region of MYH11 (distal to the p-ibc) produced the 5′-CBFB/MYH11–3′ chimeric transcript in inv(16)/del patients. These data confirm that the 5′-CBFB/MYH11–3′ chimeric transcript, rather than the reciprocal 5′- MYH11/CBFB-3′, is the critical product for chromosome 16-related leukemogenesis.