S Daw

Dual-probe fluorescence in situ hybridization assay for detecting deletions associated with VCFS/DiGeorge syndrome I and DiGeorge syndrome II loci

Over 90% of patients with DiGeorge syndrome (DGS) or velocardiofacial syndrome (VCFS) have a microdeletion at 22q11.2. Given that these deletions are difficult to visualize at the light microscopic level, fluorescence in situ hybridization (FISH) has been instrumental in the diagnosis of this disorder. Deletions on the short arm of chromosome 10 are also associated with a DGS-like phenotype. Since deletions at 22q11.2 and at 10p13p14 result in similar findings, we have developed a dual-probe FISH assay for screening samples referred for DGS or VCFS in the clinical laboratory. This assay includes two test probes for the loci, DGSI at 22q11.2 and DGSII at 10p13p14, and centromeric probes for chromosomes 10 and 22. Of 412 patients tested, 54 were found to be deleted for the DGSI locus on chromosome 22 (13%), and a single patient was found deleted for the DGSII locus on chromosome 10 (0. 24%). The patient with the 10p deletion had facial features consistent with VCFS, plus sensorineural hearing loss, and renal anomalies. Cytogenetic analysis showed a large deletion of 10p [46, XX,del(10)(p12.2p14)] and FISH using a 10p telomere region-specific probe confirmed the interstitial nature of the deletion. Analysis for the DGSI and the DGSII loci suggests that the deletion of the DGSII locus on chromosome 10 may be 50 times less frequent than the deletion of DGSI on chromosome 22. The incidence of deletions at 22q11.2 has been estimated to be 1 in 4000 newborns; therefore, the deletion at 10p13p14 may be estimated to occur in 1 in 200,000 live births.

Isolation of a new marker and conserved sequences close to the DiGeorge syndrome marker HP500 (D22S134).

End fragment cloning from a YAC at the D22S134 locus allowed the isolation of a new probe HD7k. This marker detects hemizygosity in two patients previously shown to be dizygous for D22S134. This positions the distal deletion breakpoint in these patients to the sequences within the YAC, and confirms that HD7k is proximal to D22S134. In a search for coding sequences within the region commonly deleted in DGS we have identified a conserved sequence at D22S134. Although no cDNAs have yet been isolated, genomic sequencing shows a short open reading frame with weak similarity to collagen proteins.

A novel C2H2 zinc-finger protein gene (ZNF160) maps to human chromosome 19q13.3-q13.4.

The human genome may contain up to 400 genes encoding zinc-finger (ZNF) proteins; a high proportion of those mapped have been localized to human chromosome 19. Heubner and colleagues have mapped 6 cDNAs containing a ZNF motif to 19q, and one to 19p, using somatic cell hybrids. Ten further sequences were regionally assigned by Lichter and colleagues, with a retinoic acid inducible ZNF gene being mapped to 19q13.2-q13.4. Thirty-nine cosmids identified on the basis of cross-hybridization to a ZNF {open_quotes}linker{close_quotes} sequence were mapped by FISH, 24 mapping to 19p and 15 to 19q. However, it is not known which of these sequences are transcribed or encode the cDNAs mentioned above. Forty Kruppel C{sub 2}H{sub 2}-related genes have been mapped to a cluster on 19p12-p13.1. It is interesting that none of these genes is detectable in the mouse or rat genomes, suggesting a relatively recent evolutionary origin for this cluster. 12 refs., 1 fig.