A dual FISH assay for detecting deletions associated with VCFS/DiGeorge syndrome I and DiGeorge syndrome II loci.

Abstract

DiGeorge syndrome (DCS) is a developmental field defect of the 3rd and 4th pharyngeal pouches. This syndrome is characterized by dysmorphic features, hypoplasia of the thymus and parathyroid glands, and conotruncal heart defects. Over 90% of patients with the syndrome have a microdeletion at 22qll.2. This deletion occurs in about 1 in 4000 live births. Since 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. Another less frequent chromosomal abnormality associated with the DGS phenotype is a deletion at 10pl3pl4. Since both deletions are associated with a similar phenotype, we have developed a dual FISH assay in our laboratory for screening samples referred for DGS or velocardiofacial syndrome (VCFS). This assay includes two test probes: a cosmid (F5) located in the DGSI critical region on chromosome 22, a PAC (72-A7) that is contained within the DGSII critical region on chromosome 10, and control probes specific for chromosomes 10 and 22. Since 1996, over 400 patients have been tested with the dual FISH assay. Recently, one patient was identified who was deleted for the DGSII locus at 10pl3pl4. This child had facial features of VCFS, sensorineural hearing loss, and renal anomalies. Cytogenetic analysis revealed a large deletion of 10p [46, XX, del(10)(pl2.2pl4)] and FISH using a 10p telomere-specific probe confirmed the interstitial nature of the deletion. The identification of this case prompted us to review the results of samples submitted to our laboratory for the dual probe assay. 412 patients have been screened with the dual assay and 54 were found to be deleted for 22q11.2 (13%), whereas only one patient was found to be deleted for the locus on chromosome 10 (0.24%). Hence, the deletion on chromosome 10p may be 50 times less frequent than the deletion on chromosome 22. Based on a frequency of 22q11.2 deletions of 1 in 4000, the incidence of deletions in the DGSII critical region on chromosome 10 is estimated to be about 1 in 200,000.