Inhibition of replicon initiation in human cells following stabilization of topoisomerase-DNA cleavable complexes.

Diploid human fibroblast strains were treated for 10 min with inhibitors of type I and type II DNA topoisomerases, and after removal of the inhibitors, the rate of initiation of DNA synthesis at replicon origins was determined. By alkaline elution chromatography, 4'-(9-acridinylamino)methanesulfon-m-anisidide (amsacrine), an inhibitor of DNA topoisomerase II, was shown to produce DNA strand breaks. These strand breaks are thought to reflect drug-induced stabilization of topoisomerase-DNA cleavable complexes. Removal of the drug led to a rapid resealing of the strand breaks by dissociation of the complexes. Velocity sedimentation analysis was used to quantify the effects of amsacrine treatment on DNA replication. It was demonstrated that transient exposure to low concentrations of amsacrine inhibited replicon initiation but did not substantially affect DNA chainelongation within operating replicons. Maximal inhibition of replicon initiation occurred 20 to 30 min after drug treatment, and the initiation rate recovered 30 to 90 min later. Ataxia telangiectasia cells displayed normal levels of amsacrine-induced DNA strand breaks during stabilization of cleavable complexes but failed to downregulate replicon initiation after exposure to the topoisomerase inhibitor. Thus, inhibition of replicon initiation in response to DNA damage appears to be an active process which requires a gene product which is defective or missing in ataxia telangiectasia cells. In normal human fibroblasts, the inhibition of DNA topoisomerase I by camptothecin produced reversible DNA strand breaks. Transient exposure to this drug also inhibited replicon initiation. These results suggest that the cellular response pathway which downregulates replicon initiation following genotoxic damage may respond to perturbations of chromatin structure which accompany stabilization of topoisomerase-DNA cleavable complexes.

Interstitial deletion of distal chromosome 4p in a patient without classical Wolf-Hirschhorn syndrome.

We report on a patient with a de novo interstitial deletion of chromosome 4p; 46,XY,del(4) (p15.31p16.3). The cytogenetic diagnosis would predict a patient with the Wolf-Hirschhorn syndrome (WHS) since deletions of 4p16 are associated with WHS [Wilson et al., 1981]. This patient lacks the facial characteristics of WHS, but has some anomalies of WHS that are also commonly seen in other syndromes, i.e., severe growth retardation, developmental delay, and hypospadias. His molecular distal breakpoint occurs in 4p16.3 as defined by fluorescence in situ hybridization and Southern blot analysis, and his deletion does not overlap with the currently proposed WHS critical region. This case gives further support to the distal position of the WHS critical region and demonstrates some of the WHS associated phenotypes that can be attributed to a deletion of the proximal third of 4p16.3.

Holoprosencephaly in an infant with a minute deletion of chromosome 21(q22.3).

We evaluated an infant because holoprosencephaly had been detected by prenatal ultrasound examination and magnetic resonance imaging (MRI). Postnatally, high-resolution cytogenetic studies showed a minute deletion of chromosome 21(q22.3). This patient lacks many of the characteristics associated with monosomy 21, partial monosomy 21, and ring 21 chromosome patients. She also lacks the midline facial abnormalities often seen with holoprosencephaly. The similarity in facial appearance between this case and one previously reported patient with holoprosencephaly and a ring chromosome 21 suggests a causal relationship between holoprosencephaly and deletion of chromosome 21(q22.3). These findings also suggest that infants and children with developmental delay and apparently normal facial appearance should be examined for holoprosencephaly and that all identified patients with holoprosencephaly need high-resolution cytogenetic studies with careful attention to the terminal portion of 21q.

Preliminary phenotypic map of chromosome 4p16 based on 4p deletions.

We have collected and analyzed clinical information from 11 patients with chromosome 4p deletions or rearrangements characterized by various molecular techniques. Comparing the extent of these patients' deletions with their respective clinical presentations led to the proposal of a preliminary phenotypic map of chromosome 4p. This map consists of regions which, when deleted, are associated with specific clinical manifestations. Nonspecific changes such as mental and growth retardation are not localized, and probably result from the deletion of more than one gene or region. The region associated with most of the facial traits considered typical in Wolf-Hirschhorn syndrome (WHS) patients coincides with the currently proposed WHS critical region (WHSCR), but some anomalies commonly seen in WHS appear to map outside of the WHSCR. The observation of clinodactyly in 2 patients with nonoverlapping deletions allows assignment of these defects to at least 2 separate regions in 4p16. These initial observations and attempts at genotype/phenotype correlation lay the groundwork for identifying the genetic basis of these malformations, a common objective of gene mapping efforts and chromosome deletion studies.

Albright hereditary osteodystrophy and del(2) (q37.3) in four unrelated individuals.

Albright hereditary osteodystrophy (AHO) is a condition with characteristic physical findings (short stature, obesity, round face, brachydactyly) but variable biochemical changes (pseudohypoparathyroidism, pseudopseudohypoparathyroidism). Most patients with AHO have decreased activity of the guanine nucleotide-binding protein (GS protein) that stimulates adenylyl cyclase. The gene encoding the alpha subunit of the GS protein (GNAS1) has been mapped to the long arm of chromosome 20. We describe 4 unrelated individuals with apparent AHO, associated with small terminal deletions of chromosome 2. All 4 patients had normal serum calcium levels consistent with pseudopseudohypoparathyroidism. Del(2) (q37) is the first consistent karyotypic abnormality that has been documented in AHO [Phelan et al., 1993: Am J Hum Genet 53:484]. The finding of the same small terminal deletion in 4 unrelated individuals with a similar phenotype suggests that a gene locus in the 2q37 region is important in the pathogenesis of Albright syndrome. The association of Albright syndrome and the GNAS1 locus on chromosome 20 is well documented. The observation of a second potential disease locus on chromosome 2 may help explain the heterogeneity observed in this disorder.

Summary of the 1993 ASHG ancillary meeting "recent research on chromosome 4p syndromes and genes".

The following is a summary of presentations given during an ancillary meeting to the 1993 American Society of Human Genetics Meeting in New Orleans, LA. This ancillary meeting, entitled "Recent Research on Chromosome 4p Syndromes and Genes," reviewed the history of the Wolf-Hirschhorn syndrome (WHS), the natural history of patients with WHS, and the smallest region of deletion associated with the WHS. The proximal 4p deletion syndrome and the duplication 4p syndrome were also described and advice was offered regarding detection of chromosome 4p deletions, duplications, and rearrangements. The current status of the physical map of chromosome 4p with emphasis on the genes that map to the 4p16 region was presented along with a preliminary phenotypic map of 4p16. The goal of this format was to provide a comprehensive review of the clinical presentations, diagnostic capabilities, and genetic mapping advances involving chromosome 4p.

Antibody deficiency in Wolf-Hirschhorn syndrome.

We identified antibody deficiencies in 9 of 13 infection-prone children with Wolf-Hirschhorn syndrome (4p-monosomy). Eight of the immunodeficient children were identified by a questionnaire sent to 190 families with an affected child. Two of the children had common variable immunodeficiency, one had IgA and IgG2 subclass deficiency, three had IgA deficiency, and three had impaired polysaccharide responsiveness. T-cell immunity was normal. The association of antibody defects with Wolf-Hirschhorn syndrome suggests a regulatory gene within the deleted chromosome region that affects the B cell system.

Molecular characterisation of chromosome 4p deletions resulting in Wolf-Hirschhorn syndrome.

We present three patients with Wolf-Hirschhorn syndrome with small cytogenetic deletions of 4p16. One case is a de novo translocation and two cases represent de novo deletions. Using molecular techniques we determined the extent of these deletions and attempted to ascertain parental origin. Case 1 had a deletion of 4p16.3 with a breakpoint proximal to D4S10, case 2 had a larger deletion including D4S62 in 4p16.2, and case 3 had the largest deletion which included D4S240, but not the Raf2 locus in 4p16.1. The parental origin of the deletion in case 3 was paternal; the other two cases were indeterminable. Our results show that these three deletions include the currently proposed Wolf-Hirschhorn syndrome critical region within the most distal 2 Mb of 4p16.3 and offer supportive evidence for continuous terminal deletions.

A molecular deletion of distal chromosome 4p in two families with a satellited chromosome 4 lacking the Wolf-Hirschhorn syndrome phenotype.

We report two families with a satellited chromosome 4 short arm (4ps). Satellites and stalks normally occur on the short arms of acrocentric chromosomes; however, the literature cites several reports of satellited nonacrocentric chromosomes, which presumably result from a translocation with an acrocentric chromosome. This is the first report of 4ps chromosomes. Our families are remarkable in that both unaffected and affected individuals carry the 4ps chromosome. The phenotypes observed in affected individuals, although dissimilar, were sufficient to encourage a search for a deletion of chromosome 4p. By Southern blot analysis and fluorescence in situ hybridization, a deletion of material mapping approximately 150 kb from chromosome 4pter was discovered. This deletion is notable because it does not result in the Wolf-Hirschhorn syndrome and can result in an apparently normal phenotype. We speculate that homology between subterminal repeat sequences on 4p and sequences on the acrocentric short arms may explain the origin of the rearrangement and that position effect may play a role in the expression of the abnormal phenotype.

Rapid detection of chromosome aneuploidies by prenatal interphase FISH (fluorescence in situ hybridization) and its clinical utility in Japan.

OBJECTIVE: The purpose of this study was to assess the accuracy, informative rate, detection rate, and clinical utility of prenatal interphase fluorescence in situ hybridization (FISH) analysis of amniotic fluid samples from Japanese women. METHODS: Amniotic fluid specimens from 2,639 Japanese women were received for prenatal interphase FISH and chromosome analysis. A questionnaire was designed to evaluate FISH clinical utility by collaboration sites. RESULTS: Based on 2,319 tested samples, the accuracy (100%), informative (94%), and detection (87.6%) rates were all high. The accuracy (100%), informative (90.2%), and detection (90.0%) rates were also remarkable in third-trimester pregnancies. We perceive significant advantages from this test regarding medical management and patient satisfaction. CONCLUSIONS: This novel report shows that in Japan prenatal interphase FISH testing is highly informative and accurate, not only in second-trimester pregnancies but also in third-trimester pregnancies. This test provides advantages to both physicians and patients, provided that its capabilities and limitations are understood.