Is the genetic basis of DiGeorge syndrome in HAND?

Cdc45 degradation during differentiation and apoptosis

Cell division cycle protein 45 (Cdc45) is crucial for the initiation as well as the elongation process of eukaryotic DNA replication. Our findings suggested that the Cdc45 protein is ubiquitylated and degraded by the proteasome pathway in human cells. Firstly, the fate of Cdc45 protein after induction of terminal differentiation of cultured human cells was significantly decelerated by application of proteasomal inhibitors. Secondly, we identified various putative destruction boxes and one KEN-box in the amino acid sequence of vertebrate Cdc45, which indicate that Cdc45 seems to be a novel substrate of the anaphase promoting complex/cyclosome. Thus, the evidences for ubiquitylation of Cdc45 refer the first posttranslational modification of this essential replication factor. We also found, that the human Cdc45 protein was not cleaved during apoptosis of cultured cells. This is in accordance with reports demonstrating the absence of indiscriminative cleavage of replication proteins during the programmed cell death.

Apparently unrelated cytogenetic abnormalities among 462 probands referred for the detection of del(22q) by FISH

Laboratory-based reports of the cytogenetic abnormalities detected during the course of testing for deletion del(22q) are scant. We report our findings from the testing with FISH of 462 patients suspected to have del(22q) between 1994 and 2000. Of these, 447 had a normal karyotype. An apparently unrelated cytogenetic abnormality was detected in 15 (3.2%). Two of these abnormalities involved reciprocal translocation with chromosome 22q and one of these showed del(22q) with FISH. The other abnormalities included sex chromosome aneuploidies and unbalanced rearrangements of various chromosomal segments. There was no commonality among these abnormalities and no correlation with other reported cases. Among those with a normal karyotype, an unexpected deletion of the control arylsulphatase A (ARSA) probe was found, providing a definite frequency of 1/262 for ARSA deletions among patients suspected to have del(22q). Of the 462 referrals, 48 (10%) had one or more additional diagnoses, and in this group, 4 (8%) had del(22q) and 2 (4%) had an apparently unrelated cytogenetic abnormality. The data highlight the importance of initial cytogenetic analysis in patients suspected of del(22q) and negates the use of interphase FISH screening by itself for del(22q). The finding of 3.2% unrelated cytogenetic abnormalities is noteworthy. FISH should be used in any structural rearrangement to ascertain if the relevant locus is deleted or not. The continued reporting of patients diagnosed with del(22q) found to have an unrelated cytogenetic abnormality will expand the phenotypic spectrum and possible gene mapping may refine phenotypic specificity.

Pulmonary atresia with hypoplastic right ventricle. A clinical embryological study

An unusual case of pulmonary atresia with an aberrant karyotype of 46,XX,t(6;8)(p21.2;q11.2) is reported. Fetal ultrasonic examination at the 20th week of gestation revealed a hypoplastic right ventricle and an intact interventricular septum. Authors summarize their postnatal findings in fetal heart and the large adjacent vessels with special reference to the pathogenesis of this rare congenital heart defect. The observation delineates right-ventricular outflow tract obstruction associated with an abnormal pulmonary blood supply. The anatomy of the systemic pulmonary collaterals was studied and correlated with multifocal disorders in the system of the pharyngeal arch arteries in the early embryonic development.

Isolation of novel cDNA encompassing the ADU balanced translocation break point in the DiGeorge critical region

DiGeorge syndrome (DGS) is a developmental field defect of the third and fourth pharyngeal pouches that are associated with congenital heart defects, hypoparathyroidism, cell-mediated immunodeficiency, velopharyngeal insufficiency, and craniofacial anomalities. Approximately 90% of patients exhibit monosomy in the 22q11 region. In order to isolate the critical gene responsible for DGS, the cDNA libraries were screened with a probe containing the ADU balanced translocation break point, that is a locus reported in one patient (ADU) caused by a balanced translocation between chromosomes 22 and 2. Out of 10(6) clones, three independent overlapping clones were isolated, which were assumed to have originated from a single transcript, DGCR7. This transcript contained a 175-aa long open reading frame (ORF), encoding an acidic (pI = 5.81) and a proline-rich peptide, which are often found in the activation domain of several transcription factors. Also, it was predicted to be a nuclear protein. Northern hybridization detected an approx 1.9 kb transcript in all fetal and adult tissues tested, with strong expression in the fetal liver and kidney. In the case of adult tissues, strong expression was also detected in areas such as the heart, skeletal muscle, liver, and kidney.

A complex of mammalian ufd1 and npl4 links the AAA-ATPase, p97, to ubiquitin and nuclear transport pathways

The AAA-ATPase, p97/Cdc48p, has been implicated in many different pathways ranging from membrane fusion to ubiquitin-dependent protein degradation. Binding of the p47 complex directs p97 to act in the post-mitotic fusion of Golgi membranes. We now describe another binding complex comprising mammalian Ufd1 and Npl4. Yeast Ufd1p is required for ubiquitin-dependent protein degradation whereas yeast Npl4p has been implicated in nuclear transport. In rat liver cytosol, Ufd1 and Npl4 form a binary complex, which exists either alone or bound to p97. Ufd1/Npl4 competes with p47 for binding to p97 and so inhibits Golgi membrane fusion. This suggests that it is involved in another cellular function catalysed by p97, the most likely being ubiquitin-dependent events during mitosis. The fact that the binding of p47 and Ufd1/Npl4 is mutually exclusive suggests that these protein complexes act as adapters, directing a basic p97 activity into different cellular pathways.

Structure of chromosomal duplicons and their role in mediating human genomic disorders

Chromosome-specific low-copy repeats, or duplicons, occur in multiple regions of the human genome. Homologous recombination between different duplicon copies leads to chromosomal rearrangements, such as deletions, duplications, inversions, and inverted duplications, depending on the orientation of the recombining duplicons. When such rearrangements cause dosage imbalance of a developmentally important gene(s), genetic diseases now termed genomic disorders result, at a frequency of 0.7-1/1000 births. Duplicons can have simple or very complex structures, with variation in copy number from 2 to >10 repeats, and each varying in size from a few kilobases in length to hundreds of kilobases. Analysis of the different duplicons involved in human genomic disorders identifies features that may predispose to recombination, including large size and high sequence identity between the recombining copies, putative recombination promoting features, and the presence of multiple genes/pseudogenes that may include genes expressed in germ cells. Most of the chromosome rearrangements involve duplicons near pericentromeric regions, which may relate to the propensity of such regions to accumulate duplicons. Detailed analyses of the structure, polymorphic variation, and mechanisms of recombination in genomic disorders, as well as the evolutionary origin of various duplicons will further our understanding of the structure, function, and fluidity of the human genome.