New alpha 2 globin chain variant with low oxygen affinity affecting the N-terminal residue and leading to N-acetylation [Hb Lyon-Bron alpha 1(NA1)Val --> Ac-Ala]

Hemoglobin Lyon-Bron was found in two members of a family of German ascent presenting with a moderate normocytic anemia. In this alpha 2 globin variant, the N-terminal valine of the chain was replaced by an alanine. Electrospray mass spectrometry of the alpha chain showed that, as normally, the initiator methionine was cleaved during globin processing but that the N alpha-terminal group was totally acetylated. This resulted in structural modifications of a region crucial for oxygen binding. As a consequence, hemoglobin Lyon-Bron displayed both a reduced chloride effect and a decreased oxygen affinity, this last point explaining the apparent anemia.

Alternative promoter usage and splicing of ZNF74 multifinger gene produce protein isoforms with a different repressor activity and nuclear partitioning

We have previously shown that ZNF74, a candidate gene for DiGeorge syndrome, encodes a developmentally expressed zinc finger gene of the Kruppel-associated box (KRAB) multifinger subfamily. Using RACE, RT-PCR, and primer extension on human fetal brain and heart mRNAs, we here demonstrate the existence of six mRNA variants resulting from alternative promoter usage and splicing. These transcripts encode four protein isoforms differing at their N terminus by the composition of their KRAB motif. One isoform, ZNF74-I, which corresponds to the originally cloned cDNA, was found to be encoded by two additional mRNA variants. This isoform, which contains a KRAB motif lacking the N terminus of the KRAB A box, was devoid of transcriptional activity. In contrast, ZNF74-II, a newly identified form of the protein that is encoded by a single transcript and contains an intact KRAB domain with full A and B boxes, showed strong repressor activity. Deconvolution immunofluorescence microscopy using transfected human neuroblastoma cells and nonimmortalized HS68 fibroblasts revealed a distinct subcellular distribution for ZNF74-I and ZNF74-II. In contrast to ZNF74-I, which largely colocalizes with SC-35 in nuclear speckles enriched in splicing factors, the transcriptionally active ZNF74-II had a more diffuse nuclear distribution that is more characteristic of transcriptional regulators. Taken with the previously described RNA-binding activity of ZNF74-I and direct interaction with a hyperphosphorylated form of the RNA polymerase II participating in pre-mRNA processing, our results suggest that the two ZNF74 isoforms exert different or complementary roles in RNA maturation and in transcriptional regulation.

ZNF74, a gene deleted in DiGeorge syndrome, is expressed in human neural crest-derived tissues and foregut endoderm epithelia

DiGeorge syndrome (DGS) is a developmental disorder associated with large hemizygous deletions on chromosome 22q11.2. ZNF74 zinc finger gene is a candidate from the commonly deleted region. To address the potential involvement of ZNF74 in DGS, its human developmental expression pattern has been assessed. In situ hybridization on Carnegie Stage 18 embryos revealed that ZNF74 expression is limited to specific neural crest-derived tissues and neuroepithelium of the spinal cord as well as to foregut endoderm epithelia (esophagus and respiratory tract). Interestingly, ZNF74 expression was detected in the wall of the pulmonary artery and aorta and in the aortic valve, which are populated by neural crest-derived cells. This finding is significant, considering that DGS is believed to result from defective neural crest contributions and that outflow tract and aorticopulmonary septation defects are typical features of the DGS phenotype. Thus, the restricted expression of ZNF74 in structures affected in DGS suggests a role for this putative regulator of gene expression in aspects of the DGS phenotype.

Two new alpha chain variants: Hb Boghé [alpha58(E7)His-->Gln, alpha2], a variant on the distal histidine, and Hb CHarolles [alpha103(G10)His-Tyr, alpha1]

The present paper reports two new alpha-globin chain variants: Hb Boghé [alpha58(E7)His-->Gln, alpha2] and Hb Charolles [alpha103(G10)His-->Tyr, alpha1]. Hb Boghé was found in a 12-month-old girl who was treated for malignant histiocytosis at 9 months of age and received a bone marrow transplant from her sister. Hb Boghé was undetectable by isoelectrofocusing and high performance liquid chromatography of hemoglobins. It was only revealed by polyacrylamide gel electrophoresis of globin chains in the presence of urea-Triton X-100 and accounted for 10% of the total hemoglobin. Hb Charolles was detected in a 46-year-old patient who presented with microcytosis and hypochromia. It was easily detected by isoelectrofocusing and high performance liquid chromatography. Hb Charolles accounted for 11% of the total hemoglobin. Characterization of the two hemoglobin variants was achieved by DNA and restriction enzyme analyses. Oxygen equilibrium curves measured on whole blood with Hb Boghé were normal. DNA sequencing revealed the association of Hb Charolles with a common mutation of the alpha2 polyadenylation site: AATAAA-->AATAAG.

Direct interaction of the KRAB/Cys2-His2 zinc finger protein ZNF74 with a hyperphosphorylated form of the RNA polymerase II largest subunit

We previously identified ZNF74 as a developmentally expressed gene commonly deleted in DiGeorge syndrome. ZNF74 encodes an RNA-binding protein tightly associated with the nuclear matrix and belongs to a large subfamily of Cys2-His2 zinc finger proteins containing a KRAB (Kruppel-associated box) repressor motif. We now report on the multifunctionality of the zinc finger domain of ZNF74. This nucleic acid binding domain is shown here to function as a nuclear matrix targeting sequence and to be involved in protein-protein interaction. By far-Western analysis and coimmunoprecipitation studies, we demonstrate that ZNF74 interacts, via its zinc finger domain, with the hyperphosphorylated largest subunit of RNA polymerase II (pol IIo) but not with the hypophosphorylated form. The importance of the phosphorylation in this interaction is supported by the observation that phosphatase treatment inhibits ZNF74 binding. Double immunofluorescence experiments indicate that ZNF74 colocalizes with the pol IIo and the SC35 splicing factor in irregularly shaped subnuclear domains. Thus, ZNF74 sublocalization in nuclear domains enriched in pre-mRNA maturating factors, its RNA binding activity, and its direct phosphodependent interaction with the pol IIo, a form of the RNA polymerase functionally associated with pre- mRNA processing, suggest a role for this member of the KRAB multifinger protein family in RNA processing.

The KRAB zinc finger gene ZNF74 encodes an RNA-binding protein tightly associated with the nuclear matrix

We previously cloned ZNF74, a developmentally expressed zinc finger gene commonly deleted in DiGeorge syndrome. Here, the intron/exon organization of the human gene and the functional properties of the expressed protein are presented. This zinc finger gene from the transcription factor IIIA/Kruppel family contains three exons. A truncated Kruppel-associated box (KRAB) located at the N terminus of the predicted 64-kDa zinc finger protein is encoded by exon 2. The remainder of the protein including the zinc finger domain as well as the 3'-untranslated region (UTR) is encoded by exon 3. Both 5'-UTR (exon 1) and 3'-UTR contain repetitive Alu elements. In vitro translation of a cDNA encoding the entire ZNF74 coding region produced a 63-kDa protein as determined on sodium dodecyl sulfate-polyacrylamide gel. A bacterially expressed fusion protein shown to bind tightly to 65zinc was used to test the nucleic acid binding properties of ZNF74. By RNA binding assays, ZNF74 was found to bind specifically to poly(U) and poly(G) RNA homopolymers. The restricted binding to these homopolymers and not to poly(A) and poly(C) suggested that ZNF74 displays RNA sequence preferences. RNA binding was mediated by the zinc finger domain. Immunofluorescence studies on transfected cells revealed ZNF74 nuclear localization. The labeling pattern observed in the nuclei clearly excluded the nucleoli. The zinc finger region lacks a classical nuclear localization signal but was found to be responsible for nuclear targeting. Subcellular and in situ sequential fractionations further showed that ZNF74 is associated with the nuclear matrix. The RNA binding properties of this protein and its tight association with the nuclear matrix, a subnuclear compartment involved in DNA replication as well as RNA synthesis and processing, suggest a role for ZNF74 in RNA metabolism.

Excess of deletions of maternal origin in the DiGeorge/velo-cardio-facial syndromes. A study of 22 new patients and review of the literature

We have determined the parental origin of the deleted chromosome 22 in 29 cases of DiGeorge syndrome (DGS) using a CA-repeat mapping within the commonly deleted region, and in one other case by using a chromosome 22 short arm heteromorphism. The CA-repeat was informative in 21 out of 29 families studied and the deleted chromosome was of maternal origin in 16 cases (72%). When these data are pooled with recent results from the literature, 24 de novo DGS, velo-cardio-facial syndrome (VCFS) and isolated conotruncal cardiac disease deletions are found to be of maternal origin and 8 of paternal origin, yielding a chi 2 of 8 with a probability level lower than 0.01. These data, and review of the literature on familial DGS/VCFS and isolated conotruncal cardiopathies suggest that there is a strong tendency for the 22q11.2 deletions to be of maternal origin.

Isolation of a zinc finger gene consistently deleted in DiGeorge syndrome

DiGeorge syndrome is a human developmental disorder resulting in hypoplasia of the thymus and parathyroids, and conotruncal heart defects. We recently isolated four genes with zinc finger DNA binding motifs mapping to chromosome 22q11.2 DiGeorge critical region. We now report that one of them, ZNF74 gene, is hemizygously deleted in 23 out of 24 DiGeorge syndrome patients tested. ZNF74 mRNA transcripts are detected in human and mouse embryos but not in adult tissues. Sequence analysis of a corresponding cDNA reveals an an open reading frame encoding 12 zinc finger motifs of the Kruppel/TFIIIA type as well as N-terminal and C-terminal non-zinc finger domains. These results suggest that changes in the dosage of a putative transcription factor through ZNF74 hemizygous deletion may be critical for DiGeorge developmental anomalies.

Cloning of six new genes with zinc finger motifs mapping to short and long arms of human acrocentric chromosome 22 (p and q11.2)

Zinc finger genes encode proteins containing tandemly repeated zinc-mediated folded structures that are found in several transcriptional regulatory proteins. To identify new zinc finger genes, we have screened at low stringency human cosmid libraries enriched in chromosome 22 sequences with a probe derived from the finger region of the mouse Kruppel-like gene, mKr2. We identified 23 nonoverlapping human cosmids cross-hybridizing with the probe. All sequences obtained from cosmid subclones hybridizing with the probe revealed Kruppel-type consensus sequences. Hybridizations to somatic cell hybrid panels and to metaphase chromosomes revealed that 2 nonoverlapping zinc finger cosmids map to chromosome 22p and 4 map to 22q11.2. The 17 other nonoverlapping cosmids most likely map to other chromosomes. The short arms of acrocentric chromosomes are thought to encode only ribosomal RNA genes. Therefore, the identification of two zinc finger genes on chromosome 22p represents an unexpected finding of unknown significance. The four zinc finger genes that map to 22q11.2 are within the cat eye and DiGeorge syndrome regions and thus provide us with potential candidate genes for these developmental malformations.

Tubulin is phosphorylated at tyrosine by pp60c-src in nerve growth cone membranes

We show here that tubulin is the major in vivo substrate of the tyrosine-specific protein kinase pp60c-src in nerve growth cone membranes. Phosphotyrosine antibodies were used to demonstrate phosphotyrosyl residues in a subpopulation of alpha- and beta-tubulin that was highly enriched in a subcellular fraction of growth cone membranes from fetal rat brain. The presence of phosphotyrosine-modified isoforms of alpha- and beta-tubulin in vivo was confirmed by 32p labeling of rat cortical neurons in culture. Tubulin in growth cone membranes was phosphorylated at tyrosine in endogenous membrane phosphorylation reactions (0.068 mol phosphotyrosine/mol alpha-tubulin and 0.045 mol phosphotyrosine/mol beta-tubulin), and phosphorylation was specifically inhibited by antibodies directed against pp60c-src, which is localized in the growth cone membranes. pp60c-src was capable of directly phosphorylating tubulin as shown in immune complex kinase assays with purified brain tubulin. Phosphopeptide mapping revealed a limited number of sites of tyrosine phosphorylation in alpha- and beta-tubulin, with similar phosphopeptides observed in vivo and in vitro. These results reveal a novel posttranslational modification of tubulin that could regulate microtubule dynamics at the growth cone.

Monoclonal antibodies as probes for the transmembrane structure of neutral endopeptidase 24.11 ('enkephalinase')

The neutral endopeptidase (EC 3.4.24.11) ('enkephalinase') is a membrane-bound metalloendopeptidase that is present in large amounts in the microvilli of the kidney proximal tubules. By immunizing mice with purified rabbit kidney brush-border membranes, we have obtained four different monoclonal antibodies that recognize this enzyme in dot-blot and Western-blot assays and can be used for immunoprecipitation of neutral endopeptidase from crude kidney solubilizates. One of these monoclonal antibodies (2B12) allows the labeling of proximal tubule cells with colloidal gold particles. This monoclonal antibody also binds to native brush-border membrane vesicles (which are mostly in the right-side-out configuration) and recognizes an epitope which is destroyed after reduction and alkylation of the protein. By contrast, all three other monoclonal antibodies (21G10, 23B11 and 22E2) compete for another epitope of neutral endopeptidase that is not exposed at the extracytoplasmic surface either in intact cells or in sealed brush-border vesicles. Permeabilization of the vesicles with digitonin, however, restores the full binding activity. Binding of these antibodies is not altered by prior reduction and alkylation of the protein. Taken together, these results strongly suggest that the 2B12 monoclonal antibody binds a conformational epitope located on the ectodomain of the enzyme, whereas the three others (21G10, 23B11 and 22E2) bind to a common or to overlapping epitopes located on the cytosolic domain. These results also demonstrate unambiguously the transmembrane nature of neutral endopeptidase.

Determination of brush border membrane vesicle orientation using monoclonal antibodies recognizing extracytoplasmic and cytoplasmic domains of neutral endopeptidase-24.11

A method for determination of the orientation and integrity of brush border membrane vesicles is described. The method takes advantage of the availability of two monoclonal antibodies, 23B11 and 2B12, which recognize a cytoplasmic and an extracytoplasmic domain, respectively, of the neutral endopeptidase-24.11. Specific binding of the antibodies to intact kidney brush border vesicles or to vesicles permeabilized by digitonin is detected by fluorescence using an anti-mouse immunoglobulin G-fluorescein isothiocyanate conjugate. The method allows discrimination between right side out, inside out, and unsealed vesicles. It requires limited amounts of material and can be completed the day of the brush border vesicle preparation. Application to rabbit kidney brush border membranes freshly prepared led to values of 89, 8, and 3% for right side out, inside out, and unsealed vesicles, respectively. Storage at low temperature was associated with a marked increase in the proportion of unsealed vesicles.

Developmental regulation of protein tyrosine phosphorylation in rat brain

Proteins phosphorylated at tyrosine residues in the developing rat brain have been identified with a focus on the nerve growth cone and synaptic terminal. Endogenous protein phosphorylation in membranes from a subcellular growth cone fraction of fetal rat brain revealed prominent 55-60 kD phosphotyrosine-containing proteins. Proteins of similar size were recognized by phosphotyrosine antibodies in isolated growth cone membranes, indicating that they contained phosphotyrosine in vivo. Proteins of 55-60 kD were not highly phosphorylated in synaptosomes from adult brain, suggesting a growth cone-specific function. Generally, tyrosine phosphorylation was much lower in adult brain than in fetal brain fractions. Although some synaptosomal membrane proteins that contained phosphotyrosine corresponded in size with those in growth cone membranes (92 kD, 41 kD), others were unique to synaptosomal membranes (38 kD and 30 kD). Immunoperoxidase staining of fetal rat neocortex with phosphotyrosine antibodies at embryonic day 19 revealed immunoreactivity in presumptive migratory neuroblasts in the intermediate zone and in processes of the molecular layer. Proliferating neuroepithelial cells of the ventricular zone showed little immunoreactivity. Lower levels of phosphotyrosine immunoreactivity were seen until postnatal day 10, correlating with the period of maximal process outgrowth. These results indicate that protein tyrosine phosphorylation in the developing nervous system may be functionally significant in an aspect of neuronal differentiation such as growth cone-mediated process extension and cell migration. An analogous role in the mature brain may be related to synaptic plasticity or function.

c-src gene product in developing rat brain is enriched in nerve growth cone membranes

Differentiating rat neurons express high levels of the protooncogene product pp60c-src, a 60-kDa tyrosine kinase of unknown function encoded by c-src. pp60c-src was found to be concentrated at least 9-fold in membranes from a subcellular fraction of nerve growth cones, the motile tips of outgrowing neuronal processes. Indirect immunofluorescence staining of cultured chick retinal explants showed pp60c-src in neuronal growth cones and processes, with the antigen particularly concentrated in growth cones of long neurites. pp60c-src in growth cone membranes was an active tyrosine-specific protein kinase with elevated tyrosine-specific protein kinase activity and reduced electrophoretic mobility characteristic of the form of pp60c-src in central nervous system neurons. pp60c-src was present at lower levels in subcellular fractions from mature rat brain but synaptosomal membranes were not enriched. Preferential localization of an active form of pp60c-src in nerve growth cone membranes and persistence of pp60c-src in mature neurons suggest that this tyrosine kinase is important in growth cone-mediated neurite extension and synaptic plasticity.

The use of a monoclonal antibody for the rapid purification of kidney neutral endopeptidase ("enkephalinase") solubilized in octyl glucoside

The neutral endopeptidase ("enkephalinase") of the rabbit brush border membrane has been purified to homogeneity by a rapid immunoaffinity method using a monoclonal antibody. In contrast with other methods used so far, a complete extraction of enkephalinase from the brush border membrane can be achieved with octyl glucoside, without loss of activity. The solubilized enzyme can be selectively separated from the other proteins in a single step using an immunoaffinity column consisting of the monoclonal antibody covalently linked to Sepharose CL-4B. It is demonstrated that enkephalinase can then be recovered in an active form by elution at low pH. The purified enzyme obtained by this method is completely inhibited by thiorphan and appears as a single 94,000 dalton protein after polyacrylamide gel electrophoresis under denaturing and reducing conditions.