Insulin receptor isoforms: an integrated view focused on gestational diabetes mellitus

The human insulin receptor (IR) exists in two isoforms that differ by the absence (IR-A) or the presence (IR-B) of a 12-amino acid segment encoded by exon 11. Both isoforms are functionally distinct regarding their binding affinities and intracellular signalling. However, the underlying mechanisms related to their cellular functions in several tissues are only partially understood. In this review, we summarize the current knowledge in this field regarding the alternative splicing of IR isoform, tissue-specific distribution and signalling both in physiology and disease, with an emphasis on the human placenta in gestational diabetes mellitus (GDM). Furthermore, we discuss the clinical relevance of IR isoforms highlighted by findings that show altered insulin signalling due to differential IR-A and IR-B expression in human placental endothelium in GDM pregnancies. Future research and clinical studies focused on the role of IR isoform signalling might provide novel therapeutic targets for treating GDM to improve the adverse maternal and neonatal outcomes.

Mice engineered for an obligatory Mdm4 exon skipping express higher levels of the Mdm4-S isoform but exhibit increased p53 activity

Mdm4, a protein related to the ubiquitin-ligase Mdm2, is an essential inhibitor of tumor suppressor protein p53. In both human and mouse cells, the Mdm4 gene encodes two major transcripts: one encodes the full-length oncoprotein (designated below as Mdm4-FL), whereas the other, resulting from a variant splicing that skips exon 6, encodes the shorter isoform Mdm4-S. Importantly, increased Mdm4-S mRNA levels were observed in several human cancers, and correlated with poor survival. However, the role of Mdm4-S in cancer progression remains controversial, because the Mdm4-S protein appeared to be a potent p53 inhibitor when overexpressed, but the splice variant also leads to a decrease in Mdm4-FL expression. To unambiguously determine the physiological impact of the Mdm4-S splice variant, we generated a mouse model with a targeted deletion of the Mdm4 exon 6, thereby creating an obligatory exon skipping. The mutant allele (Mdm4(ΔE6)) prevented the expression of Mdm4-FL, but also led to increased Mdm4-S mRNA levels. Mice homozygous for this allele died during embryonic development, but were rescued by a concomitant p53 deficiency. Furthermore in a hypomorphic p53(ΔP/ΔP) context, the Mdm4(ΔE6) allele led to p53 activation and delayed the growth of oncogene-induced tumors. We next determined the effect of Mdm4(+/ΔE6) heterozygosity in a hypermorphic p53(+/Δ31) genetic background, recently shown to be extremely sensitive to Mdm4 activity. Mdm4(+/ΔE6) p53(+/Δ31) pups were born, but suffered from aplastic anemia and died before weaning, again indicating an increased p53 activity. Our results demonstrate that the main effect of a skipping of Mdm4 exon 6 is not the synthesis of the Mdm4-S protein, but rather a decrease in Mdm4-FL expression. These and other data suggest that increased Mdm4-S mRNA levels might correlate with more aggressive cancers without encoding significant amounts of a potential oncoprotein. Hypotheses that may account for this apparent paradox are discussed.

Severe sorafenib-induced hand-foot skin reaction

Sorafenib is a new drug, multikinase inhibitor, which has been recently approved for the treatment of metastatic renal cell carcinoma and hepatocellular carcinoma. Up to 90 percent of patients receiving this drug have been reported to develop dermatological symptoms. Recently, it has been suggested that the appearance of skin toxicity during therapy may indicate antitumor activity. We report a new case of sorafenib-induced severe hand-foot skin reaction, which hindered the patient's normal life. The reaction was successfully treated with topical costicosteroids and discontinuation of sorafenib. However, the patient died one month later.

The origin of chromosome rearrangements at early stages of AMPD2 gene amplification in Chinese hamster cells

BACKGROUND

Gene amplification and chromosomal rearrangements are frequent properties of cancer cells, provoking considerable interest in the mechanism of gene amplification and its consequences - particularly its relationship to chromosomal rearrangements. We recently studied the amplification of the gene for adenylate deaminase 2 (AMPD2) in Chinese hamster cells. Using fluorescent in situ hybridization (FISH), we found that early amplification of the AMPD2 gene is based on unequal gene segregation at mitosis, rather than local over-replication. We observed large inverted repeats of the amplified sequences, consistent with an amplification mechanism involving cycles of chromatid breakage, followed by fusion after replication and, in mitosis, the formation of bridges between the fused sister chromatids that leads to further breaks - a process we refer to as chromatid breakage-fusion-bridge (BFB) cycles. Our previous work left open the question of how this mechanism of gene amplification is related, if at all, to the chromosomal rearrangements that generate the dicentric, ring and double-minute (DM) chromosomes observed in some AMPD2-amplified metaphase cells, which are not predicted intermediates of chromatid BFB cycles, although they could be generated by related chromosome BFB cycles.

RESULTS

We have addressed this question using FISH with probes for the AMPD2 gene and other markers on the same chromosome. Our results are not consistent with the chromosome BFB cycle mechanism, in which two chromatids break simultaneously and fuse to generate, after replication, a dicentric chromosome. Rather, they suggest that dicentric chromosomes are generated by secondary events that occur during chromatid BFB cycles. Our results also suggest that DM chromosomes are generated by the 'looping-out' of a chromosomal region, generating a circular DNA molecule lacking a centromere; in this case, gene amplification would result from the unequal segregation of DM chromosomes at mitosis.

CONCLUSION

We conclude that, at early stages of AMPD2 gene amplification, chromatid BFB cycles are a major source of both 'intrachromosomal' gene amplification and genomic rearrangement, which are first limited to a single chromosome but which can then potentially spread to any additional chromosome. It also seems that, occasionally, a DNA sequence including the AMPD2 gene can be excised, generating a DM chromosome and thus initiating an independent process of 'extrachromosomal' amplification.

Enhanced flexibility and aphidicolin-induced DNA breaks near mammalian replication origins: implications for replicon mapping and chromosome fragility

Common fragile sites are chromosomal loci prone to breakage and rearrangement that can be induced by aphidicolin, an inhibitor of DNA polymerases. Within these loci, sites of preferential DNA breaks were proposed to correlate with peaks of enhanced DNA flexibility, the function of which remains elusive. Here we show that mammalian DNA replication origins are enriched in peaks of enhanced flexibility. This finding suggests that the search for these features may help in the mapping of replication origins, and we present evidence supporting this hypothesis. The association of peaks of flexibility with replication origins also suggests that some origins may associate with minor levels of fragility. As shown here, an increased sensitivity to aphidicolin was found near two mammalian DNA replication origins.

Initiation of DNA replication at the Chinese hamster origin oriGNAI3 relies on local sequences and/or chromatin structures, but not on transcription of the nearby GNAI3 gene

We recently identified a region of preferential replication initiation, oriGNAI3, near the 3' end of the Chinese hamster GNAI3 gene. oriGNAI3 is co-amplified in mutants selected for AMPD2 amplification, a process generating chromosomal rearrangements. In this report we have taken advantage of cell lines with truncated and translocated amplified units to show that these rearrangements do not alter the function of ori GNAI3. These results indicate that replication initiation at this locus relies essentially on local features. Interestingly, the study of one line in which a rearrangement has disrupted the GNAI3 gene shows that ongoing transcription of this gene is not required for initiation at oriGNAI3. In order to obtain further insight into the sequences and/or chromatin structures required for oriGNAI3 function, we have analyzed the DNase I sensitivity and nucleotide sequence of the region. The features important for replication initiation appear to cluster in a 7-12 kb region which includes oriGNAI3.

Gene amplification mechanisms: the role of fragile sites

We studied the early stages of gene amplification in a Chinese hamster cell line and identified two distinct amplification mechanisms, both relying on an unequal segregation of gene copies at mitosis. In some cases, a sequence containing the selected gene is looped out, generating an acentric circular molecule, and amplification proceeds through unequal segregation of such extrachromosomal elements in successive cell cycles. In other cases, the accumulation of intrachromosomally amplified copies is driven by cycles of chromatid breakage, followed by fusion of sister chromatids devoid of a telomere, which leads to bridge formation and further break in mitosis (BFB cycles). We showed that some clastogenic drugs specifically trigger the intrachromosomal amplification pathway and strictly correlated this induction of BFB cycles to the ability of these drugs to activate fragile sites. In three model systems, we also established, that the location of centromeric and telomeric fragile sites relative to the selected genes determines the size and sequence content of the early amplicons.

A new role for hypoxia in tumor progression: induction of fragile site triggering genomic rearrangements and formation of complex DMs and HSRs

Genome rearrangements including gene amplification are frequent properties of tumor cells, but how they are related to the tumor microenvironment is unknown. Here, we report direct evidence for a causal relationship between hypoxia, induction of fragile sites, and gene amplification. Recently, we showed that breaks at fragile sites initiate intrachromosomal amplification. We demonstrate here that hypoxia is a potent fragile site inducer and that, like fragile sites inducing drugs, it drives fusion of double minutes (DMs) and their targeted reintegration into chromosomal fragile sites, generating homogeneously staining regions (HSRs). This pathway operates efficiently for DMs bearing different sequences, suggesting a model of hypoxia-driven formation of the HSRs containing nonsyntenic sequences frequently observed in solid tumors.

oriGNAI3: a narrow zone of preferential replication initiation in mammalian cells identified by 2D gel and competitive PCR replicon mapping techniques

The nature of mammalian origins of DNA replication remains controversial and this is primarily because two-dimensional gel replicon mapping techniques have identified broad zones of replication initiation whereas several other techniques, such as quantitative PCR, have disclosed more discrete sites of initiation at the same chromosomal loci. In this report we analyze the replication of an amplified genomic region encompassing the 3'-end of the GNAI3 gene, the entire GNAT2 gene and the intergenic region between them in exponentially growing Chinese hamster fibroblasts. These cells express GNAI3 but not GNAT2 . The replication pattern was first analyzed by two-dimensional neutral-alkaline gel electrophoresis. Surprisingly, the results revealed a small preferential zone of replication initiation, of at most 1.7 kb, located in a limited part of the GNAI3 - GNAT2 intergenic region. Mapping of this initiation zone was then confirmed by quantitative PCR. The agreement between the two techniques exploited here strengthens the hypothesis that preferred sites of replication initiation do exist in mammalian genomes.

Matrix attachment regions and transcription units in a polygenic mammalian locus overlapping two isochores

Eukaryotic chromosomes are ponctuated by specialized DNA sequences (MARs) characterized by their ability to bind the network of nonhistone proteins that form the nuclear matrix or scaffold. We previously described an amplifiable cluster of genes with different tissue-specific expression patterns, located on Chinese hamster chromosome 1q. This model is especially appropriate to study the relationships between MARs and transcription units. We show here that four attachment regions, with sequences exhibiting motifs specific to MARs, are present within the 100 kb of screened DNA. Three of them are relatively short sequences localized in intergenic regions. The last one extends over one of the transcription units and contains a region previously identified as a recombination hot spot. Moreover, the analysis of a DNA sequence extending over some 50 Kb of this region and spanning at least four genes, disclosed a strikingly sharp change in G + C content. This strongly suggests that the studied region contains the boundary of two isochores. We propose that the frequency and the size of MARs are correlated to their localization in G + C rich or poor domains.

Expression of fragile sites triggers intrachromosomal mammalian gene amplification and sets boundaries to early amplicons

Drug-selected intrachromosomal gene amplification by breakage-fusion-bridge (BFB) cycles is well documented in mammalian cells, but factors governing this mechanism are not clear. Here, we show that only some clastogenic drugs induce drug resistance through intrachromosomal amplification. We strictly correlate triggering of BFB cycles to induction of fragile site expression. We demonstrate a dual role for fragile sites in intrachromosomal amplification: a site telomeric to the selected gene is involved in initiation, while a centromeric site defines the size and organization of early amplified units. The positions of fragile sites relative to boundaries of amplicons found in human cancers support the hypothesis that fragile sites play a key role in the amplification of at least some oncogenes during tumor progression.

GNAI3, GNAT2, AMPD2, GSTM are clustered in 120 kb of Chinese hamster chromosome 1q

We studied a polygenic region located on Chromosome (Chr) 1q in Chinese hamster cells that is coamplified along with the AMPD2 gene. Previous sequence analysis identified both members of the GSTM family and the GNAI3 gene within a cloned 120-kb region surrounding the AMPD2 locus. We show here that the GNAT2 gene, which is inactive in the fibroblastic cells, lies within the 20 kb separating the transcriptionally active GNAI3 and AMPD2 genes. We map most gene ends by sequence comparison with human homologs; one is inferred from the presence of an unmethylated CpG island. This Chinese hamster locus corresponds to a region of conserved linkage between human Chr 1 (locus 1p13) and mouse Chr 3 (position 52.5 cM), where Gnai-3 and Gnat-2 have been mapped. The AMPD2 gene is presently unlocalized in human genome; its proposed position on mouse Chr 3 is at 53.4 cM. Our results, obtained by physical mapping, strongly suggest that the order and possibly the tight linkage of these genes are conserved on all three genomes.

The highly conserved Chinese hamster GNAI3 gene maps less than 60 kb from the AMPD2 gene and lacks the intronic U6 snRNA present in its human counterpart

The identity of a gene coamplified with the adenylate deaminase 2 gene (AMPD2) in coformycin-resistant cells was determined by analysis of its genomic sequence. Sequence comparisons reveal a significant homology with the 3' terminal part of the gene encoding the alpha i3 subunit of Gi proteins from several species (GNAI3). Identification of the gene was confirmed by Western blot analysis of its products. A precise sequence comparison was performed with the human genomic sequence. It showed that conservation remains important in noncoding exons as well as in introns. However, sequences corresponding to combined U6 snRNA and E protein pseudogene, previously identified inside intron 7 of the human gene, were not found in the Chinese hamster gene. GNAI3 is mapped to a region of conserved linkage between human chromosome 1 (locus 1p13) and mouse chromosome 3 (at 48.4 cM). The Chinese hamster GNAI3 gene maps to chromosome 1 within a 120-kb fragment that also comprises the AMPD2 and GSTM genes.

[Gene amplification and chromosomal rearrangements during acquisition of cellular resistance to the antimetabolite coformycin]

We studied the early stages of gene amplification in a Chinese hamster cell line and we show that two distinct mechanisms can operate at a single locus. Both of them rely on an unequal segregation of gene copies at mitosis. We conclude that cycles of chromatid breakage, followed by fusion of sister chromatids devoid of a telomere that lead to further breaks in mitosis, have a key role in the coupling of gene amplification and genome remodeling. Rearrangements are first limited to a single chromosome but can then potentially spread to any additional chromosome. Occasionally, a sequence containing the selected gene can be looped out, generating a "double minute" and thus initiating an independent process of extrachromosomal amplification.

DNA puff C4 of Bradysia hygida (Diptera: Sciaridae) contains genes unequally amplified and differentially expressed during development

We report here the isolation and characterization of a 2.3 kb genomic EcoRI fragment that co-localizes in the DNA puff C4 of Bradysia hygida with a 4 kb EcoRI fragment previously characterized as containing part of a gene amplified and expressed in the salivary gland at the time when puff C4 expands. Verification of the relative amount of DNA complementary to these two genomic fragments shows that they are unequally amplified in the salivary gland. The fragment containing part of the gene expressed when puff C4 expands amplifies about eight times more than the 2.3 kb fragment. This 2.3 kb fragment also carries sequences complementary to RNA species present in the gland in a period when puff C4 has already receded. Based on these data we discuss the nature of the DNA puff and the possible way in which amplification is occurring at these sites.

Co-amplified markers alternate in megabase long chromosomal inverted repeats and cluster independently in interphase nuclei at early steps of mammalian gene amplification

Two-colour in situ hybridization with probes for two co-amplified markers located several megabases apart on chromosome 1 has been used to analyse early stages of adenylate deaminase 2 (AMPD2) gene amplification in Chinese hamster cells. In the amplified chromosomal structures, the distribution of hybridization spots identifies megabase-long inverted repeats. Their organization is remarkably well accounted for if breakage-fusion-bridge cycles involving sister chromatids drive the amplification process at these early stages. During interphase the markers often segregate into distinct nuclear domains. Many nuclei have bulges or release micronuclei, carrying several copies of one or both markers. These observations indicate that the amplified units destabilize the nuclear organization and eventually lead to DNA breakage during interphase. We propose a model in which interphase breakage has a role in the progression of gene amplification.

The evolution of the amplified adenylate deaminase 2 domains in Chinese hamster cells suggests the sequential operation of different mechanisms of DNA amplification

Fluorescent in situ hybridization was used to localize the adenylate deaminase 2 (AMPD2) genes and flanking sequences on the chromosomes of the Chinese hamster line GMA32 and to study the distribution of additional copies of these genetic sequences in amplified mutants selected at several early stages of the amplification process. The synteny of AMPD2 genes and MDR1 genes, located on chromosomes 1, was demonstrated; in GMA32 the existence of a rearrangement positioning the two AMPD2 genes at different distances from the telomeres was disclosed. Using this structural marker, we showed that the amplified copies distribute along only one of the chromosomes 1. Their organization in different cells of clonal mutant populations at a very early stage of amplification was extremely heterogeneous; classes of organization could be recognized however. Their quantitative distribution at this stage and in cells which went through 10 more division cycles suggests an evolution pathway common to the mutant clones under study: as a rule, tandems of few units of identical and very large size (47 Mb) appear to be the first detected product of amplification; this organization is progressively overtaken by structures with more units of reduced and irregular size, while, in a growing number of cells, clusters of much shorter units can be observed. The nature of segregative amplification mechanisms operating in these processes and the possible involvement of replicative ones are discussed.

Congenital medullary tubular stenosis. A case of Caffey-Kenny syndrome

We report a case of congenital medullary stenosis of the tubular bones, or Caffey-Kenny syndrome. The patient was referred to our department with retarded psychomotor development and contractures.

Connection between two peripherical markers in a group of asthmatic patients

We have studied the goniometric value of the tda angle and the alpha 1 antitrypsin phenotypes in a group of thirty atopic patients with the following patterns: 1) they were affected with asthma, atopic dermatitis and rhinitis. 2) they presented positive prick skin tests for pneumo-allergens and had. 3) positive allergic antecedents. There exists a significant statistical value for the lower tda angle in the right hand, in the MZ phenotypes of alpha 1 antitrypsin bearers, with P less than 0.05 significance. In the left hand there are many individual values that are identical (62% of the total test effected) thus the statistical methods cannot be applied satisfactorily, bearing in mind the significant difference as the statistical group recognized. From this and other previous works, we can conclude that an individual whose right hand tda angle reaches values near 75%, and with a MZ phenotype of alpha 1 antitrypsin, has a greater possibility of pertaining to one of the groups of asthma, atopic dermatitis and rhinitis symptomatology.

[Achondrogenesis type I and II and hypochondrogenesis (author's transl)]

A study is made of achondrogenesis in relation to four observations of early fatal development. One case corresponds to type I (Parenti-Fraccaro); another to type II (Langer-Saldino); the final two, brothers, seem to come under the variation of hypochondrogenesis. In this study, authors stress the heterogenous nature of lethal, neonatal (short-limb) nanisms of which currently include: Type I and II achondrogenesis, hypochondrogenesis, homozygote achondroplasia, classical Torrance-type and San Diego-type thanatophoric dysplasia.

[Diaphysary tubular stenosis (Kenny-Caffey's syndrome): presentation of four observations (author's transl)]

Four new observations of diaphysary tubular stenosis or Kenny-Caffey's syndrome are studied, the first in the spanish literature. They belong to two unrelated families. Cases 1 and 2 are a son and his mother; cases 3 and 4 are first cousins of, apparently, normal consanguineous parents. In the last two cases hypocalcemic tetany of recurrent evolution and congenital glaucoma were verified. For the purpose of this serie a revision is made of the cases reported on the literature. They are a total of eight observations, including the cases studied in this paper. A discussion is made, especially, on the genetic mechanism of this skeletal dysplasia, which in cases 3 and 4 present remarkable peculiarities.