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J Parma

Publications and source records attributed to J Parma.

At least 55 records · Page 3Linked to original sources
PubMed1995

Somatic mutations causing constitutive activity of the thyrotropin receptor are the major cause of hyperfunctioning thyroid adenomas: identification of additional mutations activating both the cyclic adenosine 3',5'-monophosphate and inositol phosphate-Ca2+ cascades.

A series of somatic mutations of the TSH receptor gene have been demonstrated in hyperfunctioning thyroid adenomas. The mutations studied up to now cause constitutive (i.e. TSH-independent) activation of the cAMP-regulatory cascade only. As a follow-up to our original study, we have now completely sequenced exon number 10 of the TSH receptor gene in the same series of toxic adenomas. An activating mutation was found in nine of 11 tumors. In addition to the mutations already described, two isoleucine residues belonging to the first and second extracellular loops of the receptor (Ile486 and Ile568) were found mutated. Two different adenomas were found to harbor a different amino acid substitution at residue 486 (Ile486Phe, Ile486Met). Ile568 was mutated to threonine in one. When studied by transfection in COS-7 cells, all three mutations caused very strong activation of the cAMP-regulatory cascade. In addition, the Ile486Phe and, to a lesser extent, the Ile486Met and Ile568Thr mutants stimulated constitutively the inositol phosphate-diacylglycerol cascade. Our results demonstrate that 1) the first and second extracellular loops contribute to the silencing of the unliganded TSH receptor; 2) the two regulatory cascades normally under TSH control can be constitutively activated by somatic mutations of the receptor; 3) the TSH receptor can be activated by mutation of a large number of residues distributed over the first and second extracellular loops, the third intracellular loop, and the third, sixth, and seventh transmembrane segments; 4) activating mutations of the TSH receptor constitute the major cause of toxic adenomas, accounting for about 80% of the cases.

Adenoma↗
PubMed1994

[Genes, heredity and colorectal cancer].

All cancers result from the accumulation of mutations of proto-oncogenes and tumor suppressor genes. Sporadic and familial colorectal cancers result from the accumulation of the following genes, in a relatively stereotyped chronological order: the tumor suppressor gene apc whose mutations are responsible for the familial adenomatous polyposis; the proto-oncogene K-ras which is mutated in 50% of large adenomas (> 1 cm) and adenocarcinomas; the tumor suppressor gene dcc; and the tumor suppressor gene p53 whose inactivation in a factor of bad prognosis. While some of them are induced by mutagens, others result from an instability of the genome. Two types of instability are observed in both sporadic and familial colorectal cancer. The first type, which is found in 25-50% of cases, appears as cytogenetic abnormalities with aneuploidy and allelic losses. The second type of instability is induced by mutations of the hMSH2 or hMLH1 genes which code for proteins involved in the mechanism of DNA repair.

Chromosome Deletion↗
PubMed1994

Constitutively active receptors as a disease-causing mechanism.

Membrane receptors have appeared early in evolution as the means for the unicellular organism to sense its environment. With the emergence of social cellular life in multicellular organisms, membrane receptors have acquired the additional functions of sensing the presence of similar cells (as in the aggregation phenomenon of Dictyostelium discoideum) (Klein et al., 1988) or the presence of the mate (Saccharomyces cerevisiae) (Cross et al., 1988), and to detect endocrine signals emitted by cells in distant tissues. As the latter function is central to homeostasis and regulation of cell growth, the downstream regulatory cascades under receptor control are the subject of intense research with implications in virtually all fields of biomedical science. The impact of the analysis of tyrosine kinase-activated cascades on our understanding of carcinogenesis is but one example of such an advance.

Animals↗
PubMed1994

Germline mutations in the thyrotropin receptor gene cause non-autoimmune autosomal dominant hyperthyroidism.

The thyrotropin receptor (TSHR), a member of the large family of G protein-coupled receptors, controls both the function and growth of thyroid cells via stimulation of adenylyl cyclase. We report two different mutations in the TSHR gene of affected members of two large pedigrees with non-autoimmune autosomal dominant hyperthyroidism (toxic thyroid hyperplasia), that involve residues in the third (Val509Ala) and seventh (Cys672Tyr) transmembrane segments. When expressed by transfection in COS-7 cells, the mutated receptors display a higher constitutive activation of adenylyl cyclase than wild type. This new disease entity is the germline counterpart of hyperfunctioning thyroid adenomas, in which different somatic mutations with similar functional characteristics have been demonstrated.

Adenoma↗
PubMed1994

Isodisomy of chromosome 6 in a newborn with methylmalonic acidemia and agenesis of pancreatic beta cells causing diabetes mellitus.

Isodisomy (ID) is a genetic anomaly defined as the inheritance of two copies of the same genetic material from one parent. ID in an offspring is a rare cause of recessive genetic diseases via inheritance of two copies of a mutated gene from one carrier parent. We studied a newborn female with a mut(o) of methylmalonic acidemia and complete absence of insulin-producing beta cells in otherwise normal-appearing pancreatic islets, causing insulin-dependent diabetes mellitus. The patient died 2 wk after birth. Serotyping of the HLA antigens, DNA typing of HLA-B and HLA class II loci, study of polymorphic DNA markers of chromosome 6, and cytogenetic analysis demonstrated paternal ID, involving at least a 25-centiMorgan portion of the chromosome pair that encompasses the MHC. ID probably caused methylmalonic acidemia by duplication of a mutated allele of the corresponding gene on the chromosome 6 inherited from the father. It is also very likely that ID was etiologically related to the agenesis of beta cells and consequent insulin-dependent diabetes mellitus in our patient. We thus speculate on the existence of a gene on chromosome 6 involved in beta cell differentiation.

Amino Acid Metabolism, Inborn Errors↗
PubMed1994

Identification and functional characterization of two new somatic mutations causing constitutive activation of the thyrotropin receptor in hyperfunctioning autonomous adenomas of the thyroid.

It has recently been shown that somatic and germ line mutations of the TSH receptor gene cause autonomous hyperfunctioning thyroid adenomas and nonautoimmune toxic thyroid hyperplasia by constitutive activation of the TSH receptor. A "saturated" map of these mutations is a prerequisite for a systematic screening for these clinically important mutations. In this context, it is also of interest to determine whether different amino acid substitutions at the same residue cause constitutive activation of the TSH receptor, as suggested by site-directed mutagenesis of the alpha 1 beta-adrenergic receptor. We, therefore, screened further hyperfunctioning autonomous adenomas of the thyroid for constitutively activating mutations. We identified two new somatic mutations, changing alanine in position 623 to valine (A623V) and threonine in position 632 to isoleucine (T632I). Both mutations constitutively activated cAMP when transiently expressed in COS cells. Together with neighboring mutations, the T632I mutation demonstrates the importance of transmembrane domain VI for the activation of the TSH receptor and characterizes it as a hot spot for constitutively activating mutations. The previously identified A623I and the newly identified A623V mutations demonstrate that several amino acid substitutions at the same residue can cause constitutive activation of the TSH receptor.

Adenoma↗
PubMed1994

Molecular genetics of thyroid diseases.

Over the past 20 years, recombinant deoxyribonucleic acid technology has led to the cloning of three major thyroid specific protein genes (thyroglobulin, thyroperoxidase and thyrotrophin receptor), which happen to be also the main thyroid autoantigens implicated in thyroid diseases. In this context, the impact that molecular genetics has had on the understanding of aetiopathogeny and diagnosis of thyroid diseases is summarized, with special emphasis on a recently discovered genetic mechanism responsible for toxic nodules. One fruitful outcome of the basic research on thyroid-specific gene expression has been the possibility of targeting the expression of a series of genes to the thyroid in transgenic mice. The result is the availability of mouse models mimicking human thyroid diseases such as destructive hypothyroidism, hyperactive thyroid adenomas and thyroid cancers exhibiting varying levels of differentiation.

Animals↗
PubMed1993

Somatic mutations in the thyrotropin receptor gene cause hyperfunctioning thyroid adenomas.

The pituitary hormone thyrotropin stimulates the function, expression of differentiation and growth of thyrocytes by cyclic AMP-dependent mechanisms. Tissue hyperplasia and hyperthyroidism are therefore expected to result when activation of the adenylyl cyclase-cAMP cascade is unregulated. This is observed in several situations, including when somatic mutations impair the GTPase activity of the G protein Gsa (ref 6, 7). Such a mechanism is probably responsible for the development of a minority of monoclonal hyperfunctioning thyroid adenomas. Here we identify somatic mutations in the carboxy-terminal portion of the third cytoplasmic loop of the thyrotropin receptor in three out of eleven hyperfunctioning thyroid adenomas. These mutations are restricted to tumour tissue and involve two different residues (aspartic acid at position 619 to glycine in two cases, and alanine at position 623 to isoleucine in one case). The mutant receptors confer constitutive activation of adenylyl cyclase when tested by transfection in COS cells. This shows that G-protein-coupled receptors are susceptible to constitutive activation by spontaneous somatic mutations and may thus behave as proto-oncogenes.

Adenoma↗
PubMed1992

Localization of adenylyl and guanylyl cyclase in rat brain by in situ hybridization: comparison with calmodulin mRNA distribution.

Cyclic nucleotides are major intracellular mediators in the signal transduction events in synaptic neurotransmission of the CNS. Intracellular Ca2+ is known to regulate adenylyl cyclase (AC) in a calmodulin (CaM)-dependent manner, and guanylyl cyclase (GC), in an indirect manner through CaM-sensitive nitric oxide synthase. To ascertain the physiological significance of cyclic nucleotide second messenger systems, we have localized the mRNAs encoding AC, GC, and CaM in the rat brain by in situ hybridization using 35S-labeled RNA probes. The AC mRNA is widely distributed throughout the brain; strong hybridization signal was observed in the granular layers of the cerebellum, in the pyramidal and granule cells of the hippocampus, and in the olfactory system. These AC mRNA localizations are compatible with the distribution of Ca2+/CaM-sensitive AC activities. In contrast to AC mRNA distribution, GC mRNA has a more limited distribution. Significant signals were observed in the striatum, in the pyramidal and granule cells of the hippocampus, in the olfactory system, in the inferior and superior colliculus, in the Purkinje cells of the cerebellum, in the locus coeruleus, and in many pyramidal cells in the layers II-III and V of the cerebral cortex, and mainly, in the occipital cortex. In some discrete brain regions, a close correlation was found between enzyme activity and mRNA hybridization signal of GC. The distinct distribution of AC and GC mRNAs suggests that different cyclic nucleotide second messenger systems have specialized functions. On the other hand, CaM mRNA was colocalized with the AC and GC mRNA, but its distribution was more abundant and specific for neuronal cells, since there was little hybridization signal with CaM probe in neuronal fiber regions such as the corpus callosum and the anterior commissure. The high expression of CaM mRNA in neuronal cells is in agreement with its biochemical role in the regulation of various enzymes. Results of the present study should help in analyzing the role of cyclic nucleotides and CaM in physiological and pathological situations in the CNS.

Adenylyl Cyclases↗
PubMed1991

Sequence of a human brain adenylyl cyclase partial cDNA: evidence for a consensus cyclase specific domain.

A cDNA coding for a human brain adenylyl cyclase was isolated and sequenced. The deduced partial 675 amino-acid sequence was compared with those of other known adenylyl and guanylyl cyclases. Comparison of this predicted amino-acid sequence with that of bovine brain (type I) and rat olfactory (type III) adenylyl cyclase indicated a significant homology with the carboxyl-terminal halves of both enzymes. The homology between the human adenylyl cyclase and the other two mammalian adenylyl cyclase also appears at the topographic level. Indeed, the human enzyme includes a extremely hydrophobic region containing six potential membrane-spanning segments followed by a large hydrophilic domain. At the beginning of the hydrophilic domain, there is a 250 amino-acid region which shows not only a striking homology with the bovine and rat adenylyl cyclase (86% of similarity and 57% of identity), but also a significant homology with non-mammalian adenylyl cyclase and guanylyl cyclases. We found that this 250 amino-acid domain contains a sequence of about 165 amino-acids which is highly conserved in most of the known nucleotide cyclases suggesting that it includes residues that are critical for the function of the enzymes.

Adenylyl Cyclases↗
PubMed1987

Structural organization of the 5' region of the thyroglobulin gene. Evidence for intron loss and "exonization" during evolution.

More than one third of thyroglobulin (1190 residues out of 2750) is made of one peptide motif repeated ten times in tandem. Segments unrelated to the motif interrupt this structure at various places. The corresponding gene region, which extends over 40 x 10(3) bases, was studied in detail. All exon borders and exon/intron junctions were localized precisely and sequenced, and their positions were correlated with the repetitive organization of the protein. When intron positions were compiled on a consensus sequence of all repeats, three categories of introns were observed. Except between repeats numbers 5 and 6, an intron was invariably found within the Cys codon making the limit of each motif. This category of intron most probably reflects the serial duplication events responsible for the evolution of this region of the gene. All other introns, except no. 2, are found at positions were the repetitive structure is disrupted by "inserted" peptides. We present the hypothesis that this second category of introns was already present in the original unit before the first duplication. Thereafter, they would have experienced either complete loss (some units do not contain any intron) or partial or total exonization, resulting in the slipping of intronic material into coding sequence. Intron no. 2, finally, separates motif no. 1 at a position on the boundary between two segments presenting sequence homology. This last type of intron probably reflects an initial duplication event at the origin of a primordial thyroglobulin gene motif. With all these characteristics, the thyroglobulin gene is presented as a paradigm for the analysis of the fate of introns in gene evolution.

Amino Acid Sequence↗
PubMed1987

A nonsense mutation causes hereditary goitre in the Afrikander cattle and unmasks alternative splicing of thyroglobulin transcripts.

The hereditary goitre of Afrikander cattle is an autosomal recessive disease characterized in homozygotes by the production of abnormal thyroglobulin (Tg) and the coexistence in the thyroid of normal-sized 8.4-kilobase (kb) Tg mRNA with a misspliced 7.3-kb message having lost exon 9. We have cloned and sequenced the cDNA segment corresponding to the abnormal exon 8-exon 10 junction and the relevant genomic DNA region. The mutation responsible for the disease is a cytosine to thymine transition creating a stop codon at position 697 in exon 9. The original reading frame is maintained in the 7.3-kb mRNA, which, as it lacks the mutated exon, is translatable into a potentially functional protein. This puzzling phenotype in which a mutated exon is apparently removed selectively from transcripts by alternative splicing leads us to suggest that the 7.3-kb transcript could be present in normal animals. Using a sensitive oligonucleotide hybridization assay, we have demonstrated that a 7.3-kb mRNA lacking exon 9 does exist in normal thyroids as a minor mRNA species. As it is fully translatable, the 7.3-kb mRNA is expected to be more stable than the normal-sized 8.4-kb message. This probably accounts for the higher proportion of 7.3-kb transcript found in the goitre.

Amino Acid Sequence↗
PubMed1987

Normal and defective expression of the thyroglobulin gene.

Molecular studies of the thyroglobulin (Tg) gene have progressed significantly in recent years. Cloning and sequencing the complete bovine Tg cDNA led to the knowledge of the primary structure of the Tg subunit. This large polypeptidic chain displays a repetitive structure, especially in its amino-terminal half, and bears a striking homology with the acetylcholinesterase molecule of Torpedo californica in its carboxy-terminal portion. The four specific domains known to be involved in the formation of the thyroid hormones have been assigned to both terminal parts of the polypeptide, a location which could play a role in the process leading to hormone release. The very large (greater than 250 kb) Tg gene has been localized on the long arm of chromosome 8 in man, in close linkage with the c-myc oncogene. The study of its structure allowed the characterization of the molecular defect responsible for a congenital flaw in Tg gene expression in a herd of South-African cattle. This work led to the unexpected finding that the Tg pre-mRNA undergoes alternative splicing in normal animals, too. A DNA segment involved in the transcriptional control of Tg gene expression by cAMP has been identified by transfecting primary cultured thyrocytes with recombinant genes.

Animals↗