Comparative study of p120 GTPase-activating protein and its point mutant in the pleckstrin homology domain

GTPase-activating protein (GAP) enhances the intrinsic GTPase activity of cellular Ras. In addition to two Src homology 2 (SH2) domains and one Src homology 3 (SH3) domain, it contains a pleckstrin homology (PH) domain. The wild-type or point mutant in the PH domain of p120 GAP (W568A) was expressed by using the baculovirus/Sf9 cell system. Direct effects of the G protein beta gamma subunit (G beta gamma) and several sphingolipids and the effects of phosphorylation by c-Src on the GTPase-stimulating activity of these GAPs on Ras were examined by using immunoprecipitates of these GAPs. The activities of neither of these GAPs were affected by the addition of G beta gamma, although the W568A mutant bound less to G beta gamma compared with the wild type. Several sphingolipids had no effect on the activity of these GAPs. Only in the W568A mutant was GTPase-stimulating activity reduced by tyrosine phosphorylation by c-Src.

Tumor suppressor p16INK4A: determination of solution structure and analyses of its interaction with cyclin-dependent kinase 4

The solution structure of the tumor suppressor p16INK4A has been determined by NMR, and important recognition regions of both cdk4 and p16INK4A have been identified. The tertiary structure of p16INK4A contains four helix-turn-helix motifs linked by three loops. Twelve tumorigenic mutants of p16INK4A have been constructed and analyzed for their structure and activity, and new mutants have been designed rationally. A fragment of 58 residues at the N terminus of cdk4 important for p16INK4A binding has been identified. The importance of this region was further verified by mutational analysis of cdk4. These results and docking experiments have been used to assess possible modes of binding between p16INK4A and cdk4.

Integrated amplification and detection of the C677T point mutation in the methylenetetrahydrofolate reductase gene by fluorescence resonance energy transfer and probe melting curves

A microvolume fluorimeter integrated with a rapid thermal cycler allows both amplification and point mutation detection from genomic DNA in approximately 30 min. This homogeneous method combines rapid cycle DNA amplification with allele-specific fluorescent probe melting profiles for product genotyping. The amplification reaction includes a primer internally labeled with Cy5 and a 3'-fluorescein-labeled probe that spans the region of interest. During asymmetric amplification, the probe hybridizes to excess Cy5-labeled strand and is observed as fluorescence resonance energy transfer. Resonance energy transfer increases each cycle as product accumulates during amplification. When fluorescence is monitored as the temperature increases through the Tm of the probe/product duplex, a characteristic melting profile for each genotype is obtained. Fluorescence genotyping of the common C677T base substitution in the methylenetetrahydrofolate reductase gene in 110 DNA samples correlated perfectly with genotyping by restriction enzyme digestion and gel electrophoresis. The relatively stable G:T mismatch of this example gave a 3 degrees C difference in Tm from complete Watson-Crick pairing, suggesting that this homogeneous fluorescence method can be used for all single-base mismatches.

Point mutations in v-Myb disrupt a cyclophilin-catalyzed negative regulatory mechanism

The c-Myb protein is controlled by intramolecular interactions, and point mutations can enhance its oncogenic activity. We tested whether conformational changes regulate c-Myb and found that Cyp-40, a widely distributed cyclophilin and peptidyl-prolyl isomerase, could inhibit c-Myb DNA binding activity. Inhibition by Cyp-40 required both its C-terminal protein-interaction domain, which bound specifically to c-Myb, and its N-terminal catalytic domain and was blocked by the competitive inhibitor cyclosporin A. Cyp-40 failed to bind or inhibit the oncogenic derivative v-Myb, which has a mutated Cyp-40 binding site. These results suggest that mutations in v-Myb allow it to evade a negative regulatory mechanism mediated by enzymes such as Cyp-40, and implicate peptidyl-prolyl isomerases in the regulation of transcription, transformation, and differentiation.

Determination of the migratory capacity of embryonic cortical cells lacking the transcription factor Pax-6

The cerebral cortex forms by the orderly migration and subsequent differentiation of neuronal precursors generated in the proliferative ventricular zone. We studied the role of the transcription factor Pax-6, which is expressed in the ventricular zone, in cortical development. Embryos homozygous for a mutation of Pax-6 (Small eye; Sey) had abnormalities suggesting defective migration of late-born cortical precursors. When late-born Sey/Sey precursors were transplanted into wild-type embryonic rat cortex, they showed similar integrative, migrational and differentiative abilities to those of transplanted wild-type mouse precursors. These results suggest that postmitotic cortical cells do not need Pax-6 to acquire the capacity to migrate and differentiate, but that Pax-6 generates a cortical environment that permits later-born precursors to express their full developmental potential.

Familial ALS is associated with mutations in all exons of SOD1: a novel mutation in exon 3 (Gly72Ser)

Mutations of the SOD1 gene, which encodes the enzyme copper/zinc superoxide dismutase, are associated with familial amyotrophic lateral sclerosis (ALS). SOD1 consists of five exons, and over 50 different mutations have been described involving exons 1,2,4 and 5. The absence of mutations in exon 3 has been attributed to a critical function of this exon, its integrity being necessary for the toxic effect of mutant SOD1, and it has been suggested that such mutations may be lethal rather than leading to adult onset disease. We identified the heterozygote mutation Gly72Ser (exon 3) in a family with two individuals affected by ALS. SOD enzyme activity was reduced by 45% when measured in erythrocytes indicating reduced enzyme activity, or reduced stability of the mutant protein. These findings indicate that exon 3 is not a privileged region from mutation; that all five exons should be investigated when seeking SOD1 mutations in human disease; and may help in a better understanding of the pathogenicity of these mutations in ALS.

Co-inheritance of the 20210A allele of the prothrombin gene increases the risk of thrombosis in subjects with familial thrombophilia

The presence of the 20210A allele of the prothrombin (PT) gene has recently been shown to be a risk factor for venous thromboembolism. This is probably mediated through increased plasma prothrombin levels. The aim of this study was to compare the prevalence of the prothrombin 20210A allele in control subjects and in subjects with recognised thrombophilia and to establish whether the additional inheritance of the PT 20210A allele is associated with an increased risk of venous thromboembolism. 101 subjects with a history of venous thromboembolism and diagnosed as having either factor V Leiden (R506Q) or heritable deficiencies of protein C, protein S or antithrombin were studied. The prevalence of the PT 20210A allele in this group was compared with the results obtained for 150 control subjects. In addition, the relationships were examined between genetic status and the number of documented thromboembolic episodes, and between plasma prothrombin levels and possession of the PT 20210A allele. 8 (7.9%) of the 101 patients were also heterozygous for the PT 20210A allele. This compares with 0.7% in the control subjects (p = 0.005). After exclusion of patients on warfarin, the mean plasma prothrombin of 113 subjects without 20210A was 1.09 U/ml, as compared with 1.32 U/ml in 8 with the allele (p = 0.0002). Among the 101 patients with either factor V Leiden, protein S deficiency, protein C deficiency or antithrombin deficiency, the age adjusted mean (SD) number of venous thromboembolic episodes at diagnosis was 3.7 (1.5) in those with the PT 20210A allele, as compared with 1.9 (1.1) in those without (p = 0.0001). We have demonstrated that the prevalence of the PT 20210A allele is significantly greater in subjects with venous thrombosis and characterised heritable thrombophilia than in normal control subjects and that the additional inheritance of PT 20210A is associated with an increased risk of venous thromboembolism. We have also confirmed that plasma prothrombin levels are significantly greater in subjects possessing the PT 20210A compared with those who do not.

Mutation in the thyroid hormone receptor beta gene (A317T) in a Thai subject with resistance to thyroid hormone

Analysis of the thyroid hormone receptor beta (TRbeta) gene of a Thai female with the syndrome of resistance to thyroid hormone (RTH) revealed a missense mutation at codon 317, changing the guanine in nucleotide 1234 to an adenine that results in the replacement of the normal alanine (GCT) with a threonine (ACT). The proposita was heterozygous, and this mutation was not present in her parents and her sister, compatible with a neomutation. This is the first report of TRbeta gene mutation causing RTH in an individual of Thai origin.

Prevalence of the prothrombin gene variant (nt20210A) in venous thrombosis and arterial disease

The prothrombin gene variant resulting form a G-->A transition at position 20210 has been described as a common genetic risk factor for venous thrombosis. However, the risk for developing arterial disease is unknown. In this investigation, we studied 116 patients with venous thrombosis and 71 with arterial disease, all of whom were compared with 295 controls. Additionally, we also investigated the distribution of the prothrombin alleles among African descendents and Amazonian Indians from Brazil. The prevalence of 0.7% for 20210A allele in the control group increased to 4.3% (P = 0.021) among patients with venous thrombosis. There was also a high prevalence of the mutated allele in a selected arterial disease group (5.7%) without hyperlipoproteinemia, hypertension, and diabetes mellitus when compared to the controls (P = 0.013). Heterozygotes for the allele 20210A were common among individuals of African descent (2%) and rare among Indians. These data support the hypothesis that the prothrombin variant is a risk factor for venous thrombosis and suggest that it may also be a risk factor for arterial disease.

Electrospray ionisation mass spectrometry facilitates detection of fibrinogen (Bbeta 14 Arg --> Cys) mutation in a family with thrombosis

We report the first direct detection of a fibrinogen mutation by electrospray ionisation mass spectrometry. The propositus, from a family with a history of thrombosis, came to attention after a pulmonary embolism subsequent to a spontaneous abortion. Prolonged thrombin (41 s) and reptilase times (26 s) together with an impairment of fibrinopeptide B release suggested a mutation at the thrombin cleavage site of the Bbeta chain. Direct mass analysis of purified fibrin chains from a thrombin induced clot showed that 50% of the Bbeta chains remained uncleaved. The measured mass of the mono sialo isoform of this uncleaved chain was 54150 Da, compared to a value of 54198 Da for normal Bbeta chains. This decrease of 48 Da in the intact protein is indicative of either a Bbeta 14 Arg to Cys, or Arg to Leu substitution. Heterozygosity for the Bbeta 14 Arg --> Cys mutation was verified by PCR amplification and DNA sequence analysis.

Selective screening for the Factor V Leiden mutation: is it advisable prior to the prescription of oral contraceptives?

The cumulative thrombotic risk of Factor V (FV) Leiden and oral contraceptives (OC) recommends screening for the mutation. Assuming that a family history of thrombosis increases the patient's likelihood of bearing FV Leiden, a selective rather than universal screening would be performed. We studied the utility of a family history of thrombosis for screening of FV Leiden before prescription of OC and, furthermore, the utility of screening even if oral contraception is favoured. 101 patients who had their first and single thromboembolic event while using OC were interviewed. 609 women without any history of thromboembolism recruited by gynecologists completed a standard questionnaire. 101 of these women, age-matched and currently using OC, were selected for a case-control study. Regarding patients with previous thromboembolism, a family history in a first-degree relative had a positive predictive value (PPV) of only 14% for FV Leiden. A PPV of 12% was calculated by investigating the 609 thrombosis-free women. Inherited FV Leiden (odds ratio = 4.9) and acquired risk factors (odds ratio = 10.1) were both found to be the most prominent, but independent additional risks. Nevertheless, FV Leiden carriers, both heterozygotes and homozygotes, did not suffer earlier from thromboembolism than patients without the mutation. In conclusion, family history is an unreliable criterion to detect FV Leiden carriers. Screening for factor V Leiden can be worthwhile even if the advantages of oral contraception are higher assessed than the thrombotic risk. Affected women knowing about their additional risk could contribute to the prevention of thrombosis in risk situations.

Identification of the glycine-to-arginine substitution G2043R in type VII collagen in a family with dominant dystrophic epidermolysis bullosa from Hungary

Epidermolysis bullosa (EB) represents a group of genodermatoses characterized by fragility and easy blistering of the skin. In the dystrophic forms of EB (DEB), blisters occur below the basement membrane, at the level of the anchoring fibrils. In the dominantly inherited forms (DDEB), the predominant type of mutation detected thus far is the substitution of a glycine residue which occurs within the collagenous domain of the molecule characterized by the repeating Gly-X-Y amino acid sequence. In this study, we searched for mutations in DDEB in a family from Hungary, by PCR amplification of segments of COL7A1, followed by heteroduplex analysis. Examination of the PCR fragment corresponding to exon 73 revealed a heteroduplex in affected individuals from the family. Sequence analysis revealed a G-to-A transition at nucleotide 6127 in the triple-helical domain of COL7A1, which converted a glycine residue at amino acid position 2043 to an arginine. This report represents the second incidence of this mutation, G2043R, described first in a family with DDEB from Italy.

Allosteric interaction of the 1alpha,25-dihydroxyvitamin D3 receptor and the retinoid X receptor on DNA

Genomic actions of the hormone 1alpha,25-dihydroxy-vitamin D3(VD) are mediated by the transcription factor VDR, which is a member of the nuclear receptor superfamily. VDR acts in most cases as a heterodimeric complex with the retinoid X receptor (RXR) from specific DNA sequences in the promoter of VD target genes called VD response elements (VDREs). This study describes a mutation (K45A) of the VDR DNA binding domain that enhances the affinity and ligand responsiveness of VDR-RXR heterodimers on some VDREs. In analogy to a homologous mutation in the glucocorticoid receptor (K461A), this lysine residue appears to function as an allosteric 'lock'. Interestingly, overexpression of RXR was found to reduce the responsiveness and sensitivity of wild type VDR to VD, but enhance the response of VDRK45A. Moreover, the transactivation domains of both VDR and RXR were shown to be essential for obtaining responsiveness of the heterodimers to VD and 9- cis retinoic acid (the RXR ligand). This indicates that RXR is an active rather than silent partner of the VDR on the VDREs tested. Taken together, transactivation by VDR-RXR heterodimers can be triggered individually by all components of the protein-DNA complex, but full potency appears to be reached through allosteric interaction.

Regulation of the human TRH (hTRH) gene by human thyroid hormone receptor beta 1 (hTR beta 1) mutants

TRH is negatively regulated by T3 both in the hypothalamic paraventricular nucleus and transient transfection models. Mutations in hTR beta 1 genes are associated with the syndrome of generalized resistance to thyroid hormone. To investigate potential effects of mutant TRs on T3 regulation of the hTRH gene, transient gene expression assays were performed in human neuroblastoma (HTB-11) cells with an hTRH promoter-luciferase construct, wild type (WT) hTR beta 1, and three qualitatively distinct hTR beta 1 mutant forms (ED, OK and PV). In the presence of T3 (10(-9) M), liganded WT-hTR beta 1 inhibited hTRH promoter activity significantly (40%). Cotransfection of each of the two mutants (ED and OK) achieved similar levels of inhibition only at 10 to 100 fold increased T3 concentrations. Of interest, a 10x excess of mutant ED or OK could also exert dominant negative effects upon WT hTR beta 1-T3 mediated inhibitory actions on the hTRH promoter. In contrast, mutant TR-PV exerted neither inhibitory nor dominant negative effects at even higher concentrations of T3. Moreover, all three unliganded mutant forms stimulated TRH promoter activity significantly in the absence of T3, despite their different mutations in the ligand-binding domain (LBD). These data demonstrate that thyroid hormone resistance at the level of TRH gene regulation, due to reduced inhibitory actions of mutant TR-T3 complexes, as well as dominant negative effects upon WT hTR beta 1 mediated inhibition, likely contribute to elevated TSH values observed in the syndrome of thyroid hormone resistance.

Functional analysis of the nucleotide binding domains of the multidrug resistance protein (MRP)

HeLa cells were transfected with full-length multidrug resistance protein (MRP) cDNA and with MRP cDNAs that had been mutated at certain nucleotide binding domains. Stable transfectants were isolated and those producing equivalent amounts of P190 were tested in cytotoxicity assays using a variety of chemotherapeutic agents. The results demonstrate that deletions in the C-motif of NBD1 or the A-motif of NBD2 have a pronounced effect in reducing resistance levels to adriamycin, vincristine, or etoposide (VP-16). Single-site mutations of lysine in these same motifs reduce IC50 values but less than that observed with the deletion mutants. Additional studies have demonstrated an increase in drug accumulation and reduction in drug efflux in NBD deletion and single-site mutants. The results of this study therefore identify two lysines of the NBD A- and C-motifs that are critical for MRP-mediated multidrug resistance. The results also provide definitive evidence that resistance occurring as a result of MRP overexpression is related to enhanced levels of an ATP-dependent efflux pump.

A new point mutation in a hypoxanthine phosphoribosyltransferase-deficient patient

A 12-year-old boy was referred because of abdominal pain, gross hematuria, and passage of stones. Further evaluation showed growth delay, low average range of intellectual functioning, and a speech articulation disorder. No signs of self-mutilation or self-injurious behavior were present. He had hyperuricemia, hyperuricosuria, uric acid crystalluria, uric acid calculi, macrocytosis, megaloblastic bone marrow changes, and mild anemia. Hypoxanthine phosphoribosyltransferase (HPRT) enzyme activity was reduced to approximately 26% of normal. Polymerase chain reaction-single strand conformational polymorphism analysis of the HPRT gene in DNA isolated from the patient's blood lymphocytes revealed a single nucleotide substitution at codon 200 in exon 8. The base change was a guanine to cytosine transversion, resulting in the conservative amino acid substitution of threonine in place of arginine. To our knowledge, this mutation has not previously been reported.

Transport of myosin II to the equatorial region without its own motor activity in mitotic Dictyostelium cells

Fluorescently labeled myosin moved and accumulated circumferentially in the equatorial region of dividing Dictyostelium cells within a time course of 4 min, followed by contraction of the contractile ring. To investigate the mechanism of this transport process, we have expressed three mutant myosins that cannot hydrolyze ATP in myosin null cells. Immunofluorescence staining showed that these mutant myosins were also correctly transported to the equatorial region, although no contraction followed. The rates of transport, measured using green fluorescent protein-fused myosins, were indistinguishable between wild-type and mutant myosins. These observations demonstrate that myosin is passively transported toward the equatorial region and incorporated into the forming contractile ring without its own motor activity.

The effect of point mutations on the free energy of transmembrane alpha-helix dimerization

Glycophorin A forms homodimers through interaction of the single, helical transmembrane domains of the monomers. The dimers are stable in sodium dodecylsulfate (SDS), permitting a number of studies that have identified a critical motif of residues that mediates dimer formation. We have used analytical ultracentrifugation to measure the energy of dimerization in a non-denaturing detergent solution and have observed the changes in energy arising from two of the mutants previously studied. Use of the detergent pentaoxyethylene octyl ether (C8E5) is a great advantage, since its micelles are neutrally buoyant and the detergent allows a reversible association to occur between monomer and dimer states of the glycophorin A transmembrane helices during the time-scale of sedimentation equilibrium. Use of this detergent in analytical ultracentrifugation may enable a wide range of studies of molecular association events in membrane proteins. We find that the glycophorin A transmembrane helix dimerizes with a dissociation constant of 240(+/-50) nM, corresponding to a free energy of dissociation of 9.0(+/-0.1) kcal mol-1. Point mutants that were found to be disruptive in SDS (L75A, I76A) reduced the dimer affinity in the C8E5 detergent environment (Kd=1.7(+/-0.2) microM and 4.2(+/-0.9) microM, respectively). Thus, the earlier findings are placed on a quantitative, relative energy scale of association by our measurements. Molecular modeling and simulations suggest that the energy differences can be accounted for as changes in van der Waals interactions between helices.

Analysis of compound heterozygous mice reveals that the Trembler mutation can behave as a gain-of-function allele

The most common form of Charcot-Marie-Tooth disease, CMT1A, is correlated with a 1.5 megabase duplication on chromosome 17p.11.2 containing the peripheral myelin protein 22 (PMP22) gene. Deletion of the same region is associated with a second inherited neural disorder, the hereditary neuropathy with liability to pressure palsies (HNPP). Moreover, several distinct point mutations within the PMP22 coding region are associated with CMT1A and Dejerine-Sottas Syndrome in humans and the Trembler (Tr) and Trembler-J phenotypes in mice. Heterozygous Tr mutants (Tr/+) display severe hypomyelination of peripheral nerve fibers while heterozygous pmp22 knockout mice (pmp22+/0) are characterized by focal hypermyelination. These findings suggest that the Tr mutation does not generate a pmp22 null allele but rather produces its deleterious effects by either a dominant-negative or gain-of-function mechanism. To address this question in detail, we have compared various combinations of pmp22 alleles including Tr/+, Tr/Tr, Tr/0, pmp22+/0, and pmp22(0/0) mice with respect to the resulting myelin abnormalities. The combined analysis of these mutants demonstrates that the Tr allele can act as a true gain-of-function mutation in both, the heterozygous state on a null background (Tr/0) as well as in homozygous Tr animals (Tr/Tr). Furthermore, increasing the relative Tr gene dosage correlates with more pronounced myelin deficiencies and decreased levels of MBP and P0 in 18-day-old mice.

Mutational analysis of the RecA protein L1 region identifies this area as a probable part of the co-protease substrate binding site

Previous mutational analysis of the L1 region of the RecA protein suggested that Gly-157 and Glu-158 are 'hot-spots' for the occurrence of constitutive LexA co-protease mutants (coprt[c]). In the present study, we clearly establish that position 157 is a hot-spot for the occurrence of such mutants, as 12 of 14 and 10 of 14 substitutions result in this phenotype for UmuD and LexA cleavage respectively. The frequency of such mutations at position 158 is somewhat lower, 8 of 13 and 5 of 13 for UmuD and LexA respectively. Comparison of the UmuD vs. LexA co-protease activity for all single mutants with substitutions at positions 154, 155, 156, 157 and 158 (47 in total) reveals that, although there is good agreement among most mutants regarding their ability to cleave both LexA and UmuD, there are two in particular (Glu-154-->Asp and Glu-154-->Gln) that show a clear preference for cleavage of UmuD. We also show that three second-site mutations that completely suppress coprt(c) activity toward LexA have little or no effect on the coprt(c) activity of the primary mutant toward UmuD. In addition, we observe a high frequency of second-site suppressor mutations, suggesting a functional interaction among side-chains in this region. Together, these results support the idea that the L1 region of RecA makes up part of the co-protease substrate-binding site.

Point mutations in the transmembrane domain of DjlA, a membrane-linked DnaJ-like protein, abolish its function in promoting colanic acid production via the Rcs signal transduction pathway

DjIA is a novel DnaJ-like protein localized to the inner membrane of Escherichia coli through the single transmembrane domain (TMD) found at the N-terminus. The overproduction of DjIA activates expression of the cps operon, controlling synthesis and export of the extracellular polysaccharide colanic acid via the Rcs/B two-component signal transduction pathway. We now show that both the TMD and the J-region are essential for the induction of cps expression observed with the overproduction of DjIA. Furthermore, we describe the isolation and characterization of different point mutations in the TMD that completely or partially block the induction of cps expression associated with overproduction of DjIA. These mutations were shown not to affect the localization, stability or topology of the mutant DjIA proteins. We propose that these mutations are affecting specific interactions between the TMD of DjIA and its substrate protein(s), for example RcsC, the membrane sensor kinase partner of the Rcs/B signal transduction pathway.

Gene dosage effect in one family with myoclonic epilepsy and ragged-red fibers (MERRF)

OBJECTIVES

We analyzed the percentage of mitochondrial DNA (mtDNA) heteroplasmy in blood samples of 13 individuals belonging to a three family generation of myoclonic epilepsy with ragged-red fibers (MERRF) and compared the 5 affected patients and the 8 unaffected relatives.

MATERIAL AND METHODS

DNA was extracted from blood and muscle of the proband and from blood of 12 maternal relatives. A PCR restriction analysis method was used to detect the mutation.

RESULTS

The proband had the complete MERRF phenotype. The phenotype in three other individuals in the maternal lineage was consistent with the MERRF syndrome. The remaining were asymptomatic. The np 8344 mutation was observed in muscle and blood of the proband, and in blood from every one of 12 maternal relatives, ranging from 44% to 83% of mutated genomes. Symptomatic individuals had higher levels (P < 0.001) of mutated mtDNA than asymptomatic maternal relatives. However, high proportions of mutant genomes (up to 63%) were found in asymptomatic relatives.

CONCLUSIONS

Although there seems to be a gene dosage effect in MERRF, we found no absolute relationship between the relative proportion of mutant genomes in blood and clinical severity. Factors other than gene dosage in blood may account for the differences in clinical phenotype.

Regulation of thrombin formation by activated protein C: effect of the factor V Leiden mutation

Activated Protein C (APC) resistance, one of the most common genetic risk factors for venous thrombosis, is caused by a single base mutation (G1691-->A) in the factor V (FV) gene resulting in the replacement of Arg506 by Gln at a predominant cleavage site for APC. Great progress in understanding the mechanism of downregulation of FVa activity via the protein C pathway has been achieved by studying APC-mediated inactivation of FVa purified from homozygous APC-resistant individuals. This review briefly summarizes the role of FVa in prothrombin activation and the structure-function relationship of FV and FVa. Subsequently, APC-dependent inactivation of FVa and FVa Leiden and its modulation by protein S and factor Xa in model systems containing purified proteins is discussed. FV also has a function in increasing the inactivation of FVIII/VIIIa by APC. This cofactor activity appears diminished in FV Leiden. Thus, an intricate mechanism of regulation of thrombin formation via the protein C pathway is starting to emerge. Extensive studies in plasma milieu will be needed to gain more insight into the relation between the presence of FV Leiden and impaired downregulation of thrombin formation in APC-resistant individuals.

Nature of the 5' residue in the M2 domain affects function of the human alpha 1 beta 1 GABAA receptor

The effects on the functional properties of the alpha 1 beta 1 GABAA receptor when the 5' (alpha 1 Val260; beta 1 Ile255) hydrophobic amino acids in the second transmembrane (M2) region were changed to threonine were examined. In response to a saturating concentration of GABA, the current evoked in mutant receptors showed a decreased rate of desensitization and at equilibrium was a greater fraction of the peak current than in wild-type receptors. The half-saturation concentration of the peak current response to GABA in mutant receptors was comparable to that in wild-type receptors, but the Hill coefficient was reduced to less than one. It was concluded that the 5' amino acids in the M2 region have a role in the conformational changes that occur within the alpha 1 beta 1 GABAA receptor in response to GABA.

Antithrombin and its inherited deficiency states

Antithrombin is the primary inhibitor of thrombin that also inhibits many of the other activated serine proteinases involved in blood coagulation. A hypercoagulable state occurs when a deficiency of antithrombin exists in plasma; the deficiency may be either inherited or acquired. This failure to regulate adequately the activity of coagulation proteinases can, with additional provocation, result in clot formation and in the clinical presentation of thromboembolic disease. The structure and function of antithrombin, nature and heterogeneity of the molecular defects in the antithrombin gene associated with inherited antithrombin deficiency, prevalence and the natural history of inherited antithrombin deficiency are all reviewed here.

Protein C and protein S deficiencies

The protein C (PC) pathway, with its cofactor protein S (PS), is an important natural antithrombotic mechanism. Both PC and PS deficiencies have been implicated in thrombophilia. The molecular basis for hereditary PC and PS deficiencies is highly heterogeneous, with a large spectrum of mutations that have various effects on the expression of the relevant allele. A small subset of patients who are homozygous or compound heterozygous for a PC gene mutation have severe thrombotic complications at birth, whereas onset occurs later in the other cases. Patients heterozygous for a PC or PS gene abnormality may develop recurrent thrombosis during adulthood, with a probability of remaining free of thrombosis of about 50% at age 45. A PC or PS gene defect is associated with the factor V Arg 506 to Gln mutation in 10% to 30% of symptomatic patients, suggesting that clinical expression is controlled by several genes in heterozygous patients.

Combination of activated protein C resistance and antibodies to phospholipids in the development of thrombosis

An insufficient response to activated protein C (APC) resistance and antibodies against phospholipids (PLa) are frequent laboratory findings associated with thrombosis. Studies investigating the coexistance of these two factors in thrombophilic patients and in patients with autoimmune disorders are summarized. The investigation of thrombophilic patients has revealed PLa in 35%. About half of these patients had a combination of PLa and APC resistance proved by the Arg506-Gln mutation in factor V (FV). A combination of APC resistance with PLa in this group of patients did not increase the risk of thrombosis recurrence. In the PLa-positive patients with autoimmune disorders having thrombosis in 10% to 56%, the incidence of the Arg506-Gln mutation in FV was significantly lower and varied between 2% and 9%. A known ability of PLa to induce APC resistance experimentally was often a reason to avoid the determination of APC response in PLa-positive patients. We suggest that PLa may play a dual role in the development of thrombosis in the carriers of the Arg506-Gln mutation in FV. On the one hand, PLas favor thrombosis by maintaining the APC response at a constantly low level. On the other hand, preventing the interaction of coagulation proteins on the phospholipid surface PLa may protect them from hypercoagulation.

Characterization of alpha1(IV) collagen mutations in Caenorhabditis elegans and the effects of alpha1 and alpha2(IV) mutations on type IV collagen distribution

Type IV collagen is a major component of basement membranes. We have characterized 11 mutations in emb-9, the alpha1(IV) collagen gene of Caenorhabditis elegans, that result in a spectrum of phenotypes. Five are substitutions of glycines in the Gly-X-Y domain and cause semidominant, temperature-sensitive lethality at the twofold stage of embryogenesis. One is a glycine substitution that causes recessive, non-temperature-sensitive larval lethality. Three putative null alleles, two nonsense mutations and a deletion, all cause recessive, non-temperature-sensitive lethality at the threefold stage of embryogenesis. The less severe null phenotype indicates that glycine substitution containing mutant chains dominantly interfere with the function of other molecules. The emb-9 null mutants do not stain with anti-EMB-9 antisera and show intracellular accumulation of the alpha2(IV) chain, LET-2, indicating that LET-2 assembly and/or secretion requires EMB-9. Glycine substitutions in either EMB-9 or LET-2 cause intracellular accumulation of both chains. The degree of intracellular accumulation differs depending on the allele and temperature and correlates with the severity of the phenotype. Temperature sensitivity appears to result from reduced assembly/secretion of type IV collagen, not defective function in the basement membrane. Because the dominant interference of glycine substitution mutations is maximal when type IV collagen secretion is totally blocked, this interference appears to occur intracellularly, rather than in the basement membrane. We suggest that the nature of dominant interference caused by mutations in type IV collagen is different than that caused by mutations in fibrillar collagens.

pH-dependent interactions of Cd2+ and a carboxylate blocker with the rat C1C-1 chloride channel and its R304E mutant in the Sf-9 insect cell line

1. Gating of the skeletal muscle chloride channel (ClC-1) is sensitive to extracellular pH. In this study, whole-cell recording of currents from wild-type (WT) ClC-1 and a mutant, R304E, expressed in the Sf-9 insect cell line was used to investigate further the nature of the pH-sensitive residues. 2. Extracellular Cd2+ produced a concentration-dependent block of WT ClC-1 with an IC50 of 1.0 +/- 0.1 mM and a Hill coefficient of 2.0 +/- 0.3. This block was sensitive to external pH, reducing at low pH, with an apparent pKa of 6.8 +/- 0.1 and a Hill coefficient for proton binding of 3.0 +/- 0.3. Anthracene-9-carboxylate (A-9-C) block of WT ClC-1 was also pH sensitive, increasing at low pH, with an apparent pKa of 6.4 +/- 0.1 and a Hill coefficient for proton binding of 1.0 +/- 0.2. 3. Compared with WT ClC-1, R304E had a lower affinity for Cd2+ (IC50, 3.0 +/- 0.3 mM) but it had a similar Hill coefficient for transition metal ion binding. The Hill coefficient for proton binding to the Cd2+ binding site was reduced to 1.4 +/- 0.3. In contrast, the A-9-C binding site in R304E showed the same pH sensitivity and affinity for the blocker as that seen in WT ClC-1. 4. ClC-1 has at least two binding sites for Cd2+, each of which has at least three residues which can be protonated. Binding of A-9-C is influenced by protonation of a single residue. Arg 304 is not sufficiently close to the A-9-C binding site to affect its characteristics, but it does. alter Cd2+ binding, indicating that transition metal ions and aromatic carboxylates interact with distinct sites. 5. The block of ClC-1 by transition metal ions and the apparent pKa of this block, together with the apparent pKa for A-9-C block and gating are all compatible with the involvement of His residues in the pore and gate of ClC-1.

Insulin receptor substrate-1 gene polymorphism and cardiovascular risk in non-insulin dependent diabetes mellitus and patients undergoing coronary angiography

Clustering of risk factors for cardiovascular disease related to insulin resistance may account for the increased incidence of vascular disease in these conditions and in non-diabetic subjects. To investigate the relationship between a coding polymorphism in the insulin receptor substrate-1 gene and the presence of cardiovascular risk factors, 209 patients with NIDDM and 452 subjects investigated for coronary artery disease (CAD) were studied. In the NIDDM subjects 22 (10.5%) were heterozygous at codon 972 for a polymorphism which codes for a glycine to arginine substitution and 187 (89.5%) were homozygous for the wild type. Patients with the mutation had lower levels of cholesterol compared with wild type (mean, 95% confidence intervals), 5.3 (4.9-5.8) vs 6.0 (5.9-6.2) mmol/l, respectively (P = 0.002), triglyceride 1.7 (1.4-2.1) vs 2.2 (2.0-2.4) mmol/l (P = 0.051), factor VII:C activity 109.5 (85.5-133.5) vs 133.5 (127-140)% (P = 0.057) and PAI-1 antigen, 16.0 (10.5-24.3) vs 22.2 (20.0-24.6) ng/ml (P = 0.054). There were no differences in body mass index, indices of glycaemic control, fasting insulin or the prevalence of hypertension. In patients with CAD, 55 (12.7%) were carriers of the mutation (including three homozygotes) (NIDDM vs CAD, NS). Although similar trends in cholesterol, factor VII, PAI-1 antigen and triglyceride existed between carriers of the mutation and the wild type, none reached statistical significance. The results indicate that the IRS-1 gene is not implicated in the pathogenesis of NIDDM or CAD.

beta-Galactosidase gene mutations in patients with slowly progressive GM1 gangliosidosis

Three unrelated North American cases with slowly progressive forms of GM1 gangliosidosis were found to have two unique point mutations and a 9 bp insertion in the coding region of the gene encoding beta-galactosidase. Case 1 was noted to have a 9 bp insertion ¿CAGAATTTT¿ on one allele between nucleotides 730 and 731 with no other mutations identified in the other allele. In case 2, two point mutations were found: a unique G-->A transition at nucleotide 602 causing an Arg-->His substitution in codon 201 (mutation R201H); and a previously identified G-->T transition at nucleotide 1527 causing a Trp-->Cys substitution in codon 509 (mutation W509C), which has been noted in adult and chronic forms of GM1 gangliosidosis. Case 3 had a unique point mutation (A-->G transition at nucleotide 797) resulting in a Asn-->Ser amino acid substitution in codon 266 (mutation N266S), with no other mutations found in the same or the other allele. Single-strand conformation polymorphism performed on over 100 controls did not demonstrate the presence of the point mutations R201H or N266S. Also, the mutant proteins coded by the two point mutations did not show enzymatic activity in the Cos-1 cell expression system confirming that these mutations are associated with low enzyme activity.

Slow-channel transgenic mice: a model of postsynaptic organellar degeneration at the neuromuscular junction

The slow-channel congenital myasthenic syndrome (SCCMS) is a dominantly inherited disorder of neuromuscular transmission characterized by delayed closure of the skeletal muscle acetylcholine receptor (AChR) ion channel and degeneration of the neuromuscular junction. The identification of a series of AChR subunit mutations in the SCCMS supports the hypothesis that the altered kinetics of the endplate currents in this disease are attributable to inherited abnormalities of the AChR. To investigate the role of these mutant AChR subunits in the development of the synaptic degeneration seen in the SCCMS, we have studied the properties of the AChR mutation, epsilonL269F, found in a family with SCCMS, using both in vitro and in vivo expression systems. The mutation causes a sixfold increase in the open time of AChRs expressed in vitro, similar to the phenotype of other reported mutants. Transgenic mice expressing this mutant develop a syndrome that is highly reminiscent of the SCCMS. Mice have fatigability of limb muscles, electrophysiological evidence of slow AChR ion channels, and defective neuromuscular transmission. Pathologically, the motor endplates show focal accumulation of calcium and striking ultrastructural changes, including enlargement and degeneration of the subsynaptic mitochondria and nuclei. These findings clearly demonstrate the role of this mutation in the spectrum of abnormalities associated with the SCCMS and point to the subsynaptic organelles as principal targets in this disease. These transgenic mice provide a useful model for the study of excitotoxic synaptic degeneration.

Gestational diabetes mellitus and gene mutations which affect insulin secretion

We investigated whether genetic mutations known to impair insulin secretion and glucose tolerance are operative in a group of American women with gestational diabetes mellitus. Study groups were comprised of elderly non-diabetic controls (n = 55) with normal glucose tolerance and patients with gestational diabetes (n = 50), together with one family with maturity-onset diabetes of the young (three controls and three affected). No mutations were detected in any exon of the human glucokinase gene or the mitochondrial tRNA[Leu](UUR) gene by single strand conformational analysis and direct exon sequencing. Also, chi2 analysis showed no significant association with gestational diabetes for a polymorphism at position -30 (G --> A) of the beta-cell-specific glucokinase gene promoter. We have determined that glucokinase and mitochondrial tRNA[Leu](UUR) gene mutations, which are known to impair insulin secretion are relatively uncommon and do not constitute a large component of genetic risk for gestational diabetes in the study population.

A mild phenotype of autosomal dominant retinitis pigmentosa is associated with the rhodopsin mutation Pro-267-Leu

By screening blood samples from patients with autosomal dominant retinitis pigmentosa, we found in one of the families a rhodopsin mutation (Pro-267-Leu), which segregates with the disease in two affected and five unaffected family members. Here, we present the results of the clinical evaluation of the family, including full-field electroretinography from the two affected family members. A 25-year-old family member with the mutation had an almost normal electrophysiological retinal response. The patient's father, who was also heterozygous for the mutation and had mild subjective symptoms of retinitis pigmentosa, demonstrated a substantially preserved retinal function. Our results suggest that the Pro-267-Leu rhodopsin mutation is associated with a very mild phenotype of retinitis pigmentosa. Young patients with the disease may have minimal pathological changes in the electroretinogram and some patients with few symptoms may be affected without acquiring a diagnosis of eye disease.

Point mutations in human beta cardiac myosin heavy chain have differential effects on sarcomeric structure and assembly: an ATP binding site change disrupts both thick and thin filaments, whereas hypertrophic cardiomyopathy mutations display normal assembly

Hypertrophic cardiomyopathy is a human heart disease characterized by increased ventricular mass, focal areas of fibrosis, myocyte, and myofibrillar disorganization. This genetically dominant disease can be caused by mutations in any one of several contractile proteins, including beta cardiac myosin heavy chain (beta MHC). To determine whether point mutations in human beta MHC have direct effects on interfering with filament assembly and sarcomeric structure, full-length wild-type and mutant human beta MHC cDNAs were cloned and expressed in primary cultures of neonatal rat ventricular cardiomyocytes (NRC) under conditions that promote myofibrillogenesis. A lysine to arginine change at amino acid 184 in the consensus ATP binding sequence of human beta MHC resulted in abnormal subcellular localization and disrupted both thick and thin filament structure in transfected NRC. Diffuse beta MHC K184R protein appeared to colocalize with actin throughout the myocyte, suggesting a tight interaction of these two proteins. Human beta MHC with S472V mutation assembled normally into thick filaments and did not affect sarcomeric structure. Two mutant myosins previously described as causing human hypertrophic cardiomyopathy, R249Q and R403Q, were competent to assemble into thick filaments producing myofibrils with well defined I bands, A bands, and H zones. Coexpression and detection of wild-type beta MHC and either R249Q or R403Q proteins in the same myocyte showed these proteins are equally able to assemble into the sarcomere and provided no discernible differences in subcellular localization. Thus, human beta MHC R249Q and R403Q mutant proteins were readily incorporated into NRC sarcomeres and did not disrupt myofilament formation. This study indicates that the phenotype of myofibrillar disarray seen in HCM patients which harbor either of these two mutations may not be directly due to the failure of the mutant myosin heavy chain protein to assemble and form normal sarcomeres, but may rather be a secondary effect possibly resulting from the chronic stress of decreased beta MHC function.

A new point mutation (M313T) in the thyroid hormone receptor beta gene in a patient with resistance to thyroid hormone

Sequence analysis of the TR beta gene from a patient with the syndrome of resistance to thyroid hormone revealed a novel missense mutation in exon 9, changing thymidine in position 1123 to cytosine. The corresponding amino acid alteration is a substitution of a methionine (ATG) for a threonine (ACG) at codon 313 being the patient heterozygous for the mutation. In contrast, his parents had only the wild-type sequence, suggesting a de novo mutational event.

Influence of elevated expression of rat wild-type PMP22 and its mutant PMP22Trembler on cell growth of NIH3T3 fibroblasts

The peripheral myelin gene PMP22 is the rat and human homologue of the murine growth-arrest-specific gene gas3. The biological function of PMP22 is unknown, but recent progress in the analysis of rat Schwann cells expressing altered levels of PMP22 revealed that one role of PMP22 is as a negative growth modulator. We have investigated the influence of rat PMP22 (rPMP22) and a mutant of PMP22 (rPMP22(Tr)) resembling the murine trembler mutation on cell growth of retrovirus-vector-infected mouse NIH3T3 cells. Transduced cells carrying the two different sense constructs expressed rPMP22 and rPMP22(Tr )mRNAs and proteins. Elevated levels of rPMP22 and rPMP22(Tr )significantly reduced fibroblast growth as judged by proliferation assays. Despite a negative modulatory influence of rPMP22 and rPMP22(Tr )on cell proliferation, cell cycle analyses by flow cytometry did not reveal an influence of rPMP22 or rPMP22(Tr )on the synchronous progression of resting NIH3T3 cells from G0 into S phase. However, cell cycle analyses by flow cytometry of asynchronously dividing cultures demonstrated that the expression of rPMP22 and rPMP22(Tr )increased the fraction of cells in the G1 phase of the cell cycle. Furthermore, cell death analyses revealed that, in contrast to control cells and cells carrying the rPMP22(Tr )construct, a significantly increased fraction of NIH3T3 cells expressing rPMP22 exit the proliferation compartment showing hallmarks of programmed cell death. These results indicate that (i) rPMP22 and rPMP22(Tr )act as negative modulators of proliferation in murine fibroblasts probably through extension of the G1 phase of the cell cycle and (ii) rPMP22 but not rPMP22(Tr )promotes programmed death of these cells.

Normal temporal and spatial distribution of oligodendrocyte progenitors in the myelin-deficient (md) rat

A point mutation in exon 3 of the proteolipid protein (PLP) gene of the myelin-deficient (md) rat leads to a failure of oligodendrocyte maturation and early death of oligodendrocytes, resulting in dysmyelination. It has been suggested that an alternative-splice isoform of PLP, known as DM-20, might be expressed in oligodendrocyte progenitors in the embryonic central nervous system (CNS), raising the possibility that early development of the oligodendrocyte lineage might also be affected in the md rat. To test this suggestion, we visualized oligodendrocyte progenitors in the embryonic md rat spinal cord and brain by in situ hybridization with a probe to the platelet-derived growth factor alpha receptor (PDGFR). We could detect no abnormalities in the time of first appearance of oligodendrocyte precursors, nor in their subsequent proliferation and dispersal throughout the CNS. These data strongly suggest that the PLP mutation in the md rat primarily or exclusively affects the later stages of oligodendrocyte lineage.

Resistance to thyroid hormone in a family caused by a new point mutation L330S in the thyroid receptor (TR) beta gene

Resistance to thyroid hormone (RTH) is an inherited defect manifesting as variable tissue hyporesponsiveness to thyroid hormone, usually caused by mutations in the thyroid hormone receptor beta (TR beta) gene. Up to now 78 mutations in this gene have been identified, mostly clustered in two regions located in exon 9 and 10. We describe a new point mutation replacing the normal thymidine-1274 with a cytosine that results in the substitution of the normal leucine-330 with a serine (L330S) in the receptor protein. This mutation was identified in an 11-year-old boy who presented with symptoms and signs suggestive of both hyperthyroidism and hypothyroidism. Interestingly a mutation in the same codon (L330F) has been previously described in a patient who presented with stigmata suggestive of thyrotoxicosis.

Tau phosphorylation in cells transfected with wild-type or an Alzheimer's disease mutant Presenilin 1

We have studied the effect of overexpressing either wild-type or an Alzheimer's disease mutant Presenilin 1 (PS1) on tau phosphorylation in transfected Chinese hamster ovary (CHO) and COS cells. Tau transfected into these cells is predominantly non-phosphorylated at many PHF-tau sites but co-transfection with the tau kinase glycogen synthase kinase-3 beta (GSK-3 beta) induces phosphorylation that generates epitopes for several phosphorylation-dependent antibodies. Co-transfection of tau with either wild-type or mutant PS1 did not alter tau phosphorylation as detected by five different antibodies. Likewise, co-transfection of the PS1s did not influence GSK-3 beta-mediated tau phosphorylation. The implications of these results for the pathogenesis of Alzheimer's disease are discussed.

Frequency of delta+ 27-thalassaemia in Sardinians

To determine the incidence of delta+ 27 thalassaemia in Northern Sardinia we examined blood samples from 750 Sardinian schoolboys by PCR-based molecular analysis. The incidence of delta+ 27 mutation was 1.2% in this study, i.e. twice as high as previously described on the basis of phenotypical studies; the frequency of the beta-thalassaemia is 10.5% and their interaction has been calculated at 0.0003. The majority of delta+ 27 carriers are characterized by a HbA2 level lower than 1.9% and the mean HbA2 level is significantly lower than in normal subjects. All compound heterozygotes for delta+ 27 and beta-thalassaemia show a silent beta-thalassaemic phenotype related to normalization of their HbA2 levels. This study suggests that delta+ 27 thalassaemia should be borne in mind in counselling at-risk couples in which one member has the typical high HbA2 beta-thal trait while the other shows normal or borderline HbA2 level. In these subjects, PCR-based ECO O 109 I digestion of the delta globin gene allows rapid detection of the delta+ 27 mutation.

Formation of cadaverine derivatives in Saccharomyces cerevisiae

The higher homologues of cadaverine, aminopropylcadaverine (APC) and N,N-bis(3-aminopropyl)cadaverine (3APC) were formed by a wild-type strain of Saccharomyces cerevisiae, and by two mutant strains, spe 3-1 and spe 4-1, exhibiting point mutations in the genes for spermidine synthase and spermine synthase, respectively. This, together with the incomplete inhibition of APC and 3APC formation in the presence of inhibitors of S-adenosylmethionine decarboxylase and spermidine synthase, suggests that the cadaverine derivatives are formed partly by the operation of a different route. However, the yeast strains were unable to utilise [14C]aspartate and lysine to form APC and 3APC. Since the ornithine decarboxylase inhibitor alpha-difluoromethylornithine (DFMO) greatly reduced the formation of APC and 3APC, it is suggested that these compounds are formed preferentially in these yeast strains from cadaverine formed by ODC. APC and 3APC formation in the yeast strains was increased substantially following exposure to 37 degrees C for 2 h.

Inhibition of alanine:glyoxylate aminotransferase 1 dimerization is a prerequisite for its peroxisome-to-mitochondrion mistargeting in primary hyperoxaluria type 1

Peroxisome-to-mitochondrion mistargeting of the homodimeric enzyme alanine:glyoxylate aminotransferase 1 (AGT) in the autosomal recessive disease primary hyperoxaluria type 1 (PH1) is associated with the combined presence of a normally occurring Pro(11)Leu polymorphism and a PH1-specific Gly170Arg mutation. The former leads to the formation of a novel NH2-terminal mitochondrial targeting sequence (MTS), which although sufficient to direct the import of in vitro-translated AGT into isolated mitochondria, requires the additional presence of the Gly170Arg mutation to function efficiently in whole cells. The role of this mutation in the mistargeting phenomenon has remained elusive. It does not interfere with the peroxisomal targeting or import of AGT. In the present study, we have investigated the role of the Gly170Arg mutation in AGT mistargeting. In addition, our studies have led us to examine the relationship between the oligomeric status of AGT and the peroxisomal and mitochondrial import processes. The results obtained show that in vitro-translated AGT rapidly forms dimers that do not readily exchange subunits. Although the presence of the Pro(11)Leu or Gly170Arg substitutions alone had no effect on dimerization, their combined presence abolished homodimerization in vitro. However, AGT containing both substitutions was still able to form heterodimers in vitro with either normal AGT or AGT containing either substitution alone. Expression of various combinations of normal and mutant, as well as epitope-tagged and untagged forms of AGT in whole cells showed that normal AGT rapidly dimerizes in the cytosol and is imported into peroxisomes as a dimer. This dimerization prevents mitochondrial import, even when the AGT possesses an MTS generated by the Pro(11)Leu substitution. The additional presence of the Gly170Arg substitution impairs dimerization sufficiently to allow mitochondrial import. Pharmacological inhibition of mitochondrial import allows AGT containing both substitutions to be imported into peroxisomes efficiently, showing that AGT dimerization is not a prerequisite for peroxisomal import.

Homocysteine and risk of premature coronary heart disease. Evidence for a common gene mutation

BACKGROUND

Plasma homocysteine levels are modulated by nutritional and genetic factors, among which is the enzyme methylenetetrahydrofolate reductase (MTHFR). A common defective (thermolabile) variant of this enzyme is causally associated with elevated plasma homocysteine, itself an independent risk factor for coronary heart disease.

METHODS AND RESULTS

To examine the hypothesis that the allele (T) that codes for the thermolabile defect increases the risk of coronary heart disease, we studied 111 patients with clinical and objective investigational evidence of coronary heart disease and 105 control subjects. The frequencies of the thermolabile defect (T) in patients and control subjects were measured, and the prevalence of elevated plasma total homocysteine according to genotype was assessed. The frequency of the defective allele was higher in patients than in control subjects with an OR of 1.6 (95% CI, 1.1 to 2.4; P = .02). The OR in the coronary heart disease group for the homozygous TT genotype was 2.9 (95% CI, 1.2 to 7.2; P = .02); 17% of patients and 7% of control subjects had the TT genotype. Plasma total homocysteine levels were significantly associated with disease status, a relationship that matched the strength of the association between disease and homozygous inheritance of the defective enzyme.

CONCLUSIONS

Homozygotes for the defective allele (T) are at increased risk of premature coronary heart disease. MTHFR, which modulates basal plasma homocysteine concentration, is folate dependent, and dietary supplementation or fortification with folic acid may reduce plasma homocysteine levels and consequent coronary risk in a significant proportion of the general population.

Paramyotonia congenita: genotype to phenotype correlations in two families and report of a new mutation in the sodium channel gene

Sodium channel disorders include hyperkalemic periodic paralysis (hyperPP), paramyotonia congenita (PC) and potassium-aggravated myotonia (PAM). PC is a myotonic syndrome characterized by cold-induced muscle stiffness and weakness. In this paper, we report two families. The first is affected by PC with cold-induced stiffness and no weakness, in addition to hyperPP. This family displays the Arg1448Cys mutation in the sodium channel gene originally described in pure PC families. The fact that families with the same mutation present distinct phenotypes indicates that other factors, genetic or environmental, may modulate the expression of the disease in sodium channel disorders. The second family was unusual because patients presented cold-induced weakness without stiffness. A mutation was found in the sodium channel gene that changed an isoleucine into a threonine at position 693. These two families demonstrate that sodium channel mutations may cause either cold-induced stiffness or weakness.

Pathology of skeletal muscle and impaired respiratory chain function in long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency with the G1528C mutation

Lactic acidosis and mitochondrial abnormalities have been reported in long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency. We studied muscle morphology and the respiratory chain function in ten patients with LCHAD deficiency and the G1528C mutation. In eight cases the light microscopy of muscle specimens showed fatty infiltration and fibre degeneration. The degenerated fibres appeared as ragged red fibres in four cases. Electron microscopy revealed enlarged mitochondria often with swollen appearance in four out of seven patients. The number of mitochondria had also increased. Complex I associated enzyme activities in muscle mitochondria were decreased in five out of seven patients, and in three of them Complex II or II + III associated activities were also affected. We suggest that the reason for respiratory chain dysfunction and structural changes of mitochondria is the accumulation of toxic intermediates of fatty acid beta-oxidation in mitochondria. Because these changes may confound the differential diagnostics between LCHAD deficiency and respiratory chain defects, awareness of their frequency is important.