Novel mutations affecting LRP5 splicing in patients with osteoporosis-pseudoglioma syndrome (OPPG).

Osteoporosis-pseudoglioma sydrome (OPPG) is an autosomal recessive disorder with early-onset severe osteoporosis and blindness, caused by biallelic loss-of-function mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene. Heterozygous carriers exhibit a milder bone phenotype. Only a few splice mutations in LRP5 have been published. We present clinical and genetic data for four patients with novel LRP5 mutations, three of which affect splicing. Patients were evaluated clinically and by radiography and bone densitometry. Genetic screening of LRP5 was performed on the basis of the clinical diagnosis of OPPG. Splice aberrances were confirmed by cDNA sequencing or exon trapping. The effect of one splice mutation on LRP5 protein function was studied. A novel splice-site mutation c.1584+4A>T abolished the donor splice site of exon 7 and activated a cryptic splice site, which led to an in-frame insertion of 21 amino acids (p.E528_V529ins21). Functional studies revealed severely impaired signal transduction presumably caused by defective intracellular transport of the mutated receptor. Exon trapping was used on two samples to confirm that splice-site mutations c.4112-2A>G and c.1015+1G>T caused splicing-out of exons 20 and 5, respectively. One patient carried a homozygous deletion of exon 4 causing the loss of exons 4 and 5, as demonstrated by cDNA analysis. Our results broaden the spectrum of mutations in LRP5 and provide the first functional data on splice aberrations.

A novel Refsum-like disorder that maps to chromosome 20.

OBJECTIVE: Clinical and genetic characterization of a neurologic disorder resembling Refsum disease in a Norwegian consanguineous family. METHODS: The affected individuals comprise a brother and sister and their third cousin. The family comes from a small island community and genealogic studies showed that both sets of parents are descendants of a man born in 1585. Based on the hypothesis that this is an autosomal recessive disease and that the patients were homozygous for the same mutation (identical by descent), we used homozygosity mapping to define the genetic locus of this disorder. RESULTS: This slowly progressive disorder starts in childhood with signs of peripheral neuropathy (pes cavus, tendoachilles contracture). Hearing loss and cataract become evident in the third decade. Subsequently, patients develop a disorder of gait due to the combination of ataxia and spasticity, and a pigment retinopathy. While the clinical picture is reminiscent of Refsum disease, affected individuals have normal phytanic and pristanic acid levels in plasma, as well as normal enzymatic activity for alpha-oxidation. We mapped the disease to a 15.96 Mb region on chromosome 20 (20p11.21-q12), containing approximately 200 genes (maximum lod score = 6.3). Sequencing of 23 candidate genes failed to demonstrate detrimental sequence variants. CONCLUSIONS: Our findings show that the clinical syndromes that include Refsum disease are more heterogeneous than previously recognized. We have chosen to report the clinical features and mapping of this novel disorder in the hope that this will permit identification of other families and thus proper genetic characterization.

Co-ordinate variations in methylmalonyl-CoA mutase and methionine synthase, and the cobalamin cofactors in human glioma cells during nitrous oxide exposure and the subsequent recovery phase.

We investigated the co-ordinate variations of the two cobalamin (Cbl)-dependent enzymes, methionine synthase (MS) and methylmalonyl-CoA mutase (MCM), and measured the levels of their respective cofactors, methylcobalamin (CH3Cbl) and adenosylcobalamin (AdoCbl) in cultured human glioma cells during nitrous oxide exposure and during a subsequent recovery period of culture in a nitrous oxide-free atmosphere (air). In agreement with published data, MS as the primary target of nitrous oxide was inactivated rapidly (initial rate of 0.06 h(-1)), followed by reduction of CH3Cbl (to <20%). Both enzyme activity and cofactor levels recovered rapidly when the cells were subsequently cultured in air, but the recovery was completely blocked by the protein-synthesis inhibitor, cycloheximide. During MS inactivation, there was a reduction of cellular AdoCbl and holo-MCM activity (measured in the absence of exogenous AdoCbl) to about 50% of pre-treatment levels. When the cells were transferred to air, both AdoCbl and holo-MCM activity recovered, albeit more slowly than the MS system. Notably, the regain of the holo-MCM and AdoCbl was enhanced rather than inhibited by cycloheximide. These findings confirm irreversible damage of MS by nitrous oxide; hence, synthesis of the enzyme is required to restore its activity. In contrast, restoration of holo-MCM activity is only dependent on repletion of the AdoCbl cofactor. We also observed a synchronous fluctuation in AdoCbl and the much larger hydroxycobalamin pool during the inactivation and recovery phase, suggesting that the loss and repletion of AdoCbl reflect changes in intracellular Cbl homoeostasis. Our data demonstrate that the nitrous oxide-induced changes in MS and CH3Cbl are associated with reversible changes in both MCM holoactivity and the AdoCbl level, suggesting co-ordinate distribution of Cbl cofactors during depletion and repletion.

Disruption of a regulatory system involving cobalamin distribution and function in a methionine-dependent human glioma cell line.

Cobalamin metabolism and function were investigated at the levels from transcobalamin II (TCII) receptor to the cobalamin-dependent enzymes, methionine synthase and methylmalonyl-CoA mutase, in a methionine-dependent (P60) and a methionine-independent (P60H) glioma cell line. Using P60H as reference, the P60 cells cultured in a methionine medium had slightly lower TCII receptor activity and normal total cobalamin content, a moderately reduced microsomal and mitochondrial cobalamin(III) reductase activity but only trace amounts of the methylcobalamin and adenosylcobalamin cofactors. When transferred to a homocysteine medium without methionine, P60H cells showed a slightly enhanced TCII receptor activity, but the other cobalamin-related functions were essentially unchanged. In contrast, the methionine-dependent P60 cells responded to homocysteine medium with a nearly 6-fold enhancement of TCII receptor expression and a doubling of both the hydroxycobalamin content and the microsomal reductase activity. The mitochondrial reductase and the cobalamin-related processes further down the pathway did not change markedly. In both cell lines, TCII receptor activity was further increased when growth in homocysteine medium was combined with N2O exposure. These data suggest that low methionine and/or high homocysteine exert a positive feedback control on TCII receptor activity. The concurrent increase in hydroxycobalamin content and in microsomal reductase activity are either subjected to similar regulation or secondary to increased cobalamin transport. This regulatory network is most prominent in the methionine-dependent P60 cells harboring a disruption of the network in the proximity of cobalamin(III) reductase.

Hyperhomocysteinemia in terms of steady-state kinetics.

The plasma level of homocysteine (Hcy) and its oxidized products, i.e., plasma total Hcy (tHcy), is a function of the influx rate of Hcy to plasma and the plasma tHcy clearance. In vitro experiments show that proliferating cells usually export more Hcy than stationary cells and that the Hcy export increases in response to high methionine, low folate or low cobalamin level, and to agents interfering with Hcy remethylation. Comparison between various cell types suggests that hepatocytes have a unique ability to increase the Hcy export in response to extracellular methionine, probably due to its capacity to form adenosylmethionine. Some but not all cell types have an ability to use extracellular Hcy as a methionine source. Clearance studies in healthy subjects indicate that about 1.2 mmol Hcy is supplied from the cells to plasma per 24 h, which is only about 5-10% of total Hcy formed. Comparison of area under the curves after administration of Hcy and methionine shows that about 10% of the methionine administered is released to plasma as Hcy. Notably, only a few percent of Hcy from plasma is excreted unchanged in the urine, and this shows that most tHcy in plasma is metabolized. Folate or cobalamin deficient patients have normal plasma tHcy clearance, which suggests that their elevated tHcy level is due to increased Hcy export from tissues into the plasma compartment. In contrast, the hyperhomocysteinemia in renal failure is accounted for by a marked reduction in tHcy clearance, suggesting an important role of kidney in elimination of Hcy from plasma.

Folate depletion induced by methotrexate affects methionine synthase activity and its susceptibility to inactivation by nitrous oxide.

We compared the effects of methotrexate (MTX) and nitrous oxide on the methionine (Met) synthase system in two variants of a human glioma cell line. The cells were protected from cytotoxic effect of MTX by adding thymidine and hypoxanthine to the cell culture medium. MTX (0-1 microM) was associated with a dose- and time-dependent reduction in 5-methyltetrahydrofolate (5-methyl-THF) in both cell lines. Already after 3 hr of exposure, 5-methyl-THF was reduced by 50% and after additional 48 hr, the level was undetectable. In addition to reduction in folate level, homocysteine (Hcy) remethylation in intact cells was markedly inhibited as judged by an increased export of Hcy from the cells, and Met synthase activity in cell extracts and level of cellular methylcobalamin (CH3Cbl) declined. MTX reduced Hcy remethylation and CH3Cbl level more efficiently than nitrous oxide. In both cell variants, the inactivation of Met synthase by nitrous oxide was almost completely prevented in cells pre-exposed to MTX. This indicates that there is no catalytic turnover in cells exposed to MTX, and emphasizes the importance of the sequence of administration for synergistic effect of this drug combination. In conclusion, our data show that MTX through depletion of 5-methyl-THF reduces both the Met synthase activity and the cellular CH3Cbl level. Moreover, the effect of MTX on the Hcy remethylation is more pronounced than the inhibition caused by nitrous oxide. These observations should be taken into account in studies on MTX pharmacodynamics.

Response of the methionine synthase system to short-term culture with homocysteine and nitrous oxide and its relation to methionine dependence.

We compared the metabolic response of a methionine(Met)-dependent (P60) human glioma cell line with that of a Met-independent variant (P60H) when cultured in a homocysteine (Hcy) medium and exposed to N2O. In Hcy medium (without Met), remethylation of Hcy in P60H cells was enhanced and supported growth, whereas remethylation was low in P60 cells, which failed to thrive under these conditions. Both cell types seemed to contain adequate amounts of folates and total cobalamin (Cbl). P60 cells showed increased total and methylcobalamin (CH3Cbl) content after the shift to a Hcy medium, but the high, stable level of CH3Cbl detected in P60H cells was not attained. Further metabolic differences were induced by N2O exposure, which markedly reduced Met-synthase activity in cell-free extracts in both cell lines and completely blocked intact-cell Hcy remethylation in P60, whereas Hcy remethylation was only partly inhibited in P60H cells cultured in Met medium. The residual Hcy remethylation in P60H cells may be related to only a moderate depletion of CH3Cbl. The resulting high CH3Cbl level relative to Met-synthase activity during N2O exposure was even higher in Hcy medium. These findings in P60H cells probably reflect increased provision of Cbl to support Hcy remethylation under metabolic strain. The inability of P60 to furnish CH3Cbl to the enzyme may explain both the Met-dependent phenotype and the increased sensitivity of Hcy remethylation to N2O exposure in these cells.

Assessment of homocysteine status.

Plasma total homocysteine (tHcy) determination is used in the diagnosis of homocystinuria, in cobalamin and folate deficiency and in cardiovascular risk assessment. However, determination of tHcy includes many pitfalls which complicate the assessment of homocysteine status. In the present article, we review basic knowledge for a rational use of plasma tHcy in diagnostic as well as scientific work. The subjects dealt with are procedures for sample handling and processing, the principles of tHcy analyses, and genetic and acquired determinants of the plasma tHcy concentration.

Plasma concentrations of homocysteine and other aminothiol compounds are related to food intake in healthy human subjects.

We investigated total, free and protein-bound plasma homocysteine, cysteine and cysteinylglycine in 13 subjects aged 24-29 y after a breakfast at 0900 h containing 15-18 g of protein and a dinner at 1500 h containing approximately 50 g of protein. Twelve subjects had normal fasting homocysteine (mean +/- SD, 7.6 +/- 1.1 mumol/L) and methionine concentrations (22.7 +/- 3.5 mumol/L) and were included in the statistical analyses. Breakfast caused a small but significant increase in plasma methionine (22.2 +/- 20.6%) and a brief, nonsignificant increase followed by a significant decline in free homocysteine. However, changes in total and bound homocysteine were small. After dinner, there was a marked increase in plasma methionine by 16.7 +/- 8.9 mumol/L (87.9 +/- 49%), which was associated with a rapid and marked increase in free homocysteine (33.7 +/- 19.6%, 4 h after dinner) and a moderate and slow increase in total (13.5 +/- 7.5%, 8 h) and protein-bound (12.6 +/- 9.4%, 8 h) homocysteine. After both meals, cysteine and cysteinylglycine concentrations seemed related to changes in homocysteine, because there were parallel fluctuations in the free:bound ratios of all three thiols. Dietary changes in plasma homocysteine will probably not affect the evaluation of vitamin deficiency states associated with moderate to severe hyperhomocysteinemia but may be of concern in the risk assessment of cardiovascular disease in patients with mild hyperhomocysteinemia. Synchronous fluctuations in the free:bound ratio of the plasma aminothiol compounds indicate that biological effects of homocysteine may be difficult to separate from effects due to associated changes in other aminothiol compounds.

Development and reversion of methionine dependence in a human glioma cell line: relation to homocysteine remethylation and cobalamin status.

We investigated the biochemical changes which accompanied the development and reversion of methionine dependence in a human glioma cell line GaMg. This cell line attained a higher proliferation rate and more malignant morphology with increasing passages in vitro. Early passages (P10, P25, and P45) were able to grow in a methionine-deficient medium supplemented with homocysteine (Met-Hcy+), while a later passage (P60) had lost this ability, i.e., it had become methionine-dependent. From P60 cells, a methionine-independent revertant (P60R) was established by exposing the cells to 5-aza-2-deoxycytidine, followed by culture in a Met-Hcy+ medium. In these genetically related cell lines, we investigated homocysteine remethylation and the functional state of cobalamin-dependent methionine synthase, the enzyme responsible for remethylation of homocysteine to methionine. The methionine synthase activity in cell extracts was similar in all cell sublines. Intact cell methionine biosynthesis and nitrous oxide-dependent homocysteine export reflect homocysteine remethylation in cells cultured in a Met-Hcy+ and methionine-containing (Met+Hcy-) medium, respectively. Both of these parameters, as well as the cellular content of the substrate 5-methyltetrahydrofolate, and the cofactor methylcobalamin, in addition to adenosylcobalamin, were high in P10, declined progressively in P45 and P60, and were restored in P60R. P25 cells had some unique features among the methionine-independent phenotypes because both homocysteine remethylation and the level of 5-methyltetrahydrofolate were low in Met+Hcy- medium. The maximal homocysteine export rate in the presence of nitrous oxide, which reflects the overall transmethylation rate, was high in P60 and even higher in P60R compared to the lower passages. The basis for development of methionine dependence during culture of this glioma cell line seems related to the combined effects of reduced methionine biosynthesis and an increased overall transmethylation rate. The single parameter which most closely correlated to the ability to use homocysteine for growth was methylcobalamin. These data support a model for methionine dependence, which implies impaired provision of cobalamin to methionine synthase.

Kinetics of plasma homocysteine in healthy subjects after peroral homocysteine loading.

The kinetics of plasma homocysteine were determined in 13 healthy subjects after peroral administration and in one person after intravenous injection. Various forms of homocysteine completely dissolved in an aqueous solution were rapidly absorbed after peroral administration, and the bioavailability was estimated to be 0.53. The volume of distribution was 0.66 L/kg. The area under the plasma concentration curve (AUC0-48 h) was proportional to the administered dose (33.5-134 mumol/kg body wt), and showed small interindividual variations. Plasma homocysteine showed first-order elimination kinetics for at least 6 h. The half-life (t1/2) was 223 +/- 45 min, and there was a significant correlation between t1/2 values determined on two different occasions in the same individual. The transient hyperhomocysteinemia was associated with an increase in plasma methionine, which probably reflects intracellular remethylation of homocysteine. Less than 2% of the administered homocysteine dose was recovered in the urine. These findings may form the basis for future studies on the regulation of plasma homocysteine in health and disease, and should motivate the evaluation of a homocysteine loading test as a diagnostic tool.

Redox status and protein binding of plasma aminothiols during the transient hyperhomocysteinemia that follows homocysteine administration.

We administered reduced L-homocysteine perorally (67 mumol/kg of body wt) to 12 healthy subjects and injected the same dose into one person, and determined the kinetics of the alterations in reduced, oxidized, and protein-bound concentrations of homocysteine, cysteine, and cysteinylglycine. After oral intake, reduced homocysteine increased rapidly (tmax < or = 15 min), reaching concentrations [3.97 (SD 2.99) mumol/L] 20-fold above fasting values, and then declined towards the normal concentration within 2 h. There was a similar increase in reduced cysteine and a moderate increase in reduced cysteinylglycine. During this response, we observed a positive correlation between the reduced/total ratio for homocysteine and cysteine. When homocysteine was injected, the increase in reduced homocysteine preceded the increase in reduced cysteine by about 3 min. After oral loading, oxidized homocysteine showed a transient increase (tmax = 30 min) that lagged behind the increase of reduced homocysteine. Oxidized cysteine and cysteinylglycine were stable or decreased slightly. Protein-bound homocysteine increased the least rapidly after homocysteine administration (tmax = 1-2 h), and returned to normal values slowly. Changes in protein-bound homocysteine essentially mirrored a concurrent decrease in protein-bound cysteine, suggesting displacement of bound cysteine. These data show that plasma homocysteine has a pronounced, direct effect on the redox status and protein binding of other plasma thiol components. Such effects should be recognized when studying the mechanisms behind the atherogenic effect of increased plasma homocysteine.

Proliferation, migration and invasion of human glioma cells exposed to antifolate drugs.

The present study describes the effects of 2 folate antagonists, methotrexate (MTX) and the lipophilic antifolate trimetrexate (TMX) on 2 permanent human glioma cell lines (GaMg and D-54Mg) grown as monolayers and as multicellular tumor spheroids. In addition, the effects of drug exposure on tumor cell invasion was studied using a three-dimensional organ co-culture system. In monolayer cultures, TMX was a more potent inhibitor of cell growth than MTX, especially towards the GaMg cell line. The 2 drugs, however, showed similar cytotoxicity as assessed by the plating efficiency assay. Reduced ability of directional migration of cells on a plastic surface was seen by either antifolate usually at concentrations to 10-fold higher than those exerting a cytotoxic effect in the plating efficiency assay. TMX was somewhat more potent than MTX as an inhibitor of spheroid growth. When tumor spheroids were exposed to MTX or TMX at concentrations that caused 65 to 70% inhibition of cell migration, there was a latent period of 4 to 5 days before inhibition of spheroid growth ensued. Invasion was investigated in a co-culture system, where tumor spheroids were confronted with fetal rat brain cell aggregates. Neither drug reduced tumor cell invasion, although histological examination revealed toxic effects both in GaMg and in D-54Mg spheroids. We conclude that spheroids from human glioma cells were less sensitive to the antifolates than monolayers. For both drugs a latency period was observed before inhibition of spheroid growth. The spheroids retained their ability to invade normal brain tissue when exposed to levels of folate antagonists inhibiting spheroid growth.

Homocysteine and other thiols in plasma and urine: automated determination and sample stability.

We have developed a modified version of our fully automated column-switching HPLC method for determining total plasma homocysteine based on single-column (reversed-phase) separation. Homocysteine, cysteine, and cysteinylglycine in plasma (total concentrations), acid-precipitated plasma (non-protein-bound concentrations), and urine can be determined. The derivatization and chromatography were performed automatically by a sample processor. The successful separation of all thiol species (within 15 min) was accomplished by accurate adjustment of the pH of the mobile phase to 3.65 (plasma) or 3.50 (acid-precipitated plasma, urine). Maximal fluorescence yield of cysteine, cysteinylglycine, and, to a lesser degree, homocysteine was dependent on optimal concentrations of EDTA and dithioerythritol during reduction (with NaBH4) and derivatization (with monobromobimane). The method is sensitive (detection limit approximately 0.05 pmol) and has a high degree of precision (CV < 5%). The sample output is approximately 70 samples in 24 h. Serum and heparin plasma can also be analyzed. Hemolysis up to approximately 2.0 g/L of hemoglobin did not interfere with the analytical recovery of homocysteine or cysteine. Collection of blood, separation of plasma from whole blood, and acid precipitation must be standardized to obtain reproducible thiol results. Our modifications and the standardization of blood-sampling procedures have substantially improved the method and broadened its applications.