A pilot study to estimate incidence of guanidinoacetate methyltransferase deficiency in newborns by direct sequencing of the GAMT gene

BACKGROUND

GAMT deficiency is an autosomal recessive disorder of creatine biosynthesis causing developmental delays or intellectual disability in untreated patients as a result of irreversible brain damage occurring prior to diagnosis. Normal neurodevelopmental outcome has been reported in patients treated from neonatal period highlighting the importance of early treatment.

METHODS

Five hundred anonymized newborns from the National Newborn Screening Program of The Netherlands were included into this pilot study. Direct sequencing of the coding region of the GAMT gene was applied following DNA extraction. The disease causing nature of novel missense variants in the GAMT gene was studied by overexpression studies. GAA and creatine was measured in blood dot spots.

RESULTS

We detected two carriers, one with a known common (c.327G>A) and one with a novel mutation (c.297_309dup (p.Arg105Glyfs*) in the GAMT gene. The estimated incidence of GAMT deficiency was 1:250,000. We also detected five novel missense variants. Overexpression of these variants in GAMT deficient fibroblasts did restore GAMT activity and thus all were considered rare, but not disease causing variants including the c.131G>T (p.Arg44Leu) variant. Interestingly, this variant was predicted to be pathogenic by in silico analysis. The variants were included in the Leiden Open Variation Database (LOVD) database (www.LOVD.nl/GAMT). The average GAA level was 1.14μmol/L±0.45 standard deviations. The average creatine level was 408μmol/L±106. The average GAA/creatine ratio was 2.94±0.136.

CONCLUSION

The estimated incidence of GAMT deficiency is 1:250,000 newborns based on our pilot study. The newborn screening for GAMT deficiency should be implemented to identify patients at the asymptomatic stage to achieve normal neurodevelopmental outcome for this treatable neurometabolic disease. Biochemical investigations including GAA, creatine and GAMT enzyme activity measurements are essential to confirm the diagnosis of GAMT deficiency. According to availability, all missense variants can be assessed functionally, as in silico prediction analysis of missense variants is not sufficient to confirm the pathogenicity of missense variants.

N-acetylaspartylglutamate in CNS hypomyelination

CSF N-acetylaspartylglutamate (NAAG) has been found to be elevated in some hypomyelinating disorders. This study addressed the question whether it could be used as a marker for hypomyelination and as a means to distinguish between hypomyelinating disorders biochemically. We have measured CSF NAAG in a cohort of 28 patients with hypomyelination with known and unknown aetiology. NAAG was found to be elevated in 7 patients, but was normal in the majority, including patients with defined hypomyelinating disorders. CSF NAAG is not a universal marker of hypomyelination, and the mechanism of its elevation remains poorly understood.

A non-radioactive sensitive assay to measure 5-fluorouracil incorporation into DNA of solid tumors

A non-radioactive method to determine 5-Fluorouracil (5FU) incorporation into DNA has been developed. Isolated DNA was enzymatically degraded to bases and the resulting 5FU was measured with standard gas-chromatography coupled to mass spectrometry (GC-MS) and compared with that of radioactive 5FU in a cell line. Incorporation into DNA of the murine Colon 26-B tumor treated with maximal tolerated doses of 5FU and fluorodeoxyuridine (FUdR) was maximal after 2 hour and was 15.4 and 71.0 fmol/microg DNA, respectively. After a plateau for about 3 days a decrease was observed to +/- 2 fmol/microg DNA after 10 days. The assay is very sensitive and reproducible and can be used in a clinical setting.

5-Fluorouracil incorporation into RNA and DNA in relation to thymidylate synthase inhibition of human colorectal cancers

BACKGROUND

The mechanism of action of 5-fluorouracil (5-FU) has been associated with inhibition of thymidylate synthase (TS) and incorporation of 5-FU into RNA and DNA, but limited data are available in human tumor tissue for the latter. We therefore measured incorporation in human tumor biopsy specimens after administration of a test dose of 5-FU alone or with leucovorin.

PATIENTS AND METHODS

Patients received 5-FU (500 mg/m(2)) with or without high-dose leucovorin, low-dose leucovorin or l-leucovorin, and biopsy specimens were taken after approximately 2, 24 or 48 h. Tissues were pulverized and extracted for nucleic acids. 5-FU incorporation was measured using gas chromatography/mass spectrometry after complete degradation to bases of isolated RNA and DNA.

RESULTS

Maximal incorporation into RNA (1.0 pmol/micrograms RNA) and DNA (127 fmol/micrograms DNA) of 59 and 46 biopsy specimens, respectively, was found at 24 h after 5-FU administration. Incorporation into RNA but not DNA was significantly correlated with intratumoral 5-FU levels. However, DNA incorporation was significantly correlated with the RNA incorporation. Primary tumor tissue, liver metastasis and normal mucosa did not show significant differences, while leucovorin had no effect. Neither for RNA (30 patients) nor DNA (24 patients) incorporation was a significant correlation with response to 5-FU therapy found. However, in the same group of patients, response was significantly correlated to TS inhibition (mean TS in responding and non-responding groups 45 and 231 pmol/h/mg protein, respectively; P=0.001).

CONCLUSIONS

5-FU is incorporated at detectable levels into RNA and DNA of human tumor tissue, but no relation between the efficacy of 5-FU treatment and incorporation was found, in contrast to TS.

Pharmacokinetics of S-1, an oral formulation of ftorafur, oxonic acid and 5-chloro-2,4-dihydroxypyridine (molar ratio 1:0.4:1) in patients with solid tumors

S-1 is an oral formulation of ftorafur (FT), oxonic acid and 5-chloro-2,4-dihydroxypyridine (CDHP) at a molar ratio of 1:0.4:1. FT is a 5-fluorouracil (5-FU) prodrug, CDHP is a dihydropyrimidine dehydrogenase (DPD) inhibitor and oxonic acid is an inhibitor of 5-FU phosphoribosylation in the gastrointestinal mucosa and was included to prevent gastrointestinal toxicity. We determined the pharmacokinetics of S-1 in 28 patients at doses of 25, 35, 40 and 45 mg/m(2). The plasma C(max) values of FT, 5-FU, oxonic acid and CDHP increased dose-dependently and after 1-2 h were in the ranges 5.8-13 microM, 0.4-2.4 microM, 0.026-1.337 microM, and 1.1-3.6 microM, respectively. Uracil levels, indicative of DPD inhibition, also increased dose-dependently from basal levels of 0.03-0.25 microM to 3.6-9.4 microM after 2-4 h, and 0.09-0.9 microM was still present after 24 h. The pharmacokinetics of CDHP and uracil were linear over the dose range. The areas under the plasma concentration curves (AUC) for CDHP and uracil were in the ranges 418-1735 and 2281-8627 micromol x min/l, respectively. The t(1/2) values were in the ranges 213-692 and 216-354 min, respectively. Cumulative urinary excretion of FT was predominantly as 5-FU and was 2.2-11.9%; the urinary excretion of both fluoro-beta-alanine and uracil was generally maximal between 6 and 18 h. During 28-day courses with twice-daily S-1 administration, 5-FU and uracil generally increased. Before each intake of S-1, 5-FU varied between 0.5 and 1 microM and uracil was in the micromolar range (up to 7 microM), indicating that effective DPD inhibition was maintained during the course. In a biopsy of an esophageal adenocarcinoma metastasis that had regressed, thymidylate synthase, the target of 5-FU, was inhibited 50%, but increased four- to tenfold after relapse in subsequent biopsies. In conclusion, oral S-1 administration resulted in prolonged exposure to micromolar 5-FU concentrations due to DPD inhibition, and the decrease in uracil levels after 6 h followed the pattern of CDHP and indicates reversible DPD inhibition.

Quantitative analysis of urinary acylglycines for the diagnosis of beta-oxidation defects using GC-NCI-MS

The analysis of acylglycines is an important biochemical tool for the diagnosis of inherited disorders of mitochondrial fatty acid beta-oxidation. A stable isotope dilution gas chromatography negative chemical ionisation mass spectrometry method for the quantitative analysis of short- and medium-chain acylglycines as their bis(trifluoromethyl)benzyl (BTFMB) ester derivatives is described. The diagnostic usefulness of the method was demonstrated in nine patients with medium-chain acyl-coenzyme A (CoA) dehydrogenase (MCAD) deficiency, and seven patients with multiple acyl-CoA dehydrogenation defect (MAD). The urinary acylglycine profiles in these patients were compared to those in controls (n = 19), children on a medium-chain triglyceride (MCT) supplemented diet (n = 4), and patients with various other diseases (n = 5).

Simultaneous analysis of plasma free fatty acids and their 3-hydroxy analogs in fatty acid beta-oxidation disorders

We present a new derivatization procedure for the simultaneous gas chromatographic-mass spectrometric analysis of free fatty acids and 3-hydroxyfatty acids in plasma. Derivatization of target compounds involved trifluoroacetylation of hydroxyl groups and tert-butyldimethylsilylation of the carboxyl groups. This new derivatization procedure had the advantage of allowing the complete baseline separation of free fatty acids and 3-hydroxyfatty acids while the superior gas chromatographic and mass spectrometric properties of tert-butyldimethylsilyl derivatives remained unchanged, permitting a sensitive analysis of the target compounds. Thirty-nine plasma samples from control subjects and patients with known defects of mitochondrial fatty acid beta-oxidation were analyzed. A characteristic increase of long-chain 3-hydroxyfatty acids was observed for all of the long-chain 3-hydroxyacyl-CoA dehydrogenase-deficient and mitochondrial trifunctional protein-deficient plasma samples. For medium-chain acyl-CoA dehydrogenase deficiency and very-long-chain acyl-CoA dehydrogenase deficiency, decenoic and tetradecenoic acids, respectively, were the main abnormal fatty acids, whereas the multiple acyl-CoA dehydrogenase-deficient patients showed variable increases of these unusual intermediates. The results showed that this selective and sensitive method is a powerful tool in the diagnosis and monitoring of mitochondrial fatty acid beta-oxidation disorders.

Quantification of 5-fluorouracil incorporation into RNA of human and murine tumors as measured with a sensitive gas chromatography-mass spectrometry assay

5-Fluorouracil (5FU) can exert its cytotoxic activity by either inhibition of thymidylate synthase or incorporation into RNA. The extent and importance of the latter in tumors of patients are not clear, due to the lack of sensitive and reproducible methods. RNA from 5FU-treated human WiDr colon tumor cells was isolated and [(14)CL]5FU incorporation into RNA was measured by traditional scintillation counting while that of nonradiolabeled 5FU was measured with the present, new method. For the latter purpose, isolated RNA was incubated with RNase, alkaline phosphatase, and uridine phosphorylase, resulting in a complete degradation of RNA, nucleotides, and nucleoside to 5FU. 5FU was then measured with gas chromatography coupled to mass spectrometry. For both methods RNA incorporation was 0.4 pmol/h/micrograms RNA at 25 microM 5FU while a similar time (up to 4 h) and concentration dependence (25 to 50 microM) were found. Reproducibility of the assay was more than 95%. In a murine colon tumor 5FU incorporation into RNA reached a peak of 10 pmol/micron RNA at 2 h after administration of the the maximum tolerated dose of 80 mg5FU/kg, which was retained until at least 72 h at 2.5 pmol/micron. In tumors from patients treated with 500 mg5FU/m(2) incorporation into RNA after 24 h amounted to 1.0-1.5 pmol/micrograms RNA. In conclusion, a novel approach, combining different sensitive and reproducible techniques, was established to measure 5FU incorporation into RNA in clinical tumor specimens enabling determination of its clinical relevance.

The reversal of cisplatin-protein interactions by the modulating agent WR2721 and its metabolites WR1065 and WR33278

The reversibility of cisplatin-protein interactions by the modulating agent WR2721, its active thiol-metabolite WR1065, and the symmetrical disulfide WR33278 was studied using the model compounds (Pt(diethylenetriammine) monofunctionally bound to the sulfur in glutathione (Pt(dien)SG) and Pt(diethylenetriammine) monofunctionally bound to the sulfur in S-methylglutathione (Pt(dien)SMeG). Both model compounds could be quantified by high-performance liquid chromatography (HPLC) with UV detection. The Pt-cysteine-like bond in Pt(dien)SG could not be reversed by any of the WR compounds or by the strong nucleophiles thiosulfate (TS) and diethyldithiocarbamate (DDTC). However, the Pt-methionine-like bond in Pt(dien)SMeG could be reversed by WR1065, although the reversal was slow (k2 = 0.142 M-1 s-1) as compared with that obtained using the modulating agents TS (k2 = 10.1 M-1 s-1) and DDTC (k2 = 3.66 M-1 s-1). WR2721 was hardly able to reverse the Pt-S bond in Pt(dien)SMeG (k2 = 0.00529 M-1 s-1), and WR33278 showed no capacity to do so. The activity of cis-diamminedichloroplatinum(II) (CDDP)-inactivated fumarase was not appreciably restored by any of the WR compounds (16%, 7.7%, and 0 for 20 mM WR1065, WR2721, and WR33278, respectively) in contrast to the strong nucleophile DDTC (61% for 2 mM DDTC). These in vitro studies provide information at the molecular level that may explain why WR2721, in contrast to DDTC, does not provide protection against cisplatin-induced nephrotoxicity when it is given after platinum-containing chemotherapy. The results support the present clinical use of WR2721 prior to the administration of platinum compounds.

Time dependence of the selective modulation of cisplatin-induced nephrotoxicity by WR2721 in the mouse

2-(3-Aminopropylamino)ethylphosphorothioic acid (WR2721; ethiofos) was shown to selectively protect nontumor tissues from cis-diamminedichloroplatinum(II) (cisplatin)-induced toxicity, when administered 30 min prior to the platinum drug. Selectivity of protection by WR2721 is probably due to the preferential formation and uptake of the thiol metabolite 2-(3-aminopropylamino)ethanethiol (WR1065), which can inactivate toxic platinum-species inside the cell. We investigated the protective potential of WR2721, when administered at different time points relative to cisplatin. BALB/c mice treated with WR2721 (200 mg/kg i.p.) either 30 min or 5 min prior to cisplatin (i.p.) allowed a 2.2-fold increase in cisplatin dose to 19 mg/kg before the occurrence of nephrotoxicity as expressed by an increase in plasma urea. A small part of the protection could be ascribed to the mannitol (200 mg/kg), present in the formulated WR2721. WR2721 (200 mg/kg) 30 min after 14.5-16-mg/kg cisplatin did not offer any protection against the rise in plasma urea. WR2721 (200 mg/kg) 5 min before 19-mg/kg cisplatin did not cause liver toxicity (increase in serum glutamic pyruvic transaminase or serum glutamic oxaloacetic transaminase). Furthermore, WR2721 (200 mg/kg) 5 min prior to cisplatin did not reduce antitumor activity in nude mice bearing well-established human ovarian cancer xenografts. Under protection of WR2721, the dose of cisplatin could be increased by a factor of 1.6 to 8 mg/kg (administered twice weekly), resulting in an increased antitumor activity.

The chemical reactivity of the modulating agent WR2721 (ethiofos) and its main metabolites with the antitumor agents cisplatin and carboplatin

The antitumor agents cisplatin [cis-diamminedichloroplatinum(II), CDDP] and carboplatin [cis-diammine(1,1-cyclobutanedicarboxylato)platinum(II), CBDCA] can react with a nucleophilic agent by a direct ligand exchange of the (labile) anionic ligands or through hydrolysis of these ligands followed by a fast reaction of the hydration product with the nucleophile. At pH 7.4 and 37 degrees, CDDP and CBDCA were incubated with several molar excesses of the modulating agent WR2721, its active thiol metabolite WR1065 or the symmetrical disulphide WR33278. The reaction rate constants for the hydrolysis and the direct inactivation by the WR-compounds were obtained from the pseudo first-order disappearance of the intact Pt-drug, with or without the WR-compounds at molar ratios of 50, 100 and 200. The hydrolysis of carboplatin (kaq,CBDCA = 2 x 10(-8) M-1 sec-1) was 100-fold less rapid than that of cisplatin (kaq,CDDP = 2 x 10(-6) M-1 sec-1). However, direct inactivation by WR2721, WR1065 and WR33278 was only 4-, 4- and 22-fold less rapid for carboplatin than for cisplatin, respectively. This direct inactivation was slow compared to the strong nucleophiles thiosulphate (TS) and diethyldithiocarbamate (DDTC) and decreased for both Pt-drugs in the following order: WR1065 (kWR1065/CDDP = 49.1 x 10(-4) M-1 sec-1, kWR1065/CBDCA = 12.4 x 10(-4) M-1 sec-1) greater than WR2721 (kWR2721/CDDP = 25.3 x 10(-4) M-1 sec-1, kWR2721/CBDCA = 6.07 x 10(-4) M-1 sec-1) greater than WR33278 (kWR33278/CDDP = 8.60 x 10(-4) M-1 sec-1, kWR33278/CBDCA = 0.39 x 10(-4) M-1 sec-1. Thus for CDDP, the hydrolysis-mediated interaction with the WR-compounds contributed more to the disappearance of intact platinum antitumor agent than it did for CBDCA. Considering the relatively low reactivity of WR2721 and its main metabolites with the platinum antitumor agents, in addition to their pharmacokinetic behavior, a significant inactivation of the platinum antitumor drugs by WR2721 and its main metabolites is, in contrast to TS, not expected in the circulation.