Familial occurrence of formiminotransferase deficiency of syndrome

Allelic spectrum of formiminotransferase-cyclodeaminase gene variants in individuals with formiminoglutamic aciduria

Background

Elevated plasma and urine formiminoglutamic acid (FIGLU) levels are commonly indicative of formiminoglutamic aciduria (OMIM #229100), a poorly understood autosomal recessive disorder of histidine and folate metabolism, resulting from formiminotransferase‐cyclodeaminase (FTCD) deficiency, a bifunctional enzyme encoded by FTCD.

Methods

In order to further understanding about the molecular alterations that contribute to FIGLU‐uria, we sequenced FTCD in 20 individuals with putative FTCD deficiency and varying laboratory findings, including increased FIGLU excretion.

Results

Individuals tested had biallelic loss‐of‐function variants in protein‐coding regions of FTCD. The FTCD allelic spectrum comprised of 12 distinct variants including 5 missense alterations that replace conserved amino acid residues (c.223A>C, c.266A>G, c.319T>C, c.430G>A, c.514G>T), an in‐frame deletion (c.1373_1375delTGG), with the remaining alterations predicted to affect mRNA processing/stability. These included two frameshift variants (c.990dup, c.1366dup) and four nonsense variants (c.337C>T, c.451A>T, c.763C>T, c.1607T>A).

Conclusion

We observed additional FTCD alleles leading to urinary FIGLU elevations, and thus, providing molecular evidence of FTCD deficiency in cases identified by newborn screening or clinical biochemical genetic laboratory testing.

Folate-Dependent Purine Nucleotide Biosynthesis in Humans

Purine nucleotide biosynthesis de novo (PNB) requires 2 folate-dependent transformylases-5'-phosphoribosyl-glycinamide (GAR) and 5'-phosphoribosyl-5-aminoimidazole-4-carboxamide (AICAR) transformylases-to introduce carbon 8 (C8) and carbon 2 (C2) into the purine ring. Both transformylases utilize 10-formyltetrahydrofolate (10-formyl-H4folate), where the formyl-carbon sources include ring-2-C of histidine, 3-C of serine, 2-C of glycine, and formate. Our findings in human studies indicate that glycine provides the carbon for GAR transformylase (exclusively C8), whereas histidine and formate are the predominant carbon sources for AICAR transformylase (C2). Contrary to the previous notion, these carbon sources may not supply a general 10-formyl-H4folate pool, which was believed to equally provide carbons to C8 and C2. To explain these phenomena, we postulate that GAR transformylase is in a complex with the trifunctional folate-metabolizing enzyme (TFM) and serine hydroxymethyltransferase to channel carbons of glycine and serine to C8. There is no evidence for channeling carbons of histidine and formate to AICAR transformylase (C2). GAR transformylase may require the TFM to furnish 10-formyl-H4folate immediately after its production from serine to protect its oxidation to 10-formyldihydrofolate (10-formyl-H2folate), whereas AICAR transformylase can utilize both 10-formyl-H2folate and 10-formyl-H4folate. Human liver may supply AICAR to AICAR transformylase in erythrocytes/erythroblasts. Incorporation of ring-2-C of histidine and formate into C2 of urinary uric acid presented a circadian rhythm with a peak in the morning, which corresponds to the maximum DNA synthesis in the bone marrow, and it may be useful in the timing of the administration of drugs that block PNB for the treatment of cancer and autoimmune disease.

13C-enrichment of urinary uric acid after L-[Ring-2-13C]histidine dose in adult humans

We determined whether ring-2 carbon of histidine is folate-dependently transferred to carbons 8 (C₈) and/or 2 (C₂) in urinary uric acid in humans. Two adults collected each urine void for four days. Aliquots of urine for the first day were used for baseline values; then the subjects ingested 0.7 g (3.3 mmol) of l-[ring-2-¹³C]histidine and collected urine for three experimental days. Aliquots were analyzed for percentage ¹³C-content at C₂ and C₈ by a liquid-chromatography-mass spectrometry method. Percentage enrichment was determined by subtracting time-of-day paired baseline percentage ¹³C-content from experimental percentage ¹³C-content for each void. C₂ was predominantly ¹³C-enriched in the majority of voids. The percentage enrichments at C₂ for two subjects were 0.14 (±0.028 [SEM], n = 26) and 0.18 (±0.049, n = 21), whereas at C₈, they were 0.008 (±0.006) and −0.005 (±0.008), respectively. The mean C₂-enrichments were significantly greater than zero (p < 0.01), whereas those of C₈ were not (p > 0.2). The enrichment had a diurnal rhythm peaking in the morning. Our results may be useful in the estimation of the timing for the administration of drugs that interfere with purine nucleotide biosynthesis in the treatment of cancer and autoimmune disease.

The molecular basis of glutamate formiminotransferase deficiency

Glutamate formiminotransferase deficiency, an autosomal recessive disorder and the second most common inborn error of folate metabolism, is presumed to be due to defects in the bifunctional enzyme glutamate formiminotransferase-cyclodeaminase (FTCD). Features of a severe phenotype, first identified in patients of Japanese descent, include elevated levels of formiminoglutamate (FIGLU) in the urine in response to histidine administration, megaloblastic anemia, and mental retardation. Features of a mild phenotype include high urinary excretion of FIGLU in the absence of histidine administration, mild developmental delay, and no hematological abnormalities. We found mutations in the human FTCD gene in three patients with putative glutamate formiminotransferase deficiency. Two siblings were heterozygous for missense mutations, c.457C>T (R135C) and c.940G>C (R299P). Mutagenesis of porcine FTCD and expression in E. coli showed that the R135C mutation reduced formiminotransferase activity to 61% of wild-type, whereas the R299P mutation reduced this activity to 57% of wild-type. The third patient was hemizygous for c.1033insG, with quantitative PCR indicating that the other allele contained a deletion. These mutations are the first identified in glutamate formiminotransferase deficiency and demonstrate that mutations in FTCD represent the molecular basis for the mild phenotype of this disease.

Formiminoglutamic aciduria in a slightly retarded boy with chronic obstructive lung disease

A 2-year-old boy excreted massive amounts of formiminoglutamic acid in urine. The substance was identified as authentic formiminoglutamic acid by two-dimensional thin-layer chromatography, column chromatography and enzymatic determination. After alkaline hydrolysis the substance was converted to glutamic acid. Serum amino acid concentrations were normal. The patient had normal serum and erythrocyte folate levels. The red blood cell picture was normal. The leukocytes showed slight hypersegmentation. From the age of 3 months he exhibited recurrent otitis media and severe pulmonary infections. He had a peculiar narrow-headed look and marked universal hypotonia. The mental development was slightly retarded. Glutamate formiminotransferase deficiency is postulated. The findings lend support to the theory of glutamate formiminotransferase deficiency being a rather benign disorder of metabolism.