Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.

The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.

This laboratory recently identified a novel proton-coupled folate transporter (PCFT) that mediates intestinal folate absorption and transport of folates into the central nervous system. The present study focuses on the definition of the minimum transcriptional regulatory region of this gene in HeLa cells and the mechanism(s) underlying the loss of PCFT expression in the methotrexate-resistant HeLa R1-11 cell line. The PCFT transcriptional regulatory controls were localized between −42 and +96 bases from the transcriptional start site using a luciferase-reporter gene system.

. Properties of the Arg376 residue of the proton-coupled folate transporter (PCFT-SLC46A1) and a glutamine mutant causing hereditary folate malabsorption.

Hereditary folate malabsorption (HFM) is a rare autosomal recessive disorder characterized by systemic and central nervous system folate deficiency. Turkish siblings are reported with the clinical syndrome of HFM, homozygous for deletion of 2 bases (c.204_205 delCC) within the first exon of the proton-coupled folate transporter (PCFT) gene, causing a frameshift. Low blood and cerebrospinal fluid folate levels were detected at ages 3.5 and 1 month. Treatment with parenteral 5-formyltetrahydrofolate resulted in normal development now at ages 3 and 1 year. Extending current knowledge on the phenotypic manifestations of HFM and the PCFT mutation spectrum will provide opportunities to define possible genotype-phenotype correlations and clarify the basis for the phenotypic variability that is characteristic of this disorder.

Hereditary folate malabsorption (HFM) is an autosomal recessive disorder, recently shown to be due to loss-of-function mutations of the proton-coupled folate transporter (PCFT-SLC46A1), resulting in systemic and central nervous system folate deficiency. Data is emerging on the spectrum of PCFT mutations associated with this disorder. In this report, novel mutations are described in three subjects with HFM: A335D/N68Kfs (c.1004C>A/ c.204-205delCC), a compound heterozygous mutation, and two homozygous PCFT mutations, G338R (c.1012G>C) and E9Gfs (c.17-18insC). Functional assessment of A335D and G338R PCFT mutants transfected into folate transporter-deficient HeLa R1-11 cells indicated a complete loss of transport activity. There were neurological deficiencies in two of the families reported; in particular, late-onset seizures. The importance of early diagnosis and treatment to achieve physiological cerebrospinal fluid folate levels is emphasized.

Loss-of-function mutations in the proton-coupled folate transporter (PCFT, SLC46A1) result in the autosomal recessive disorder, hereditary folate malabsorption (HFM). Identification and characterization of HFM mutations provide a wealth of information on the structure-function relationship of this transporter. In the current study, PCR-based random mutagenesis was employed to generate unbiased loss-of-function mutations of PCFT, simulating the spectrum of alterations that might occur in the human disorder. A total of 26 mutations were generated and 4 were identical to HFM mutations. Eleven were base deletion or insertion mutations that led to a frameshift and, along with similar HFM mutations, are predominantly localized to two narrow regions of the pcft gene at the 5-end. Base substitution mutations identified in the current study and HFM patients were largely distributed across the pcft gene. Elimination of the ATG initiation codon by a one-base substitution (G > A) did not result in a complete lack of translation at the same codon consistent with rare non-ATG translation initiation. Among six missense mutants evaluated, three mutant PCFTs were not detected at the plasma membrane, one mutation resulted in decreased binding to folate substrate, and one had a reduced rate of conformational change associated with substrate translocation. The remaining PCFT mutant had defects in both processes. These results broaden understanding of the regions of the pcft gene prone to base insertion and deletion and inform further approaches to the analysis of the structure-function of PCFT.The proton-coupled folate transporter (PCFT) 2 (SLC46A1) plays a key role in intestinal folate absorption and folate transport into the central nervous system (1). Loss-of-function mutations in the pcft gene lead to the rare autosomal recessive disorder, hereditary folate malabsorption (HFM) characterized by markedly reduced folate levels in blood and cerebrospinal fluid (1-4). A homozygous mutation in most cases or two compound heterozygous mutations in two cases have been identified in all subjects with the clinical diagnosis of HFM indicating that this disease is caused solely by alterations of the pcft gene (1, 2, 5-12).Sixteen different loss-of-function pcft mutations have been identified to date in HFM patients. Six result in drastic changes in predicted protein sequences (nonsense). p.Y362_G398del, occurred multiple times in unrelated families and is the result of skipping of exon 3 during RNA splicing (1, 9, 13). C66X introduces a stop codon at position 66 due to a two-base substitution (5). Four frameshift mutations, p. E9Gfs, p.G65Afs, C66Lfs and N68Kfs, are due to base deletions or insertions (2,8,10,12). Ten remaining mutations resulted in a single amino acid substitution in the PCFT protein (missense). Five mutations occurred at charged resides (p.R113C, p.R113S, p.R376W, p.R376Q, and p.D156Y) (2, 6, 7, 11), whereas the other five, p.G147R, p.S318R, p.A335D, p.G338R, and p.P425R, involved substitutions of a non-charged, with a ch...

Objective To determine whether subjects of Puerto Rican heritage are at increased risk for a specific mutation of the proton-coupled folate transporter (PCFT) causing hereditary folate malabsorption (HFM). Study design Three percent of the births in Puerto Rico in 2005, with additional regional oversampling, were screened for the prevalence of the c.1082G>A; p.Y362_G389 del PCFT gene mutation. Six new subjects of Puerto Rican heritage with the clinical diagnosis of HFM were also assessed for this mutation. Results Six subjects of Puerto Rican heritage with the clinical diagnosis of HFM were all homozygous for the c.1082G>A; p.Y362_G389 del PCFT mutation. Three heterozygote carriers were identified from the 1582 newborn samples randomly selected from births in Puerto Rico in 2005. The carrier frequency for the mutated allele was 0.2% island-wide and 6.3% in Villalba. Conclusion These findings are consistent with a common mutation in the PCFT gene causing HFM that has disseminated to Puerto Ricans who have migrated to mainland United States. Because prompt diagnosis and treatment of infants with HFM can prevent the consequences of this disorder, newborn screening should be considered in high-risk populations and physicians should be aware of its prevalence in infants of Puerto Rican ancestry.