Identification of novel mutations in the proton-coupled folate transporter (PCFT-SLC46A1) associated with hereditary folate malabsorption

Abstract

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.

Identification and Functional Impact of Homo-oligomers of the Human Proton-coupled Folate Transporter

“…Given the critical roles of hPCFT in intestinal absorption of dietary folates and of mutant hPCFT in HFM (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) and in the selective delivery of cytotoxic antifolates for targeting solid tumors (21, 24 -26), our findings of a functionally important oligomerization for hPCFT are particularly significant. For instance, in HFM, our findings may explain why all HFM patients thus far described have mutations in both pcft gene alleles because loss of a single pcft allele on hPCFT function would probably not be detected (5-15).…”

“…The role of hPCFT in intestinal folate absorption was established by demonstrating loss-of-function mutations in hPCFT in patients with the rare autosomal inherited disorder, hereditary folate malabsorption (HFM) (5). To date, 17 unique hPCFT mutations have been reported in ethnically varied kindreds (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). Although proton-coupled, this transporter is also func-tional at more physiologic pH, at which it retains appreciable affinity for pemetrexed (16), a newer antifolate currently approved for treating mesothelioma and non-squamous, nonsmall cell lung cancer (17)(18)(19).…”

Random Mutagenesis of the Proton-coupled Folate Transporter (SLC46A1), Clustering of Mutations, and the Bases for Associated Losses of Function

“…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).…”

“…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)(2)(3)(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)(6)(7)(8)(9)(10)(11)(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 charged, residue (2,12). There appear to be hot spots for both nonsense and missense mutations.…”