This paper adds three new PCFT mutations to the evolving knowledge of the molecular changes in the PCFT protein associated with HFM. A335D (c.1004C>A), in the 9th
TMD, was detected in one allele of an English patient traced back to an English grandparent. A different mutation was detected in the second allele, N68Kfs (c.204-205delCC), also traced back to an English grandparent (the great-grandparents were also of English heritage). It is of interest that the latter mutation was recently detected in two siblings from a family in Turkey (in this case homozygous) [1
]. Two families from Tunisia are reported both cared for by the same physician. One of these families, in which the disorder appeared to be prevalent among eight siblings, was described prior to the cloning of PCFT [5
]. A male sibling from this family is reported with a homozygous mutation due to a C insertion in an all-C region (c.18_23) resulting in a frameshift, E9Gfs (c.17-18insC). A different homozygous mutation was detected in the second subject from a Tunisian family, c.1012G>C resulting in a G338R substitution. The current availability of a genetic test for this disorder now makes it possible to confirm the diagnosis of HFM, and identify carriers, in these and other families particularly when the mutation is known and the test should be relatively inexpensive.
Immune deficiency due to hypoglobulinemia is an important complication of HFM often manifested by Pneumocystis jiroveci
pneumonia, as was documented for P1 [4
]. The immune deficiency is rapidly reversed with systemic folate repletion and correction of the hematopoietic manifestations of HFM. Neurological disorders, in particular seizures, also frequently accompany HFM [4
]. It is unclear as to why some patients develop seizures and others do not, why the onset of seizures can be delayed, as in the first subject (P1) and a sibling of the second subject (P2), and whether this is related solely to the adequacy of treatment. CSF folate levels are very high during infancy and decrease only slowly during the first decade. CSF folate levels for the first year are 100-150 nM, decreasing to ~70-90 nM by age 5 and remaining in the 60-90 nM range well into the teens [12
]. Hence, the endpoint of treatment is to achieve and maintain normal blood, and red blood cell, folate levels and CSF folate levels appropriate to the age of the patient.
Because of the level of CSF folate required for the treatment of HFM, intramuscular dosing should be the most efficacious mode of administration. Leucovin (racemic 5-formylTHF), the folate most frequently utilized, is rapidly metabolized to the physiological 5-methyltetrahydrofolate. Isovorin (calcium levofolinate – Wyeth), the active isomer of 5-formylTHF, is available for parenteral administration. The oral formulation of 5-methyltetrahydrofolate (Metafolin – Merck) is available but the strength is insufficient for treatment of this disorder. Folic acid should not be used since it binds so tightly to folate receptors that it may impair the transport of 5-methylTHF across the blood-choroid plexus-CSF barrier [6
The first exon, particularly nucleotides encoding the first extracellular loop between the first and second transmembrane domains, has a high GC content, 65.8% (75% between residues 63 and 70) and is the most frequent site of PCFT mutations in subjects with HFM. Beyond the subjects described above, three other mutations have been reported in the first exon. These include a stop codon (c. 197 GC>AA; C66X) and three frame-shifts (G65Afs, C66Lfs and N68Kfs) [1
]. All the other mutations associated with HFM, aside from the one resulting in the formation of a splice variant [1
], have been within or at the junction of TMDs [8
]. Two residues have been the sites of two different mutations in different patients: R113S/C and R376W/Q [7