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Hereditary folate malabsorption
Hereditary folate malabsorption (HFM) is an inherited disorder of folate transport characterized by a systemic and central nervous system (CNS) folate deficiency manifesting as megaloblastic anemia, failure to thrive, diarrhea and/or oral mucositis, immunologic dysfunction and neurological disorders.
The prevalence is unknown. Approximately 30 cases have been reported to date.
Disease onset usually occurs a few months after birth. Manifestations include failure to thrive, diarrhea and/or mouth ulcers, various neurological manifestations (motor impairment, seizures, developmental delay, cognitive and behavioral disorders), megaloblastic anemia and hypoimmunoglobulinemia. Megaloblastic anemia is the primary manifestation of HFM and can be very severe if untreated. Hypoimmunoglobulinemia results in unusual infections with Pneumocystis jiroveccii, C. difficile and cytomegalovirus (CMV) which can be recurrent and life-threatening in undiagnosed infants. Neurological manifestations may be the presenting symptoms in some but are absent in others. Seizures, if present, begin in infancy or later in childhood. Intracranial calcifications have been observed in some.
HFM is caused by mutations in the SLC46A1 gene found on chromosome 17q11.2 which encodes the proton-coupled folate transporter (PCFT). PCFT is essential for intestinal folate absorption and transport of folates across the blood-cerebrospinal fluid (CSF) barrier. A defect in this protein leads to a systemic folate and CNS folate deficiency. Infants cannot absorb adequate folate from breast milk/formula and become deficient once their stores accumulated during gestation are exhausted.
Diagnosis is based on clinical and laboratory findings. It is confirmed by findings of an impaired absorption of an oral folate load (even after correction of serum folate concentration) and a low CSF folate concentration (0-1.5nM). Bone marrow biopsy confirms the presence of megaloblastic anemia. Sequence analysis of the SLC46A1 coding region can identify any mutations present in the gene, also confirming diagnosis of HFM.
The immunodeficiency seen in HFM may resemble severe combined immune deficiency (SCID; see this term). Other differential diagnoses include methionine synthase deficiency with megaloblastic anemia and developmental delay, formiminoglutamic aciduria, tyrosinemia type 1, methylenetetrahydrofolate reductase deficiency and erythroleukemia (see these terms).
Antenatal diagnosis is possible via prenatal testing. Screening of newborns with a family history of HFM allows for early diagnosis and treatment with folate immediately after birth, before symptoms occur.
HFM is inherited autosomal recessively. Genetic counseling is possible.
Management and treatment
High dose oral or parenteral 5-formyltetrahydrofolate (5-formylTHF) and oral L-5-methyltetrahydrofolate (L-5-methylTHF) are the two types of reduced folates used to treat HFM. Dosage is monitored and adjusted (individualized for each patient) so that the CSF folate levels remain within the normal range (around 100nM in infants-2 year olds). Folic acid should not be used as it binds to folate receptors and blocks folate transport. If anemia is severe, a transfusion may be necessary. Early treatment with reduced folates before the appearance of symptoms can prevent the metabolic consequences of HFM. Patients should have regular blood tests to monitor complete blood count, serum and CSF folate and homocysteine concentrations and serum immunoglobulin concentrations.
With proper treatment the prognosis is good and reversal of most of the systemic consequences of the disease is usually achieved. Only when untreated is the prognosis poor.