The Role of Folate Receptor Autoimmunity in Cerebral Folate DeficiencyT. Opladen1, 2, N. Blau1,V. Th. Ramaekers2

1Division of Clinical Chemistry and Biochemistry, University Children's Hospital Zürich, Switzerland2Division of Pediatric Neurology, Dept. Pediatrics, University Hospital Aachen, Rheinisch-Westfälische Technische Hochschule, Aachen University, Aachen, Germany

Abstract

For normal functioning, the central nervous system requires folate stores which depend on folate homeostasis and intact transport mechanisms across the choroid plexus. Cer-ebral folate deficiency (CFD) describes a neurological syndrome associated with low cer-ebrospinal fluid (CSF) 5-methyltetrahydrofolate (5MTHF) in the presence of normal folate metabolism outside the nervous system. Idiopathic cerebral folate deficiency was found in a number of patients presenting with microcephaly, irritability, psychomotor retardation, cerebellar ataxia, spastic paraplegia, dyskinesias, and occasional seizures. Treatment with folinic acid proved beneficial in most patients. Serum analysis from 25 out of 28 analysed children with CFD revealed blocking autoantibodies to the membrane-bound folate receptors. All 28 serum samples of age-matched controls were negative. Cerebral folate deficiency is more common than initially assumed and should be corrected as early as possible by oral administration of folinic acid. The finding of blocking autoantibody against the folate receptor at the choroid plexus provides an autoimmune explanation for this hitherto underestimated and treatable condition.

Introduction

Folic acid is a water-soluble vitamin that functions as one-carbon donor in various meta-bolic cycles. It is involved in the biosynthesis of thymidylates and purines, the methionine synthesis via homocysteine remethylation, the methylation of phospholipids, the serine and glycine interconversion, and the metabolism of histidine and formate. It is therefore essential for growth, reproduction, maintenance of normal body function and brain devel-opment in childhood [1, 2]. Twenty-five different compounds are referred to as the folates. They can be interconverted enzymatically, but the natural active form in serum consists mainly of 5-methyltetrahydrofolate (5MTHF). Systemic folate deficiency is associated with macrocytic anaemia, high blood levels of homocysteine, and neural tube defect in new-borns [3, 4]. In contrast cerebral folate deficiency (CFD) has been defined as any neuro-logical syndrome associated with low cerebrospinal fluid (CSF) 5-methyl-tetrahydrofolate

Che

mis

try

and

Bio

logy

of

Pte

ridin

es a

nd F

olat

es©

200

7 S

PS

Ver

lags

gese

llsch

aft

mb

H,

Hei

lbro

nn,

Ger

man

y

438

(5MTHF), the active folate metabolite, in the presence of normal folate metabolism outside the nervous system [5].

Recently differentiation of CFD in a primary (“idiopathic”) form and a secondary form was suggested [5]: The primary form becomes manifest from the age of 4 months, starting with marked unrest, irritability, and sleep disturbances followed by psychomotor retarda-tion, cerebellar ataxia, spastic paraplegia, and dyskinesia; epilepsy developed in about one third of the cases. Most children showed deceleration of head growth from the age of 4 to 6 months. Visual disturbances and progressive sensorineural hearing loss are not invariably present but may develop from the age of 3 years and 6 years, respectively. The clinical findings are summarised in figure 1. Neuroimaging does not show any specific find-ings and was normal in half of the children whereas the other children showed atrophy of frontotemporal regions and periventricular demyelination or slowly progressive supra- and infratentorial atrophy [5].

0 1 2 3 4 5 10

Unrest, irritability, insomnia

Decelerating head growth

Neurodevelopmental delay, standstill, regression

Generalised hypotonia and ataxia

Pyramidal signs in lower limbs – ascending

Dyskinesia (hand choreoathetosis, ballistic movements)

Epilepsy

Visual disturbances – VEP abnormal

Hearing loss

Age (y)

Figure 1: Overview of the main clinical features in patients with primary CFD correlated to the age of onset: CFD patients develop normally during the first four months. Character-istic symptoms start with marked unrest, irritability, sleep disturbances and deceleration of head growth from the age of 4 to 6 months followed by psychomotor retardation, cer-ebellar ataxia, spastic paraplegia, and dyskinesia. Epilepsy developed in about one third of the cases from the age of one year. Visual disturbances and progressive sensorineural hearing loss are not invariably present but may develop from the age of 3 years and 6 years, respectively. (Reprinted with permission from [5] and [12]).

Secondary forms of CFD have been recognised during chronic use of antifolate and anticonvulsant drugs and in various known conditions such as Rett syndrome, Aicardi-

439

Goutières syndrome, 3-phosphoglycerate dehydrogenase deficiency, dihydropteridine reductase deficiency, aromatic amino acid decarboxylase deficiency, and Kearns-Sayre syndrome [6, 7, 5].

Oral treatment with 5-formyltetrahydrofolate (folinic acid) resulted in a favourable clinical response in idiopathic and secondary CFD patients. Treatment should be started as soon as possible in low doses at 0.5–1 mg/kg/day since patients identified before the age of six years showed a better outcome than patients treated after the age of six years. Dose has to be titrated individually, since some patients require higher daily doses of folinic acid at 2–3 mg/kg/day to normalise CSF 5MTHF values [5]. Careful clinical, laboratory and EEG follow-up should be performed continuously during treatment to prevent over- or underdosage.

The exact pathophysiology in CFD remains unclear but based on the knowledge of normal peripheral folate homeostasis, CFD could be a consequence of either disturbed folate transport to the central nervous system (CNS) or an increased folate utilisation and catabolism within the CNS. The most important folate transport mechanisms in humans are the reduced folate carrier 1 (RFC1) and the family of folate receptor proteins (FR) which possess different binding properties and are distributed at various sites in normal human tissues and are highly expressed in a variety of cancer cells [8]. RFC1 is ubiquitously distributed and represents a low-affinity folate transporting system with bidirectional trans-port across cellular membranes. In contrast the folate receptors are high-affinity proteins which function at the physiological nanomolar range of extracellular folate concentrations. FR1 is mainly distributed at epithelial cells, such as choroid plexus, lung, thyroid, and renal tubular cells, while FR2 is mainly located within mesenchymal derived cells, such as red blood cells. For passage across the blood-CNS barrier 5MTHF is bound by the FR1, anchored to choroid epithelial cells and followed by endocytosis, storage, and sub-sequent delivery to the spinal fluid compartment where it will be transported into neuronal tissues (figure 2). A specific disorder of folate transport across the choroid plexus was described for the first time in an adult male with a slowly progressive neurological disease characterized by a cerebellar syndrome, distal spinal muscular atrophy, pyramidal tract dysfunction, and perceptive hearing loss [9]. A diminished expression and/or secretion of FR1 and impaired folate release from its binding site were suggested but the exact cause remained unclear.

To exclude genetic alterations of FR1 or FR2 in our patients the folate receptor genes were analyzed by DNA sequencing and found to be normal. An alternative possibility is that impaired folate transport across the blood-CSF barrier is caused by circulating serum autoantibodies that block the binding of folate to the receptor. Such blocking autoanti-bodies were found in serum from women with a pregnancy complicated by a neural tube defect [10]. We therefore tested serum from patients with idiopathic CFD for the presence of autoantibodies to the folate receptor.

440

Basolateral surface

PlasmaBlood

[–]

5-MTHF(monoglutamate) FR

5-MTHFFolate

Fol Glut [n](polyglutamate)

12Endocytosis

Potocytosis

Tight junction

Choroid epithelium

FRexpression

RFC1

RFC1

CSF

Apical surface

(Passive diffusion)

(Active transport)

Figure 2: Schematic presentation of the active transport for folates across the blood-CSF-barrier localised at the epithelium of the choroid plexus. The folate receptor protein 1 (FR1) binds and incorporates 5-methyltetrahydrofolate. 5MTHF is converted to folyl-polyglutamate by the action of folylpoly-gamma-glutamate synthetase (1). Stored folylpolyglutamate can be converted again to its monoglutamate form by gamma-glutamyl-hydrolase (2) after which the monoglutamate form of folate can leave the choroid plexus to the spinal fluid compartment. Expression of FR1 is correlated inversely with extracellular folate concentrations. (Reprinted with permission from [5], [7] and [13]).

Patients and methods

After exclusion of other neurodegenerative diseases and inborn errors of metabolism 28 patients (20 boys and 8 girls; median age, 7.1 years, range 2.5 to 19.3) fulfilled the clini-cal criteria of cerebral folate deficiency with normal levels of serum and erythrocyte folate. The births and neonatal histories of these patients and the pregnancies of their mothers had been normal except in the case of one child, who had been born prematurely, at 28

441

weeks of gestation. All the parents were healthy and unrelated except for the parents of one patient, who were first cousins.

A lumbar punction was performed according to a standardized protocol. 5MTHF was measured with HPLC using electrochemical detection and compared with values derived from 99 normal controls, as previously described. [11, 6]. After the diagnosis had been established, treatment with folinic acid (0.5 to 1 mg per kilogram of body weight daily in two divided doses) was started according treatment protocol [5]. The history and neuro-logical examination of the 28 age-matched control subjects (17 boys and 11 girls; median age, 7.6 years; range, 1.9 to 19.0) did not reveal any of the symptoms of the cerebral folate deficiency syndrome [12].

Serum samples of patients, age-matched controls and five mothers of patients with CFD were kindly analyzed for autoantibodies against the folate receptors in the laboratory of the Department of Medicine and Biochemistry, State University New York according to the procedures published previously [10]. Additionally serum from 41 subjects with central nervous system disease unrelated to CFD was examined [12].

Results

CSF analysis confirmed the clinical diagnosis of CFD showing reduced 5MTHF concen-tration in all patients compared with normal reference values (Figure 3/Table 1). In nine patients CSF analysis showed a reduced concentration of 5-hydroxy-indoleacetic acid (5HIAA) in the presence of normal homovanillic acid (HVA) concentrations. Six patients had isolated reduction of neopterin (Table 1).

n HVA 5HIAA HVA/ 3OMD 5HTP L-DOPA Neo Bio 5MTHF (nmol/l) (nmol/l) HIAA (nmol/l) (nmol/l) (nmol/l) (nmol/l) (nmol/l) (nmol/l)

CFD 28 n n (↓) n n n n n (↓) n ↓

28 245–537 58–207 1.9–5.2 11–79.5 1.6–12 5.2–14 5.9–20.5 14.5–20.5 11.4–38.7

Norm: 0–0.5J 720(310–1100) 280(150–800) <300 <10 <25 15–35 20–70 0.5–1J 660(295–932) 224(114–336) <100 <10 <25 12–30 15–40 64–182 2–4J 603(211–871) 202(105–299) 1.5–3.5 <50 <10 <25 9–30 10–30 63–111 5–10J 523(144–801) 133(88–178) <50 <10 <25 9–20 10–30 41–117 11–16J 399(133–551) 118(74–163) <50 <10 <25 9–20 10–30 41–117 >16J 261(115–488) 103(66–141) <50 <10 <25 9–20 19–30

Table 1: Overview of the CSF analysis in 28 patients with CFD: All patients showed reduced CSF 5MTHF concentration. One third of CFD patients had further a reduced concentration of 5-hydroxy-indoleacetic acid (5HIAA) in the presence of normal homova-nillic acid (HVA) concentration. CSF pterin analysis showed isolated reduction of neopterin concentration in one fourth of the patients (Reference values from [11]). (reprinted with permission from [12]).

442

020406080

100120140160

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Age (years)

CS

F 5-

MT

HF

(nm

ol/

l)

Figure 3: 5MTHF concentrations in cerebrospinal fluid from 28 patients with CFD compared to the age-related reference values (grey shaded area)

Blocking autoantibodies against the folate receptors were identified in serum specimens from 25 of 28 children with cerebral folate deficiency and in 0 of 28 matched control subjects (P < 0.001 by the chi-square test) (table 2). No autoantibodies against the folate receptors were detected in serum from 5 mothers of children with CFD and 41 subjects with central nervous system disease unrelated to this syndrome. The mean titer of block-ing autoantibodies in the serum of cerebral folate deficiency subjects was 0.87 pmol of folate receptor blocked per millilitre of serum (table 2). The mean apparent Ka for the bind-ing of these autoantibodies to the folate receptor was 5.54 x 1010 liters per mole [12]

Serum specimens from three children with cerebral folate deficiency did not contain these autoantibodies. Patient 9, who had four of the clinical criteria for the syndrome, had frank autistic behaviour and recovered completely after receiving a daily dose of 400 µg of folic acid contained in a multivitamin formula; he currently attends a regular school. He was the first child identified to have the syndrome and received a multivitamin containing folic acid, whereas all the other children were treated with folinic acid. Patients 7 and 21 also had remarkable improvements with folinic acid, although the changes were not as dramatic as those in Patient 9.

Discussion

Idiopathic cerebral folate deficiency (CFD), defined as isolated lowering of CSF 5MTHF in the presence of a normal systemic folate pool and metabolism, is more common than initially assumed. After the first report on five children [13], a total of thirty-one children are known with the typical uniform history and clinical phenotype. CFD is further associated with autism and shows in some patients very dramatic response to folinic acid supple-mentation [5, 12, 14]. So far the exact cause for the clinical and biochemical finding were

443

unknown, although already at the first report a disturbed folate transport was suggested [13].

The finding of blocking autoantibodies against folate receptors in serum from children with infantile-onset cerebral folate deficiency supports the hypothesis that this neurologi-cal syndrome can be a consequence of autoantibody-impaired folate transport into the cerebrospinal fluid. The high affinity of the autoantibodies (mean Ka, 5.54 x 1010 litres per mole) allows them to prevent folate from binding to the receptors on the epithelial cells of the choroid plexus. Since autoantibodies with a mean Ka of 2.2 x 1010 liters per mole were shown to block the binding and cellular uptake of [3H]folic acid by KB cells [10], autoantibodies with a higher Ka, such as those in the serum from subjects with cerebral folate deficiency, would have a similar effect. Autoantibodies against GPI-anchored folate receptors preferentially bind to epithelial cells on the plasma side of the choroid plexus. Folate receptors in the lungs, kidney and thyroid gland might also be affected by these blocking autoantibodies. But recent studies could show that tissue FR expression within lung tissue is limited to the apical membrane of the epithelial cells. This means that in the blood circulating antibodies cannot gain access to them [8]. In kidneys, the folate receptors on the luminal side of the proximal renal tubules will not be affected because immunoglobulins do not pass into the renal tubules of normal kidneys.

The positive effect of treatment with folinic acid in patients with CFD remains unex-plained. Different mechanisms are possible: First, treatment with pharmacologic doses of 5-formyltetrahydrofolate (folinic acid), most of which is enzymatically converted in vivo

Patients with Age-matched Mothers of Patients with Cerebral Folate healthy controls patients with central nervous Deficiency Cerebral Folate system disease Deficiency unrelated to CFD

Number of subjects 28 28 5 41

Blocking autoantibodies detected in number of subjects 25 0 0 0

Titer* (pmol receptor blocked/ml) 0.87 (0–1.55) – – –

Affinity constant* (litres/mole) 5.54 x 1010 – – –

* (mean, range)

Table 2: Blocking autoantibodies against the folate receptor were found in serum specimens from 25 of 28 children with CFD, in 0 of 28 age-matched healthy controls, in 0 of 5 mothers of patients with CFD and in 0 of 41 patients with central nervous system dis-ease unrelated to CFD [12]. The mean apparent Ka for the binding of these autoantibodies to folate receptor was 5.54 x 1010 litres per mole. (Reprinted with permission from [12]).

444

to the physiologically active 5MTHF [15, 16] enters the cerebrospinal fluid by way of the reduced folate carrier in the choroid epithelial cells. A second possibility is displacement of blocking autoantibodies to the folate receptors by a high level of 5MTHF (approximately 2 µM or greater). A third mechanism could be diffusion or another so far unknown transport mechanism, when the plasma level of 5MTHF is very high [12].

The origin of the blocking autoantibodies remains to be explained. Because of the appear-ance of the first clinical manifestations of cerebral folate deficiency after the age of four to six months the production of autoantibodies in these children probably occurred during the first four to six months of life. A transmission of autoantibodies from the mothers to their babies is less likely because the five mothers we tested had no autoantibodies. One pos-sible induction of the autoantibody production could be contact with soluble folate-binding proteins in human or bovine milk or result from sensitization by unknown antigens with similar epitopes. Previous investigations could show that soluble-folate binding proteins in milk share amino acid sequence homology (91 percent similarity) with the membrane-bound folate receptors 1 and 2 that are expressed on human choroid plexus epithelium. [17, 18]. Moreover, the human folate receptors on the choroid plexus cross-react with rab-bit antibodies against the human-milk folate binding protein [19]. Autoantibodies against these epitopes could result in reduced folate transport into the cerebrospinal fluid.

The presence of blocking autoantibodies against the folate receptor at the choroid plexus in patients with CFD provides an autoimmune explanation for this hitherto underestimated and treatable condition. Early detection and diagnosis of cerebral folate deficiency are important because folinic acid at a pharmacologic dose can bypass autoantibody-blocked folate receptors, most likely by way of the reduced folate carrier. In this manner the folate pool within the central nervous system is restored. The clinical response in most patients is favourable, showing better social contact, motor improvements and reduction of sei-zures.

Acknowledgment

We thank Dr JM Sequeira and Prof Dr EV Quadros at the Department of Medicine and Bio-chemistry, State University New York for analysis of FR autoantibodies in serum samples. This work was supported in part by a research grant from the Medical Faculty Aachen, by the Dr. Emil-Alexander Huebner Foundation, and by the Swiss National Science Founda-tion grant no. 310000-107500

REFERENCES:

1. Greenblatt JM, Huffman LC, Reiss AL. Folic acid in neurodevelopment and child psy-chiatry, Prog. Neuropsychopharmacol. Biol. Psychiatry. 1994; 18 (4): 647–660.

445

2. Van Der Put NMJ, Van Straaten HWM, Trijbels FJM, BHJ. Folate, homocysteine and neural tube defects: An overview. Exp. Biol. Med. 2001; 226 (4): 243–270.

3. Yerby MS. Clinical care of pregnant women with epilepsy: Neural tube defects and folic acid supplementation. Epilepsia 2003; 44 Suppl 3: 33–40.

4. Moat SJ, Lang D, Mcdowell IF, Clarke ZL, Madhavan AK, Lewis MJ, Goodfellow J. Folate, homocysteine, endothelial function and cardiovascular disease. J. Nutr. Bio-chem. 2004; 15 (2): 64–79.

5. Ramaekers VT, Blau N. Cerebral folate deficiency. Dev. Med. Child Neurol. 2004; 46 (12): 843–851.

6. Blau N, Bonafe L, Krageloh-Mann I, Thony B, Kierat L, Hausler M, Ramaekers V. Cer-ebrospinal fluid pterins and folates in aicardi-goutieres syndrome: A new phenotype, Neurology 2003; 61 (5): 642–647.

7. Ramaekers VT, Hansen SI, Holm J, Opladen T, Senderek J, Hausler M, Heimann G, Fowler B, Maiwald R, Blau N. Reduced folate transport to the CNS in female rett patients. Neurology 2003; 61 (4): 506–515.

8. Parker N, MJ T, Westrick E, JD L, Low PS, Leamon CP. Folate receptor expression in carcinomas and normal tissues determined by a quantitative radioligand binding assay. Anal. Biochem. 2005; 338: 284–293.

9. Wevers RA, Hansen SI, Van Hellenberg-Hubar JL, Holm J, Hoier-Madsen M, Jongen PJ. Folate deficiency in cerebrospinal fluid associated with a defect in folate binding protein in the central nervous system. Journal of Neurology, Neurosurgery & Psychiatry 1994; 57 (2): 223–226.

10. Rothenberg SP, Da Costa MP, Sequeira JM, Cracco J, Roberts JL, Weedon J, Quadros EV. Autoantibodies against folate receptors in women with a pregnancy complicated by a neural-tube defect. [see comment]. New England Journal of Medicine 2004; 350 (2): 134–142.

11. Blau N, Duran M, Blaskovics ME. In: Physician’s guide to laboratory diagnosis of meta-bolic diseases. Berlin: Springer, London, 2003.

12. Ramaekers VT, Rothenberg SP, Sequeira JM, Opladen T, Blau N, Quadros EV, Selhub J. Autoantibodies to folate receptors in the cerebral folate deficiency syndrome. [see comment]. New England Journal of Medicine 2005; 352 (19): 1985–1991.

13. Ramaekers VT, Hausler M, Opladen T, Heimann G, Blau N. Psychomotor retardation, spastic paraplegia, cerebellar ataxia and dyskinesia associated with low 5-methyltet-rahydrofolate in cerebrospinal fluid: A novel neurometabolic condition responding to folinic acid substitution. Neuropediatrics 2002; 33 (6): 301–308.

14. Hansen FJ, Blau N. Cerebral folate deficiency: Life-changing supplementation with folinic acid. Mol. Genet. Metab. 2005; 84: 371–373.

446

15. Levitt M, Nixon PF, Pincus JH, Bertino JR. Transport characteristics of folates in cerebrospinal fluid; a study utilizing doubly labeled 5-methyltetrahydrofolate and 5-formyltetrahydrofolate. Journal of Clinical Investigation 1971; 50 (6): 1301–1308.

16. Wright AJ, Finglas PM, Dainty JR, Hart DJ, Wolfe CA, Southon S, Gregory JF. Single oral doses of 13c forms of pteroylmonoglutamic acid and 5-formyltetrahydrofolic acid elicit differences in short-term kinetics of labelled and unlabelled folates in plasma: Potential problems in interpretation of folate bioavailability studies. British Journal of Nutrition 2003; 90 (2): 363–371.

17. Pearson WR, Lipman DJ. Improved tools for biological sequence comparison. Pro-ceedings of the National Academy of Sciences of the United States of America 1988; 85 (8): 2444–2448.

18. Svendsen I, Hansen SI, Holm J, J L. Amino acid sequence homology between human and bovine low molecular weight folate binding protein isolated from milk. Carlsberg Res Commun 1982; 47: 371–376.

19. Holm J, Hansen SI, Hoier-Madsen M, Bostad L. High-affinity folate binding in human choroid plexus. Characterization of radioligand binding, immunoreactivity, molecular heterogeneity and hydrophobic domain of the binding protein. Biochemical Journal 1991; 280 (Pt 1): 267–271.

ADDRESS FOR CORRESPONDENCE:

Vincent Thomas Ramaekers, MD PhDDivision of Paediatric NeurologyUniversity Hospital AachenPauwelsstrasse 30D-52074 AachenGermanyE-Mail: vramaekers@ukaachen.de

447