Homocystinuria due to cystathionine β-synthase deficiency associated with megaloblastic anaemia
Transcript of Homocystinuria due to cystathionine β-synthase deficiency associated with megaloblastic anaemia
LETTER TO THE EDITOR
Homocystinuria due to cystathionineb-synthase de®ciency associatedwith megaloblastic anaemia
DEAREAR SIRIR,
Homocystinuria is an inborn error of amino acidmetabolism in which homocystine, the disulphide of
homocysteine, is excreted in the urine as a conse-
quence of elevated homocysteine levels in the blood.The most common cause of homocystinuria is a
cystathionine b-synthase (CBS) de®ciency, which is
inherited as an autosomal recessive disorder and ischaracterized by mental retardation, lens disloca-
tion, skeletal abnormalities and thrombotic vascular
disease [1]. We encountered a patient with CBS-defective homocystinuria complicated with mega-
loblastic anaemia due to hypovitaminosis of folate
and vitamin B12 and attempted the administrationof folate and vitamin B12. We report the bene®cial
clinical course of the patient, together with thepathogenesis of megaloblastic anaemia in this
patient.
Case report
A 20-year-old man was admitted with dyspnoea oneffort and gingival haemorrhage. He had been
diagnosed with homocystinuria at the age of two,
when his sibling was identi®ed with hypermethio-ninuria due to CBS de®ciency as a result of national
neonatal screening and following CBS assay with
cultured skin ®broblasts. He had been hospitalized inour hospital with sinus thrombosis at the age of 16,
and then administered drugs including pyridoxine
hydrochloride, aspirin and dipyridamole. The clinicalresponse to administration of vitamin B6 was not
satisfactory.
On admission, physical examination revealed alimited IQ (58), funnel chest, ectopia lentis and joint
laxity of the ankle. The initial laboratory studies
showed the following: RBC, 1.11 ´ 106 lL±1; Hb,
4.1 g dL±1; MCV, 111.7 fL; MCH, 36.9 pg; WBC,
5200 lL±1 with normal differential count; PLT,
6.9 ´ 104 lL±1; AST, 61 U L±1 (normal range8±38); ALT, 46 U L±1 (normal range 8±38); LDH,
9088 U L±1 (normal range 106±211). The serum
folate concentration was 1.2 ng mL±1 (normalrange 2.4±9.8) and vitamin B12 level was
157 pg mL±1 (normal range 249±938). Antibody
for intrinsic factor was negative. Bone marrowaspiration showed hypercellular marrow
(31.2 ´ 104lL±1), myeloid/erythroid ratio of 1.24,
and marked megaloblastic changes in both theerythroid and granulocytic series. Upper gastroin-
testinal endoscopy and radiography of the small
intestine revealed no mucosal abnormality. Thefaecal concentrations of a1-antitrypsin and lactoff-
erin were within the normal ranges. Plasma amino
acid analysis revealed a methionine level of0.26 lmol mL±1 (normal range 0.01±0.02) and a
homocystine level of 0.05 lmol mL±1 (normallyundetectable). The urine methionine level was
0.16 lmol mg±1 creatinine (normal range
0.01±0.06), and the urine homocystine level was0.26 lmol mg±1 creatinine (normally undetectable).
Cystathionine was undetectable in either plasma or
urine. The urine methylmalonate level was normal.Treatment with parenteral administration of
mecobalamin 500 lg day±1 for a week resulted in
an effective rise in PLT and a marked decrease inLDH concentration, whilst megaloblastic anaemia
persisted. After the therapy was changed to intra-
venous administration of mecobalamin 500 lg day±
1 and folic acid 15 mg day±1, prompt rises in Hb
concentration were observed. His urine methionine
level elevated further and homocystine decreased.The treatment with oral mecobalamin
1500 lg day±1 and oral folic acid 15 mg day±1 is
continuing, and the Hb level has improved to thenormal range.
In the present case and his sibling with CBS
de®ciency (18-year-old female), examinations were
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ã 2001 Blackwell Science Ltd 453
performed of a complete blood count, serum folic
acid, serum vitamin B12, three urine amino acids,
urine methylmalonate and two urine pyrimidinederivatives (Table 1). Analyses of patients' urine
were performed using a rapid and simple procedure
consisting of urease-treatment of urine, stableisotope dilution and gas chromatography-mass
spectrometry, enabling the simultaneous quanti®ca-
tion of methionine, homocystine, cystine, methyl-malonate, orotate, and uracil [2,3]. The results
of laboratory studies showed that his sibling had
a normal Hb concentration, a normal folicacid concentration, and a low serum concentration
of vitamin B12 (12.7 g dL±1, 4.4 ng mL±1 and 165
pg mL±1, respectively). Urine methionine andhomocystine levels were signi®cantly increased,
whereas methylmalonate was within the normal
range. After the treatment with oral folic acid andmecobalamin, the urine methionine level of his
sibling elevated further and homocystine decreased.
Interestingly, urine orotate concentrations weremarkedly increased in both family members, and
decreased into the normal range after treatmentwith folate and vitamin B12 (Table 1).
Besides homocystinuria due to CBS de®ciency, two
other types of inherited homocystinuria are charac-terized by defective remethylation due to N5,10-
methylenetetrahydrofolate reductase (MTHFR)
de®ciency, and caused by N5-methyltetrahydrofolatehomocysteine methyltransferase de®ciency due to
the defective synthesis of methylcobalamin and
deoxyadenosylcobalamin [4,5]. The major biochemi-cal ®ndings in both of them are moderate homocy-
stinuria with low or relatively normal levels of
plasma methionine, and the condition of the latter
is accompanied by combined homocystinuria and
methylmalonic aciduria. The present patient shouldbe diagnosed as homocystinuria due to CBS de®ci-
ency, because of his clinical symptoms, hypermethi-
oninaemia, undetectable plasma and urinecystathionine and normal urine methylmalonate.
There have been no previous reports of homocystin-
uria due to CBS de®ciency associated with megalob-lastic anaemia. Some patients with CBS de®ciency
have had mild folate de®ciency at the time of
diagnosis [6±9]. In no instance was the folatede®ciency severe enough to cause clinical
manifestations, with the single exception of moderate
macrocytic anaemia in a patient who was receivingphenytoin [6]. The cause of the mild folate de®ciency
seen in some of these untreated CBS-de®cient
patients has been proposed to be excessive utilizationof 5-methyltetrahydrofolate in the methylation of
homocysteine to form methionine. This patient had
severe de®ciency of both folate and vitamin B12. Noresponse to pyridoxine in this patient had been
observed until megaloblastic anaemia occurred. Hehad been receiving a normal diet and had not
received anticonvulsants until admission. Moreover,
he had no gastrointestinal lesions, and serum anti-body for intrinsic factor was negative.
After treatment with folate and vitamin B12, the
megaloblastic anaemia improved dramatically, theurine methionine level elevated further and homo-
cystine decreased. Thus, we speculate that the
severe folate and vitamin B12 de®ciency and meg-aloblastic anaemia of our patient had the following
cause: the excessive utilization of folate and vitamin
Table 1 Urinary metabolite levels in patients with homocystinuria
Case Hcys Met Cys MMA Orotate Uracil Hb Vitamin B12 Folate
1
Before 26.36 11.82 0.97 1.03 9.60 8.85 4.1 157 1.2
After 4.35 30.73 2.07 0.48 0.43 4.66 12.6 533 41.5
2
Before 21.09 12.55 1.35 2.04 2.16 3.38 12.7 165 4.4
After 12.55 17.32 1.02 1.57 0.21 8.83 13.4 214 5.8
Control (n � 27) (range)
Mean UD 3.00 7.7 1.83 0.72 8.25 M16.0 � 2.0 233±914 2.4±9.8
SD UD 1.58 4.84 0.94 0.33 4.6 F 14.0 � 2.0
Hcys, homocystine; Met, methionine; Cys, cystine; MMA, methylmalonate; UD, undetectable.
Values are expressed as mmol mol)1 creatinine except for Hb, folate and vitamin B12; Hb, folate and vitamin B12 were determined by
routine laboratory test (g dL)1, ng mL)1 and pg mL)1, respectively).
Case 1, the present case; Case 2, the sibling of case 1. Data for both cases are presented before and after treatment.
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B12 increased, and subsequently the accumulation
of homocysteine accelerated re-methylation of
homocysteine to methionine. We believe that folatewas highly involved in the pathogenesis of mega-
loblastic anaemia, primarily because no prompt rise
in Hb level of our patient was observed by thetreatment with vitamin B12 alone, and the serum
folate concentration of his sibling without anaemia
was normal.The mechanism by which vitamin B12 de®ciency
causes megaloblastic anaemia remains controversial.
There is, however, general agreement that DNAsynthesis is impaired due to interference with the
folate metabolism. Vitamin B12 acts as a cofactor in
the methylation reaction of homocysteine to methio-nine, in which 5-methyltetrahydrofolate (methyl-
THF) is converted to tetrahydrofolate (THF) (Fig. 1).
Methyl-THF is the form of folate which all body cells,including those of the bone marrow, receive from
plasma. It was originally postulated that vitamin B12
de®ciency produced a block in the folate metabolismby trapping folate as methyl-THF, thus depriving
cells of THF and therefore of all the other coenzymeforms of folate derived from THF [10]. A more recent
theory suggested that the reduced supply of methio-
nine leads to reduced availability of `activatedformate' and hence of formyl THF, and that it is
this defect that results in the failure of folate
coenzyme synthesis [11].
In our study, the patient and his sibling showedincreased urinary concentrations of orotate, which
decreased into the normal range after treatment
with folate and vitamin B12. Orotate was the onlyintermediate in pyrimidine biosynthesis, which was
analysable by the procedure using urease-treatment
of urine, isotope-dilution and gas chromatography[2]. Conversion of dUMP to dTMP, catalysed by
thymidylate synthase, is folate-dependent, and pyr-
imidine biosynthesis is regulated by end-productinhibition (Fig. 1). Therefore, it was suggested that
folate and vitamin B12 de®ciency caused impaired
DNA synthesis and enhanced pyrimidine biosynthe-sis, together with orotic aciduria and megaloblastic
anaemia. Analysing the urinary excretion of orotate
may be useful for monitoring the biochemicalconditions of impaired DNA synthesis in patients
with megaloblastic anaemia.
S . I S H I D AS H I D A1 , H . I S O T A N IS O T A N I
1 ,
K . F U R U K A W AU R U K A W A1 & T . K U H A R AU H A R A
2
1Department of Internal Medicine,
Hirakata City Hospital, Osaka, Japan2Division of Human Genetics,
Medical Research Institute,
Kanazawa Medical University, Ishikawa, Japan
Fig. 1 Metabolism of methionine, homocysteine and cystathionine, and its relationship to folate metabolism, a cofactor vitamin B12 and the
DNA synthesis. Glu, glutamic acid; Asp, aspartic acid; THF, tetrahydrofolate; SAM, S-adenosyl methionine; SAH, S-adenosylhomocysteine;CBS, cystathionine b-synthase.
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Received 21 September 2000; revision received 7 November
2000; accepted 20 December 2000.
Correspondence: Dr Shimon Ishida, Department of Internal Medi-
cine, Hirakata City Hospital, 2-14-1 Kinya-honmachi, Hirakata,
Osaka 573-1013, Japan (fax: +81-72-847-2825).
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