Introduction
Fructose-1,6-bisphosphatase (FBPase) deficiency (OMIM 229700; FBPase; E.C.3.1.3.11)
is a
very rare autosomal recessive disorder of gluconeogenesis with a frequency of 1–9
per
100,000, which is characterized by recurrent episodes of hypoglycemia with metabolic
and
lactic acidosis, apnea, hyperventilation, and ketosis (1, 2). Fructose intake, fasting,
and febrile
infectious disease are known to trigger these symptoms. Once the diagnosis is established,
the prognosis of this disorder is excellent if simple measures are followed such as
the
prevention of hypoglycemia (2) and avoidance of the
consumption of foods with fructose (2) and glycerol
(3).
FBPase deficiency can be definitively diagnosed by confirming mutations in
FBP1, which encodes fructose-1,6-bisphosphatase-1. FBP1
consists of 7 exons, which span more than 31 kb at chromosome 9q22.2-q22.3 and encodes
a 362
amino acid protein that is mainly expressed in the liver and kidney. Since the first
identified mutations in 1995, at least 36 additional mutations resulting in FBPase
deficiency, including those from the Japanese population (4), have been described
in the genomic region spanned by FBP1.
Excretion of glycerol and glycerol-3-phosphate in the urine may help to distinguish
this
disease from other metabolic acidosis diseases (5).
Here, we report a patient with FBPase deficiency caused by novel compound heterozygous
mutations in FBP1, who had normal urine glycerol-3-phosphate during an oral
fructose tolerance test.
Patient Report
The patient was an 18-mo-old boy born to non-consanguineous healthy Japanese parents
at
full term after an uncomplicated pregnancy and delivery. He had no remarkable medical
history during infancy.
He was admitted to our hospital at 18 mo of age with drowsiness without any obvious
fructose intake. Physical examinations revealed a pale face, hepatomegaly, and Kussmaul
respiration (respiration rate, 52/min). His body weight was 9.4 kg (–1.0 SD). FBPase
deficiency was suspected because of a combination of lactic acidosis (pH, 7.135; serum
bicarbonate level, 4.1 mmol/L; base excess, –22.4 mmol/L; plasma lactate level, 78.7
mg/dL)
and hyperuricemia (serum uric acid level, 17.1 mg/dL). Hyponatremia (serum sodium
level, 124
mEq/L) was also noted, which was probably a result of the vomiting and diarrhea. Other
blood
examination results included mild hypoglycemia (serum glucose level, 71 mg/dL), ketosis
(serum total ketone body level, 5150 μmol/L; acetoacetate, 526 μmol/L; 3-hydroxybutyrate
4630 μmol/L), and increased levels of ammonia and pyruvate (plasma ammonia level,
112 μg/dL;
serum pyruvic acid level, 2.01 mg/dL; lactate/pyruvate ratio, 39). Excretion of lactate
and
ketone bodies in his urine were also increased. Computed tomography scans of his abdomen
revealed moderate hepatomegaly and a fatty liver.
After symptomatic and biochemical improvements with a glucose infusion (GIR 3.0 mg
kg–1min–1) for one wk, an oral fructose tolerance test (1 g/kg) was
performed. In this test, hypoglycemia (serum glucose levels decreased from 70 to 45
mg/dL)
was noted, and lactate and uric acid levels were increased (lactic acid levels from
27.3 to
54.4 mg/dL, and uric acid levels from 4.4 to 8.9 mg/dL). Excretion of glycerol in
the urine
was markedly high at 293.8 mmol–1mol–1cre (control: 38.1 ± 13.4
mmol–1mol–1cre), and excretion of glycerol-3-phosphate was normal
(5.0 mmol–1mol–1cre). These levels were analyzed using gas
chromatography-mass spectrometry (GC/MS) with a urease pretreatment non-extraction
method.
Taken together, these findings supported the diagnosis of FBPase deficiency, except
for the
glycerol-3-phosphate excretion levels in the urine.
Mutational Analysis
The study was approved by the Institutional Review Board of the Tokyo Metropolitan
Children’s Medical Center, and informed consent for the molecular study was obtained
from
the parents. Genomic DNA was extracted from the peripheral blood leukocytes of the
patient
and his parents. We used PCR-direct sequencing to examine all coding exons and flanking
introns of FBP1. Direct sequencing of FBP1 revealed
compound heterozygous FBP1 mutations (c.530C>A, p.Ala177Asp; and
c.268T>G, p.Phe90Val) in the patient (Fig. 1A and
B
Fig. 1.
Mutational analysis of FBP1. A: The chromatograms of the proband and
the mother indicate a heterozygosity of aspartic acid [GAC] in place of alanine [GCC]
at codon 530. The arrow indicates the mutated nucleotide. B: The chromatograms of
the
proband and the father indicate a heterozygosity of valine [GTT] in place of
phenylalanine [TTT] at codon 268. The arrow indicates the mutated nucleotide.
). His father carried the p.Ala177Asp mutation and his mother carried the p.Phe90Val
mutation (Fig. 1A and B).
Previously, the p.Ala177Asp mutation was identified in a Japanese patient with FBPase
deficiency. The pathogenicity of the Ala177Asp mutation in FBPase was verified with
a
functional assay; the enzymatic activity was markedly reduced (0.2 units/mg in mutant,
6.8 ±
0.5 units/mg in wild type) (2). The p.Phe90Val
mutation was novel, was not detected in any of the 150 healthy controls tested, and
was
absent from various databases including dbSNP, the 1000 Genomes Project, Exome Variant
Server, NHLBI Exome Sequencing Project, and the Human Genetic Variation Database in
Japanese
Population. In silico analyses with SIFT (http://sift.jcvi.org/) and M-CAP
(http://bejerano.stanford.edu/mcap/index.html) predicted that the mutation would cause
functional damage (SIFT score 0.02, M-CAP score 0.043).
Discussion
Here, we report a case of FBPase deficiency with compound heterozygous mutations,
p.Phe90Val and p.Ala177Asp, in FBP1.
In general, urinary organic acid analysis using gas chromatography-mass spectroscopy
(GC/MS) is very useful for screening FBPase deficiency (5). The fructose tolerance
test from our patient showed a high level of glycerol
excretion in the urine, whereas the excretion of glycerol-3-phosphate was at a normal
level.
The mechanism responsible for the normal concentration of glycerol-3-phosphate was
not
immediately clear. Kato et al. (2015) reported a case of FBPase deficiency
in which the excretion level of glycerol-3-phosphate in the urine during a fasting
episode
was at a normal level based on GC/MS analysis after solvent extraction (6). However,
excretion of glycerol-3-phosphate in the same
sample was found to be increased when analyzed using GC/MS with the urease pretreatment
non-extraction method (6). In our case, the normal
value of glycerol-3-phosphate excretion in the urine was a false-negative, although
the
urine sample was analyzed using GC/MS with the urease pretreatment non-extraction
method.
Alternatively, a large amount of fructose during the oral tolerance test resulted
in the
excessive consumption of a derivatizing agent such that glycerol-3-phosphate in the
urine
was not well derivatized and could not be detected in the GC/MS analysis, thus causing
the
insufficient excretion of glycerol-3-phosphate.
FBPase deficiency is a fatal illness and is associated with a particularly high mortality
rate during the neonatal period (3). Therefore, an
early definitive diagnosis by genetic analysis is important for any suspected cases
of this
disease, which then eliminates the need to perform a potentially risky fructose tolerance
test as was done in this case. Urgent treatment of hypoglycemia and appropriate diet
control
can prevent sudden infant death and improve growth in patients with FBPase deficiency.
In
cases in which the first child is diagnosed with FBPase deficiency, genetic analysis
of the
parents is important for carrier detection to predict whether the siblings will be
affected.
In the present case, given that the parents were respective carriers for each of the
detected mutations, genetic analysis of the next child may facilitate early diagnosis
of
FBPase deficiency before the onset of symptoms.
Conflict of Interest: The authors have nothing to declare.