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Medium-Chain Acyl Coenzyme A Dehydrogenase Deficiency
Occurrence in an Infant and His Father
Marc Bodman, BS;
David Smith;
William L. Nyhan, MD, PhD;
Robert K. Naviaux, MD, PhD
Arch Neurol. 2001;58:811-814.
ABSTRACT
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Background Autosomal recessive inborn errors of metabolism often present as life-threatening
disease in infancy and have adverse effects on the nervous system. Parents
are usually heterozygotes. This is true of most disorders of fatty acid oxidation,
which are rare and present with hypoketotic hypoglycemia. However, the gene
for medium-chain acyl coenzyme A dehydrogenase (MCAD) deficiency is common
in white people, raising the possibility that a parent may be homozygous.
Objective To document the occurrence of MCAD deficiency in a 12-month-old boy
and his father, both of whom were homozygous for the A985G mutation.
Design Clinical observations and definitive biochemical testing.
Setting Children's hospital and university laboratory.
Participants One child and one adult.
Interventions Diagnosis and treatment.
Main Outcome Measures Clinical outcome; analysis results of plasma and urine for carnitine
and organic acids.
Results An infant admitted with an acute illness requiring intensive care was
found to have carnitine deficiency and dicarboxylic aciduria; MCAD deficiency
was diagnosed by assay of his DNA for the common mutation. Test results of
the father revealed him also to be homozygous.
Conclusion In MCAD deficiency, as opposed to the usual rare autosomal recessive
metabolic disease, a parent may also be an affected homozygote.
INTRODUCTION
MEDIUM-CHAIN acyl coenzyme A dehydrogenase (MCAD) (EC 1.3.99.3) catalyzes
the ß-oxidation of straight-chain acyl-coenzyme A esters from C4 to C12
units in length. An MCAD deficiency is an inborn error of metabolism in which
fatty acid oxidation is impaired. It is an autosomal recessive disorder affecting
approximately 1 in 10 000 white people. The most frequent cause of MCAD
deficiency is a point mutation that changes an adenosine (A) to a guanine
(G) at nucleotide 985, which results in a lysine to glutamic acid change in
position 329 of the protein.1 Clinical manifestations
of the disorder are typically hypoglycemia with impaired ketogenesis and secondary
deficiency of carnitine. It is the purpose of this report to describe a boy
with MCAD deficiency who was homozygous for the A985G mutation and whose father
was also homozygous for this mutation.
METHODS
Organic acid analysis was performed using gas chromatography–mass
spectrometry.2 Urine and plasma levels of carnitine
were determined with a radioisotopic enzyme assay.3, 4
Analysis of DNA for the A985G mutation was carried out by polymerase
chain reaction to amplify a 63–base pair (bp) fragment with mismatched
oligonucleotide primers.5, 6 The
product was digested with the restriction endonuclease NcoI and analyzed by electrophoresis on 12% polyacrylamide gel. With
the mismatched primer, the mutant DNA yielded a 43-bp fragment that could
be distinguished from the normal 63-bp fragment.
REPORT OF CASES
CASE 1
A 12-month-old -old white infant was admitted to the pediatric intensive
care unit at Children's Hospital and Health Center in San Diego, Calif, because
of diarrhea, an upper respiratory tract infection, and lethargy. Parents were
not consanguineous. For the first 9 months of his life, his mother had awakened
him at 2 AM for a feeding, but at age nine months he began to sleep through
the night. At about this time the mother began to incorporate meals of solid
food and to develop more of a 3-meals-a-day program. Nine days prior to admission
he had been treated with an antibiotic because of a diagnosis of otitis media,
but this treatment was discontinued after 3 days. Vomiting and diarrhea began
2 days prior to admission. He was brought to the emergency department where
a barium enema was administered because the presence of Hemoccult-positive
(Beckman Coulter Inc, Fullerton, Calif) stool and lethargy suggested the possibility
of intussusception. Urine output was thought to be reduced.
On admission he appeared sleepy but was rousable. He could track with
his eyes but had little interaction with the examiner. His temperature was
36.7°C; pulse, 105 beats/min; respirations, 30 breaths/min; and blood
pressure, 96/40 mm Hg. His extremities were cool and blue, and slow capillary
refill indicated poor perfusion. Nasal mucous membranes were inflamed. He
appeared acutely and seriously ill. The liver was palpable 4 cm below the
costal margin on the right and had a firm and smooth contour. Deep tendon
reflexes were normal, and there was no clonus. Findings of physical examination
were otherwise unremarkable.
Laboratory findings included the following values in millimoles per
liter: sodium, 136; potassium, 3.3; chloride, 104; and bicarbonate ions, 14.
The leukocyte count was 18.3 x 109/L with 56% segmented polymorphonuclear
cells and 28% band forms. The hemoglobin level was 104 g/L; hematocrit, 0.31;
and platelet count 436 x109/L. Serum activity of creatine
phospho kinase was elevated at 796 U/L and rose to 1077 U/L the next day.
The aspartate aminotransferase reading was 167 U/L; alanine aminotransferase,
120U/L; and  -glutamyltransferase, 16 U/L. The serum urea nitrogen was
15 mmol/L (40 mg/dL) and creatinine, 44 µmol/L. The initial blood glucose
level was 1.83 mmol/L (33 mg/dL) rising to 16.65 mmol/L (300 mg/dL) after
intravenous administration of a 2 mL/kg bolus of 10% glucose. The albumin
level was 33 g/L; calcium, 2.03 mmol/L (8.1 mg/dL); glucose in the cerebrospinal
fluid, 5.38 mmol/L; and ammonia, 56.9 µmol/L (97 µg/dL). Findings
of the urine test for ketones revealed a trace amount. The uric acid level
was not measured until day 3, at which time it was 172.5 µmol/L. Culture
findings of the blood, urine, and cerebrospinal fluid were negative. Results
of viral studies were positive for rotavirus and respiratory syncytial virus.
The patient was treated with parenteral fluids containing glucose, saline,
and potassium acetate, along with added sodium bicarbonate until the serum
bicarbonate became normal on day 2. In addition, ceftriaxone sodium was given
intravenously for 24 hours.
Atrial bigeminy developed on day 2 after an initial period of stability,
and a 2:1 block was noted. Findings of an echocardiogram revealed normal cardiac
anatomy. Cardiac output was low, and the patient was treated with dopamine
hydrochloride, dobutamine hydrochloride, atropine sulfate, and isoproterenol
hydrochloride. The arrhythmia resolved promptly and findings of electrocardiography
became normal. Peripheral perfusion and mental status were considered to be
normal after the initial hydration. The creatine kinase level was 174 U/L
on day 4.
Urine was analyzed for the pattern of organic acid excretion and carnitine,
and plasma for amino acid and carnitine (Table 1). The patient was discharged after 5 days in the hospital.
Once a diagnosis of MCAD deficiency was made, the parents were counseled about
the avoidance of fasting, a cornstarch supplement was prescribed at bedtime,
and 1 g of carnitine was given daily.
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Carnitine Concentrations in Plasma and Urine
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CASE 2
When patient 2 (the 29-year-old father of the patient in case 1) and
his wife were told of the diagnosis of their son's illness and its nature
and treatment, the history emerged that patient 2 at age 11 months had developed
what was called hypoglycemic shock after a period of extended fasting. This
resolved, but he experienced a second episode soon after that. Thereafter
he had studiously avoided fasting. He would eat at other than normal mealtimes
and would often wake up from sleep to eat. In addition, throughout his childhood,
family members were encouraged to give him carbohydrates to prevent a recurrence.
Blood was sent for molecular study, and he was found to be homozygous for
the A985G mutation. Carnitine therapy was initiated. Patient 2 has one sibling
to whom testing for the mutation has been offered.
RESULTS
The findings of analysis of concentrations of amino acids in the plasma
of case 1 were notable for a concentration of alanine of 642 µmol/L.
Analysis of organic acids of the urine was carried out the day prior to discharge
when he was clinically well, and the results revealed that the suberylglycine
level was 12 mmol per mole of creatinine; adipic acid, 14 mmol per mole of
creatinine; suberic acid, 12 mmol per mole of creatinine; and hexanoylglycine,
29 mmol per mole of creatinine. The carnitine level in the plasma was low
(Table 1). The excretion of esterified
carnitine in the urine was high. The esterified carnitine–free carnitine
ratios were high.
The findings of the organic acid analysis in the urine of patient 2
were unremarkable. His plasma free carnitine level was low (Table 1). In the urine, the esterified carnitine–free carnitine
ratio was high.
Analysis of genomic DNA extracted from blood and amplified prior to
restriction enzyme digestion revealed the 43-bp fragmentation in each patient
and the expected 63-bp fragment in controls, indicating that both patients
were homozygous for the A985G mutation (Figure
1). In the brother and the mother of patient 1, the presence of
both the 63-bp and 43-bp fragments indicated heterozygosity.
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Pedigree of the family and DNA analysis findings indicating homozygosity
for the proband (arrow) and his father for the A985G mutation. The mother
and brother were heterozygotes. Gel lane M shows size markers of a digest
(Msp 1) of pBR322 DNA; lane 1, the mother with 63–base pair (bp) and
43-bp fragments; lane 2, the proband; lane 3, his brother; lane 4, the father;
lane 5, a positive control (PC); and lane 6, a negative control (NC).
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COMMENT
In dealing with families of patients with inherited metabolic disease,
it is generally expected that the parents are heterozygotes and asymptomatic.
This is because most disorders are the result of rare autosomal recessive
genes. However, MCAD deficiency is different in that the gene is relatively
common in white populations. Results of a literature search showed that this
situation has occurred previously. Heptinstall et al7
described in 1995 a family in which the father and his proband daughter were
both homozygous for the A985G mutation. In this family the proband presented
at age 6 months with a life-threatening hypoglycemic episode, while the father
was asymptomatic. He had always had a predilection for sweet food. A brother
of the proband was found to be homozygous by analysis of cord blood. Nevertheless,
the possibility that a parent and child are both homozygous is not generally
considered.
In our kindred, direct transmission of the mutant alleles from each
parent to case 1 is the logical explanation. The alternative uniparental disomy
is unlikely. The importance of these experiences is the utility of investigation
for homozygosity in a parent of a child with a prevalent mutation. This is
particularly relevant to MCAD deficiency, a disorder in which sudden death
can be an initial presentation at any age.
An MCAD deficiency usually presents first in infants between ages 6
months and 2 years. In the usual situation, acute intercurrent infection is
followed by decreased oral intake and vomiting. Fatty acid oxidation is required
when glycogen stores are depleted, and this usually occurs after 12 hours
or more of fasting. At the same time, it is clear that some individuals can
live many years before experiencing a long fast as a teenager6
or adult.
Our proband was admitted to the hospital at age 12 months with dehydration
and lethargy, and 2 different viral infections were found: a rotavirus and
respiratory syncytial virus. Thus, he seemed to have adequate cause for serious
infantile illness. He was not considered clinically hypoglycemic, but the
initial concentration of glucose was 1.83 mmol/L (33 mg/dL). It was only the
actions of an alert infectious disease consultant that led to the submission
of the urine sample for organic acid analysis, which led to the diagnosis.
Classic MCAD deficiency presents with hypoketotic hypoglycemia. Initial urinalysis
indicated a trace amount of ketones, and even a small amount can be misleading.
The elevated creatine kinase levels could have served as an alerting marker.
Increased levels of creatine kinase and uric acid characterize acute episodes
of metabolic imbalance in disorders of fatty acid oxidation.1
Findings of uric acid status were not available immediately; most clinical
chemical analysis panels in children's hospitals do not include uric acid.
The significance of the elevated creatine kinase level is highlighted by the
occurrence of the cardiac arrhythmia. This is another way in which disorders
of fatty acid oxidation may become life-threatening. In the index patient,
all of the clinical manifestations of acute disease resolved promptly, and
he was sent home.
The dicarboxylic aciduria of MCAD-deficient individuals is characteristically
evident only at times of acute illness. The father's urine was unrevealing.
The pattern includes elevated amounts of adipic (C6), suberic (C8), and sebacic
(C10) acids and the glycine conjugates hexanoylglycine and suberylglycine.
Phenylpropionylglycine excretion is helpful when present, but in most acutely
ill patients in the intensive care unit, this compound is absent because of
antibiotic therapy. Both patients were found to be deficient in plasma free
carnitine and to have increased amounts of esterified carnitine in the urine.
The accumulation of coenzyme A esters of carboxylic acids results in esterification
with carnitine and preferential excretion in the urine. The consequent depletion
of body stores of free carnitine may further exacerbate the disordered fatty
acid oxidation in this disease.
The relative frequency of MCAD deficiency makes the assembly of rigorous
epidemiological data possible. In a study from the Netherlands,8
the incidence of the common A985G mutation in 6195 newborn screening cards
gave a carrier frequency of 1 in 55 persons. Calculations based on Hardy-Weinberg
equilibrium8 predict a homozygote frequency
of 1 in 12 000. The odds that a parent might be a homozygote would doubtless
be lower because there are no data on the proportion of homozygotes surviving
to a childbearing age. In a prospective surveillance report from England,
Pollit and Leonard9 found a disease frequency
of 1 in 22 222. There are few data on other ethnic groups. Among gypsies
in Spain,10 a carrier frequency for the common
mutant allele was 1 in 17, which led to an estimated frequency of homozygosity
of 1 in 1156.
AUTHOR INFORMATION
Accepted for publication June 12, 2000.
Partial funding from this work came through grant MO1 RR008727 from
the General Clinical Research Center Program, National Center for Research
Resources, National Institutes of Health, Bethesda, Md, and a grant from the
Lennox Foundation, Dallas, Tex.
From the Department of Pediatric/Biochemical Genetics, University of
California, San Diego.
Reprints: William L. Nyhan, MD, PhD, Department of Pediatric/Biochemical
Genetics, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA
92093-0830 (e-mail: wnyhan{at}ucsd.edu).
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