Cytochrome c Oxidase Deficiency Due to a Novel SCO2 Mutation Mimics Werdnig-Hoffmann Disease FREE

CYTOCHROME c oxidase (COX) deficiency is arguably the most frequent mitochondrial abnormality in children, usually presenting as Leigh syndrome. In the past few years, several molecular defects have been associated with COX deficiency, including mutations in 3 mitochondrial DNA–encoded COX subunits and in 4 nuclear COX–assembly genes: SURF1,¹SCO2,²SCO1,³ and COX10.⁴

The SCO2 gene encodes a 266 amino acid protein that is imported into the mitochondrion and is probably required for the correct assembly of copper in the holoenzyme.⁵ Mutations in the SCO2 gene have been associated with a fatal infantile form of cardioencephalomyopathy. At or soon after birth, patients develop severe hypertrophic cardiomyopathy, encephalopathy, and myopathy, and they die of cardiac failure within the first year of life. Cytochrome c oxidase activity is markedly reduced in muscle, heart, and brain, while liver and fibroblasts are less severely affected.² We report a novel SCO2 mutation associated with severe involvement of spinal cord motor neurons and clinical and pathologic features resembling spinal muscular atrophy type I (Werdnig-Hoffmann disease).

A 3.2-kg girl was born from healthy, unrelated parents at 39 weeks' gestation by cesarean section because of a breech presentation. She required delivery room resuscitation for respiratory distress. She had generalized weakness and hypotonia, with minimal spontaneous movements, weak cry, and stridor. Grasp and suck reflexes were absent and she had a high-arched palate. In the following days, tongue fasciculations became apparent.

Results of routine blood tests were normal. Serum creatinine, ammonia, amino acids, and urinary organic acid levels were normal. The blood lactate level was elevated (48.6 mg/dL; reference range, <19.8 mg/dL) while pyruvate level was normal.

A chest x-ray film was normal at birth, but at 4 weeks showed heart enlargement. Echocardiography revealed hypertrophic cardiomyopathy. Brain computed tomographic and magnetic resonance imaging scans were normal.

An electromyogram showed spontaneous activity in several muscles. Motor unit amplitudes and durations were slightly decreased. Nerve conduction studies were unsuccessful due to electrical interference in the intensive care unit. Results of the mutation screening of the SMN gene was negative.

The child's condition rapidly worsened. She was intubated after an episode of aspiration and attempts to wean her from the mechanical ventilator were unsuccessful. She died of heart failure at 7 weeks of age.

Histochemical analyses of muscle biopsy specimens and postmortem tissue samples were performed as described previously.⁶ The activities of respiratory chain enzymes were measured in muscle extracts as described.⁷

Genomic DNA was isolated from the patient's muscle as described.⁸ The SCO2 gene region was amplified as described.² Polymerase chain reaction products were sequenced with internal primers using the ABI PRISM Dye Terminator Cycle Sequencing Ready Reaction Kit and the 310 Automatic Sequencer (Applied Biosystem, Perkin Elmer, Foster City, Calif). Polymerase chain reaction fragments were also subcloned in a pCRIITOPO Vector using a Topo TA Cloning kit (Invitrogen, Carlsbad, Calif). Clones were sequenced using internal SCO2 primers.

Restriction fragment length polymorphism analysis of the E140K mutation was performed as described.²

Analysis of respiratory chain enzyme activities in muscle extracts showed severe isolated COX deficiency (0.18 µmol/min per gram of tissue; mean [SD] control value, 2.80 [0.52] µmol/min per gram of tissue).

Routine histologic examination of muscle revealed neurogenic abnormalities, with large groups of atrophic fibers and separate groups of hypertrophic fibers (Figure 1A). Histochemistry showed normal checkerboard staining for succinate dehydrogenase, while COX activity was virtually undetectable in both types I and II fibers and in intramuscular blood vessels (Figure 1B).

Findings from a pathologic examination of the brain were unremarkable. In the ventral horns of the spinal cord, there was moderate to severe loss of motor neurons and astrocytosis (Figure 1D). Sparse remaining motor neurons showed central chromatolysis and rare large eosinophilic inclusions. The ventral roots showed loss of nerve fibers and glial bundles that expressed glial fibrillary acidic protein.

Sequencing of the SCO2 gene revealed the E140K mutation (Figure 2A) and an insertion at nucleotide (nt) 1312. To better define this insertion, we amplified the patient's SCO2 gene and subcloned the fragment in pCRIITOPO Vector. Clones were screened by restriction fragment length polymorphism for the presence of the E140K mutation. Negative clones were sequenced and revealed a 10–base pair duplication of nt-1312 to nt-1321 (Figure 1 2B) that disrupts the reading frame of the messenger RNA and gives rise to a truncated 83 amino acid polypeptide in which the 21 amino acids at the C-terminus are abnormal (Figure 2C). This is the first nucleotide insertion reported in the SCO2 gene. Analysis of DNA from the parents showed that the father carried the E140K mutation and the mother carried the 10–base pair insertion.

We have also performed prenatal diagnosis on amniocytes from a second pregnancy of this couple. The fetus was heterozygous for the E140K mutation, but the other allele was normal. This second child, a boy, is 2 months old and appears healthy.

Mutations in the SCO2 gene cause fatal infantile cardioencephalomyopathy characterized by severe COX deficiency in affected organs. All patients described to date harbored a common E140K mutation. While the first 6 patients were compound heterozygotes for this mutation and another point mutation,^2,9 3 patients homozygous for the E140K mutation have also been reported. These children had a milder clinical phenotype, with later onset and slower progression.¹⁰

Hypertrophic cardiomyopathy is the clinical hallmark of the disease and is usually the cause of death. However, as illustrated by our patient, it is not always the presenting symptom. Encephalopathy is also present, but does not have the typical neuroradiological or neuropathological features of Leigh syndrome. This is in contrast to mutations in SURF-1, another COX-assembly gene, which are consistently associated with the Leigh syndrome phenotype.¹¹

Brain involvement varied in the 3 patients with SCO2 mutations previously described by us.² The first patient had early capillary proliferation reminescent of Leigh syndrome. The second patient had necrosis of the globus pallidus and athrophy of the hippocampus, deep gray nuclei, white matter, and cerebellum. The third infant had changes suggestive of a migrational disorder, with patches of cortical dysplasia in the left temporal lobe and focal heterotopia in the cerebellum. The spinal cord was studied in 2 of these patients. One showed atrophy of the descending spinal tracts, diffuse gliosis, and moderate patchy loss of motor neurons. The other had mild gliosis and white matter spongiosis.

Conversely, the patient described herein had no brain abnormalities, but the pathologic changes in the spinal cord were severe and closely resembled those of Werdnig-Hoffmann disease. There were, however, subtle differences from the typical neuropathological features of Werdnig-Hoffmann disease, including abnormal shape of many motor neurons, absence of typical ballooned cells, more severe involvement of the sensory component of the peripheral nervous system, and minor changes in the pyramidal tracts.

In the muscle biopsy specimen, neurogenic changes were striking, suggesting that the severe weakness and hypotonia of this patient were not simply due to a COX-deficient myopathy, but were largely attributable to denervation. Neurogenic changes in muscle were also seen in patients with the milder clinical phenotype, who were homozygous for the E140K mutation, but spinal cord pathology was not described. Our data indicate that denervation in our patient was not due to involvement of the peripheral nervous system, but rather to alterations of the anterior horn cell of the spinal cord, as in Werdnig-Hoffman disease.

Mutations in the SMN gene are present in more than 95% of the patients with Werdnig-Hoffmann disease,¹² but were excluded in our patient. Hence, the anterior horn disease in this child seems to be a phenocopy of Werdnig-Hoffmann disease caused by the SCO2 mutations.

The reasons for the variability in the clinical expression of SCO2 mutations are not totally clear. Patients homozygous for the E140K mutation show a milder phenotype, perhaps because the mutant protein is still partially active.¹⁰ However, there is no clear correlation between type of mutation and type of neurological involvement.

This case illustrates the importance of screening for SCO2 mutations even in patients without the typical phenotype, and especially in children with features of SMA but without mutations in the SMN gene. Knowledge of the molecular defect will, as in this case, make prenatal diagnosis and accurate genetic counseling possible for young couples who have already lost a child.

Accepted for publication January 14, 2002.

Author contributions: Study concept and design (Drs Sacconi, Kronn, Shanske, and DiMauro); acquisition of data (Drs Salviati, Sacconi, Rasalan, Kronn, Braun, Canoll, Shanske, Bonilla, and Hays); analysis and interpretation of data (Drs Salviati, Sacconi, Davidson, Shanske, Hays, and DiMauro); drafting of the manuscript (Drs Salviati, Sacconi, Rasalan, and Shanske); critical revision of the manuscript for important intellectual content (Drs Sacconi, Kronn, Braun, Canoll, Davidson, Bonilla, Hays, and DiMauro); obtained funding (Drs Shanske and DiMauro); administrative, technical, and material support (Drs Sacconi, Braun, Davidson, Shanske, and DiMauro); study supervision (Drs Sacconi, Kronn, Davidson, Shanske, Bonilla, and Hays).

This study was supported by grants PO1HD32062 and NS11766 from the National Institutes of Health, Bethesda, Md, and by a grant from the Muscular Dystrophy Association, Tucson, Ariz. Dr Salviati is supported by grant 439b from Telethon Italia, Rome, Italy.

Corresponding author and reprints: Salvatore DiMauro, MD, 4-420 College of Physicians and Surgeons, 630 W 168th St, New York, NY 10032 (e-mail:sd12@columbia.edu).