Early-onset severe neuromuscular phenotype associated with compound heterozygosity for OPA1 mutations

https://doi.org/10.1016/j.ymgme.2011.04.018Get rights and content

Abstract

Introduction

Pathogenic mutations in the OPA1 gene are the most common identifiable cause of autosomal dominant optic atrophy (DOA), which is characterized by selective retinal ganglion cell loss, a distinctive pattern of temporal pallor of the optic nerve and a typical color vision deficit, with variable effects on visual acuity. Haploinsufficiency has been suggested as the major pathogenic mechanism for DOA. Here we present two siblings with severe ataxia, hypotonia, gastrointestinal dysmotility, dysphagia, and severe, early-onset optic atrophy who were found to be compound heterozygotes for two pathogenic OPA1 mutations. This example expands the clinical phenotype of OPA1-associated disorders and provides additional evidence for semi-dominant inheritance.

Methods and results

Molecular analysis of the OPA1 gene in this family by Sanger sequencing revealed compound heterozygosity for two mutations in trans configuration, a p.I382M missense mutation and a p.V903GfsX3 frameshift deletion in both affected siblings. Electron microscopy of a skeletal muscle biopsy of the older sibling revealed dense osmiophilic bodies within the mitochondria. Mitochondrial DNA (mtDNA) content was within normal limits, and electron transport chain analysis showed no deficiencies of the mitochondrial respiratory chain enzymes. Multiple mtDNA deletions were not found.

Conclusion

Compound heterozygosity of pathogenic OPA1 mutations may cause severe neuromuscular phenotypes in addition to early-onset optic atrophy. While a role for OPA1 in mtDNA maintenance has been discussed, compound biallelic pathogenic OPA1 mutations in our patients did not result in altered mtDNA copy number, mtDNA deletions, or deficiencies of the electron transport chain, despite the severe clinical phenotype.

Introduction

Autosomal dominant optic atrophy (ADOA, OMIM 165500) is the most common inherited optic atrophy, with an estimated prevalence of 1 in 35,000 or higher [1]. In Denmark, the prevalence is estimated as high as 1 in 10,000, likely attributable to a founder effect. The optic atrophy in ADOA is characterized by insidious onset of visual impairment, typically during the first two decades of life, leading to moderate to severe loss of visual acuity, temporal (more than nasal) optic disk pallor in the papillomacular bundle, characteristic color vision (tritan) deficits, and centrocecal, central, or paracentral visual field defects [2].

As many as 20% of individuals with ADOA have additional, extra-ocular neurological signs and complications, among them sensorineural hearing loss, ataxia, and myopathy. The term “dominant optic atrophy plus” has been applied to these variants [3].

Most ADOA cases are caused by mutations in the OPA1 gene, which encodes the human homolog of the Schizosaccharomyces pombe dynamin-related protein Msp1. More than 200 different mutations of the OPA1 gene have been reported to date, most of them resulting in protein truncation. Consequently, haploinsufficiency has been proposed as the major pathogenic mechanism for ADOA. Interestingly, missense mutations have been associated with more severe phenotypes and most commonly with hearing loss (typically “ADOA plus”) when compared to nonsense mutations [3], suggesting a dominant-negative mechanism for these variants.

A total of four ADOA patients have been reported who were found to each carry two mutations in the OPA1 gene [3], [4]. A 30 year-old woman, compound heterozygous for a p.E270K and a p.R290W mutation, had both severe impairment of visual acuity and total pallor of optic nerves in each eye. She was described to have a more severe clinical phenotype than either one of her heterozygous parents or her siblings, providing some evidence for semi-dominant rather than pure dominant inheritance of ADOA. Interestingly, the compound heterozygous subject was not described to have any non-ocular neurological features [4]. Two German siblings were found to both carry a p.K212fsX4 and a p.V548I mutation; however detailed clinical information was not available and in the absence of parental samples, it could not be determined if these mutations were on the same allele or compound heterozygous. The same applies to a 60 year-old Norwegian proband heterozygous for the p.S256R and p.Q285R mutations, for whom parental follow-up was not available [3].

Here we report two siblings with severe, early-onset optic atrophy, ataxia, hypotonia, dysphagia, and gastrointestinal dysmotility, who were compound heterozygous for two pathogenic OPA1 mutations, thereby expanding the clinical phenotype of OPA1-associated disorders and providing additional evidence for semi-dominant inheritance.

An 8 year-old boy and his 3 year-old sister presented to the Pediatric Genetics Clinic (M.B.) for evaluation of ataxia and optic atrophy.

The boy was born at 30 weeks gestational age by spontaneous vaginal delivery to a 32 year-old G1 mother. The pregnancy was complicated only by preterm labor. His birth weight was 2097 g. His postnatal course was complicated by grade II intraventricular hemorrhage. Serial ophthalmoscopic examinations did not reveal retinopathy of prematurity. He had exotropia at age 1 year, for which he was treated with patching and corrective surgery. At the same time, he was noted to have optic nerve atrophy in each eye. By age 3 years, the boy was noted to have nystagmus, and his optic nerve atrophy had progressed to the point that he was declared legally blind. He was hypotonic during infancy and was noted to have substantial ataxia when attempting to stand at around 1 year of age. He was not able to walk without support until the age of 2.5 years. At the age 8 years, he is still able to walk independently but with profound ataxia and an unsteady, stomping gait with heavy heel strikes, as well as postural instability, suggestive of a sensory ataxia and peripheral neuropathy. His review of systems is significant for dysphagia, choking episodes, and vomiting, starting at age 4 years. Decreased intestinal motility with profound constipation ensued, requiring medication and intermittent disimpaction. He had brain MR imaging at age 2 years and was found to have mild periventricular leukomalacia. The cerebellum appeared normal in structure and size. Echocardiography revealed a structurally normal heart, and he had normal CK levels and a normal radiographic bone age study.

His 3 year-old sister was born at 36 weeks gestational age by spontaneous vaginal delivery to her then 36 year-old G2 mother. The pregnancy was uneventful and the postnatal course was uncomplicated. She was diagnosed with optic nerve atrophy O.U. at the age of 6 months and developed nystagmus at 1 year. She displayed profound hypotonia and ataxia and was not able to walk without support until the age of 3 years. Dysphagia and constipation were present clinically but not yet with the same severity observed in her older brother.

At age 8 years, the boy was able to count fingers at about 15 cm eccentrically with each eye. The anterior segment and biomicroscopic examination was structurally normal in each eye, except for the healed conjunctival scars over the medial and lateral rectus muscles in both eyes. Dilated fundus examination showed diffuse severe optic atrophy in each eye (Fig. 1A and B). No foveal light reflex or umbo was present in either eye, evidence of profound inner retinal atrophy. Attempts at formal color vision testing could not be completed because of his extremely poor acuity. Audiological testing revealed normal hearing sensitivity with normal middle-ear function. Ipsilateral acoustic reflexes were present but the contralateral acoustic reflexes were present only at 500 Hz with sound presented to the right ear. Distortion product otoacoustic emissions were present bilaterally but with reduced amplitudes, possibly reflecting subtle changes in cochlear mechanics not yet manifested in the behavioral audiogram.

For the 3 year-old girl, her poor visual acuity could not be measured because of her lack of cooperation; however, she avoided objects and moved around unaided. Her pupil function showed a slow direct response to light, but otherwise the anterior segments were structurally normal. Dilated fundus examination revealed diffuse advanced optic atrophy in an otherwise normal-sized optic disk of each eye. She had no foveal light reflex or umbo in either macula, consonant with her advanced optic atrophy. Due to her lack of behavioral cooperation, color vision testing and retinal photography were unobtainable. Audiological testing with condition-play audiometry revealed essentially normal hearing bilaterally. Distortion product otoacoustic emissions were present and robust. Acoustic immittance measures indicated normal middle-ear function; however there were subtle abnormalities in both the ipsilateral and contralateral acoustic reflexes.

Given the suspicion for an inherited optic atrophy, both parents underwent ophthalmologic evaluation and dilated ophthalmoscopic examination. The father had been wearing glasses since his early 20s and was aware of his difficulty with color discrimination. His best corrected visual acuity was 20/25 OD and 20/30 + 2 OS. Upon examination, results of his anterior segment, motility, and biomicroscopic examination were normal. Dilated fundus examination was normal except for the optic nerves and macular areas. The optic nerves on each side were normal in size, cup, and contour, but there was distinct temporal pallor of the papillomacular bundle of each optic nerve, with a clear demarcation between the striated ganglion cell layer, which was missing from approximately 8 o'clock to approximately 10:30 o'clock in the right eye, and in the comparable temporal sector of the left eye. The umbo was minimally detectable and there was no foveal light reflex in either eye. Formal color vision testing demonstrated a distinct tritan error with minimal effect on the protan and deutan axes. Audiological evaluation revealed mild high frequency sensorineural hearing loss in the right ear and mild low frequency sensorineural hearing loss in the left. Acoustic immittance measures indicated normal middle-ear function. Distortion product otoacoustic emissions were present and of normal amplitude in the right ear but of reduced amplitude above 4 kHz in the left ear.

The mother had a myopic refractive error of approximately − 4.75 sphere in each eye yielding normal visual acuity (20/20 O.U.). External, adnexal, and biomicroscopic examination were normal. Dilated fundus examination including each optic nerve was normal. Formal color vision testing was normal. Audiological evaluation was significant for bilateral, mild low frequency sensorineural hearing loss with normal middle-ear function. Otoacoustic emissions amplitude was below 2 kHz, suggestive of decreased outer hair cell function.

Section snippets

Patients and samples

Both probands were reassessed through the Pediatric Genetics Clinic at Texas Children's Hospital, Houston, TX. The probands and their parents were enrolled in research protocols approved by the Institutional Research Boards for Human Subject Research at Baylor College of Medicine, Houston, TX. Total DNA was extracted from muscle and blood specimens with DNA isolation kits (Gentra Systems Inc., Minneapolis, MN, USA) according to the manufacturer's protocols.

Molecular analysis

Sequence-specific oligonucleotide

Results

Sanger sequencing of DNA obtained from peripheral blood revealed compound heterozygosity of a frameshift mutation, c.2708_2711delTTAG (p.V903GfsX3) in exon 27, and a missense mutation, c.1146A > G (p.I382M) in exon 12, in both the 8 year-old boy and his 3 year-old sister. Both mutations have been reported previously in heterozygous state to be associated with ADOA [11], [12]. Concurrent parental analyses revealed that the father is heterozygous for the p.V903GfsX3 mutation and the mother is

Discussion

We present the clinical and molecular characterization of two siblings who are compound heterozygotes for disease-causing mutations of the OPA1 gene. To our knowledge, these represent the second and third reported cases of OPA1 compound heterozygosity.

While a total of four individuals with two OPA1 mutations have been reported, compound heterozygosity has only been proven for one of these four. Pesch et al. described a female who was compound heterozygous for two missense mutations (p.E270K and

Acknowledgments

We thank P. Zimmerman and V. Gudell for their help in scheduling the patients' and family's appointments. We thank Zbigniew Krason C.R.A. for the retinal imaging and color vision testing. This investigation was supported in part by an unrestricted award to R.A.L. from Research to Prevent Blindness, New York, N.Y.

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