Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease

OBJECTIVES

Homoplasmic maternally inherited, m.14674T>C or m. 14674T>G mt-tRNA(Glu) mutations have recently been identified in reversible infantile cytochrome c oxidase deficiency (or 'benign COX deficiency'). This study sought other genetic defects that may give rise to similar presentations.

PATIENTS

Eight patients from seven families with clinicopathological features of infantile reversible cytochrome c oxidase deficiency were investigated.

METHODS

The study reviewed the diagnostic features and performed molecular genetic analyses of mitochondrial DNA and nuclear encoded candidate genes.

RESULTS

Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life. Although recovery was remarkable, a mild myopathy persisted into adulthood. Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres. Biochemical studies suggested more generalised abnormalities than pure COX deficiency. Clinical improvement was reflected by normalisation of lactic acidosis and histopathological abnormalities. The m.14674T>C mt-tRNA(Glu) mutation was identified in four families, but none had the m. 14674T>G mutation. Furthermore, in two families pathogenic mutations were also found in the nuclear TRMU gene which has not previously been associated with this phenotype. In one family, the genetic aetiology still remains unknown.

CONCLUSIONS

Benign COX deficiency is better described as 'reversible infantile respiratory chain deficiency'. It is genetically heterogeneous, and patients not carrying the m.14674T>C or T>G mt-tRNA(Glu) mutations may have mutations in the TRMU gene. Diagnosing this disorder at the molecular level is a significant advance for paediatric neurologists and intensive care paediatricians, enabling them to select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy.

Muscle histology vs MRI in Duchenne muscular dystrophy

OBJECTIVE

There are currently no effective treatments to halt the muscle breakdown in Duchenne muscular dystrophy (DMD), although genetic-based clinical trials are being piloted. Most of these trials have as an endpoint the restoration of dystrophin in muscle fibers, hence requiring sufficiently well-preserved muscle of recruited patients. The choice of the muscles to be studied and the role of noninvasive methods to assess muscle preservation therefore require further evaluation.

METHODS

We studied the degree of muscle involvement in the lower leg muscles of 34 patients with DMD >8 years, using muscle MRI. In a subgroup of 15 patients we correlated the muscle MRI findings with the histology of open extensor digitorum brevis (EDB) muscle biopsies. Muscle MRI involvement was assigned using a scale 0-4 (normal-severe).

RESULTS

In all patients we documented a gradient of involvement of the lower leg muscles: the posterior compartment (gastrocnemius > soleus) was most severely affected; the anterior compartment (tibialis anterior/posterior, popliteus, extensor digitorum longus) least affected. Muscle MRI showed EDB involvement that correlated with the patient's age (p = 0.055). We show a correlation between the MRI and EDB histopathologic changes, with MRI 3-4 grades associated with a more severe fibro-adipose tissue replacement. The EDB was sufficiently preserved for bulk and signal intensity in 18/22 wheelchair users aged 10-16.6 years.

CONCLUSION

This study provides a detailed correlation between muscle histology and MRI changes in DMD and demonstrates the value of this imaging technique as a reliable tool for the selection of muscles in patients recruited into clinical trials.

Charcot-Marie-Tooth (CMT) disease 1A with superimposed inflammatory polyneuropathy in children

Charcot-Marie-Tooth (CMT) disease is genetically heterogeneous and subdivided into demyelinating (CMT 1) and axonal (CMT 2) types based on neurophysiology findings. CMT1A, the commonest form associated with duplication of the PMP22 segment on chromosome 17p, often arises in childhood but is generally a slowly progressive disease. We report 2 children presenting with clinical features of an acute inflammatory demyelinating polyneuropathy (AIDP) who were subsequently diagnosed with underlying CMT1A. Both children had neurophysiology and histopathology features consistent with CMT1. Immunoglobulin treatment was initiated considering the evidence of superimposed inflammation and appeared to modify disease progression. Our findings indicate that CMT1A predisposes to a superimposed inflammatory neuropathy. Recognition of this association is difficult, particularly in children without clear family history, but of great importance as immunomodulatory treatment may improve outcome. In addition, we postulate that an underlying genetic polyneuropathy should be suspected if the recovery from AIDP is slower than expected, or incomplete.

Joint hypermobility as a distinctive feature in the differential diagnosis of myopathies

Congenital and adult-onset inherited myopathies represent a wide spectrum of syndromes. Classification is based upon clinical features and biochemical and genetic defects. Joint hypermobility is one of the distinctive clinical features that has often been underrecognized so far. We therefore present an overview of myopathies associated with joint hypermobility: Ullrich congenital muscular dystrophy, Bethlem myopathy, congenital muscular dystrophy with joint hyperlaxity, multi-minicore disease, central core disease, and limb girdle muscular dystrophy 2E with joint hyperlaxity and contractures. We shortly discuss a second group of disorders characterised by both muscular features and joint hypermobility: the inherited disorders of connective tissue Ehlers-Danlos syndrome and Marfan syndrome. Furthermore, we will briefly discuss the extent and pattern of joint hypermobility in these myopathies and connective tissue disorders and propose two grading scales commonly used to score the severity of joint hypermobility. We will conclude focusing on the various molecules involved in these disorders and on their role and interactions in muscle and tendon, with a view to further elucidate the pathophysiology of combined hypermobility and myopathy. Hopefully, this review will contribute to enhanced recognition of joint hypermobility and thus be of aid in differential diagnosis.

Minicore myopathy with ophthalmoplegia caused by mutations in the ryanodine receptor type 1 gene

BACKGROUND

Minicore myopathy (multi-minicore disease [MmD]) is a congenital myopathy characterized by multifocal areas with loss of oxidative activity on muscle biopsy. MmD is clinically heterogeneous and distinct phenotypes have been associated with recessive mutations in either the selenoprotein N (SEPN1) or the skeletal muscle ryanodine receptor (RYR1) gene, also implicated in central core disease and malignant hyperthermia. External ophthalmoplegia is an additional finding in a subset of patients with MmD.

OBJECTIVE

To clinically and genetically examine families with MmD and external ophthalmoplegia.

METHODS

The authors investigated 11 affected individuals from 5 unrelated families. Clinical, histopathologic, and imaging studies were performed and RYR1 haplotyping and mutational analysis were carried out.

RESULTS

All patients had multiple cores involving the entire fiber diameter on longitudinal sections. Weakness and wasting in the shoulder girdle, scoliosis, moderate respiratory impairment, and feeding difficulties were prominent. In contrast to SEPN1-related myopathies, soleus was more severely affected than gastrocnemius on muscle MRI. Haplotyping suggested linkage to the RYR1 locus in informative families and mutational screening revealed four novel RYR1 mutations in three unrelated families; in addition, functional haploinsufficiency was found in one allele of two recessive cases.

CONCLUSION

These findings expand the phenotypic spectrum associated with mutations in the skeletal muscle ryanodine receptor (RYR1) gene. Recessive mutations of domains commonly affected in malignant hyperthermia appear to be particularly prevalent in multi-minicore disease with external ophthalmoplegia and might suggest a different pathomechanism from that involved in central core disease.

Expanding the phenotype of potassium channelopathy: severe neuromyotonia and skeletal deformities without prominent Episodic Ataxia

We report an unusual family in which the same point mutation in the voltage-gated potassium channel gene KCNA1 resulted in markedly different clinical phenotypes. The propositus presented in infancy with marked muscle stiffness, motor developmental delay, short stature, skeletal deformities, muscle hypertrophy and muscle rippling on percussion. He did not experience episodic ataxia. His mother presented some years later with typical features of Episodic Ataxia type 1 (EA1), with episodes of ataxia lasting a few minutes provoked by exercise. On examination she had myokymia, joint contractures and mild skeletal deformities. A heterozygous point mutation in the voltage-gated K(+) channel (KCNA1) gene (ACG-AGG, Thr226Arg) was found in both. We conclude that mutations in the potassium channel gene (KCNA1) can cause severe neuromyotonia resulting in marked skeletal deformities even if episodic ataxia is not prominent.

Pilot trial of salbutamol in central core and multi-minicore diseases

Several studies have documented positive effects of beta-adrenergic agonists on human skeletal muscle with regard to muscle mass and strength. The aim of this pilot study was to evaluate the effect of the beta2-agonist salbutamol (albuterol) in a group of children with central core disease and multi-minicore disease. Thirteen patients, 8 with central core disease (mean age 17.5 years) and 5 with minicore disease (mean age 13.6 years) received oral salbutamol at a dose of 2 mg four times a day. Measures of efficacy were the change from baseline at 3 and 6 months in muscle strength, assessed by MRC score, myometry, functional measures and forced vital capacity. Statistical analysis was performed using repeated measures ANOVA (significance level < 0.05). Two patients with central core disease stopped the medication after one month because they did not notice any improvement and another one with minicore disease after 4 months because of increased tremors and palpitations. The remaining ten (6 with central core and 4 with minicore disease) completed the course of salbutamol without any significant adverse effects. There were significant increases in myometry, MRC scores and forced vital capacity between baseline and the six-month assessments. For both myometry and MRC the difference was already significant at 3 months and this was associated with a significant increase in functional abilities assessed with a structured functional scale. Our results suggest that salbutamol was overall well tolerated and might be beneficial in both central core and minicore patients. Larger prospective randomised, double-blind, placebo-controlled trials with salbutamol will be needed to confirm these preliminary findings.

X-inactivation patterns in carriers of X-linked myotubular myopathy

X-linked myotubular myopathy is a rare severe muscle disorder in affected male neonates. Most female carriers are free from symptoms. Skewed X inactivation has been proposed to be responsible for the affected phenotype seen in some carriers. We have compared the X inactivation patterns in blood DNA with the clinical phenotype in carriers of X-linked myotubular myopathy. The X-inactivation analysis was performed using HpaII predigestion of DNA followed by polymerase chain reaction of the highly polymorphic CAG repeat of the androgen receptor (AR) gene. The frequency of skewed X inactivation was similar in the X-linked myotubular myopathy carriers (22%) and in 235 controls (18%). Three overtly affected carriers had skewed X inactivation with the mutated X as the predominantly active X in at least two of them. Four females with mild symptoms had random X inactivation. The unaffected X-linked myotubular myopathy carriers had either skewed X inactivation in favour of expression from the normal X or random X-inactivation. Thus, there was a tendency for females with a more severe phenotype to have a skewed pattern of X inactivation, while females with an intermediate phenotype had a random pattern of X-inactivation.

Principal mutation hotspot for central core disease and related myopathies in the C-terminal transmembrane region of the RYR1 gene

The congenital myopathies are a group of disorders characterised by the predominance of specific histological features observed in biopsied muscle. Central core disease and nemaline myopathy are examples of congenital myopathies that have specific histological characteristics but significantly overlapping clinical pictures. Central core disease is an autosomal dominant disorder with variable penetrance which has been linked principally to the gene for the skeletal muscle calcium release channel (RYR1). Two recent reports have identified the 3' transmembrane domain of this gene as a common site for mutations. Two other studies have reported single families that have features of both central core disease and nemaline myopathy (core/rod disease) caused by mutations in RYR1. Screening of the 3' region (exons 93-105) of the RYR1 gene for mutations in 27 apparently unrelated patients with either central core disease or core/rod disease by single strand conformation polymorphism analysis and DNA sequencing identified three described and nine novel mutations in 15 patients.