The molecular neuropathology of the muscular dystrophies: a review and update

MicroRNAs as a Tool for Differential Diagnosis of Neuromuscular Disorders

Neuromuscular disorders (NMD) are a class of progressive disorders that are characterized by wasting of the muscles. Some of the disorders like Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), congenital muscular dystrophies (CMDs), limb-girdle muscular dystrophies (LGMD), and mild spinal muscular atrophy (SMA) type III share several presenting clinical features, and hence, diagnosis is usually a challenging task. In this study, the diagnostic potential of some species of microRNAs (miRNAs) that are known to play roles in normal and pathological contexts of myocytes (myomiRs) were evaluated to assess their potential in differential diagnosis of NMDs. In this study, seventy-four patients with different neuromuscular disorders along with thirty age-matched healthy control subjects were enrolled. Peripheral blood samples were collected from enrolled subjects followed by miRNA extraction and reverse transcription followed by quantification of the circulating levels of the studied miRNAs (miR-499, miR-206, miR-208a, miR-223, miR-191, miR-103a-3p, miR-103a-5p), by real-time PCR and statistical analysis. The data indicated that miR-499 level showed high circulating levels in DMD patients as well as in patients with other related disorders such as BMD. However, the levels of miR-499 were much higher in DMD patients and it can be used to diagnose DMD. In addition, miR-206 can selectively differentiate between DMD and all other disorders. The results also revealed that miR-208a and miR-223 were significantly dysregulated in SMA patients, and miR-103a-3p could distinguish DMD from BMD. The expression levels of some miRNA species can be utilized in the process of differential diagnosis of NMDs and can serve as a diagnostic biomarker, and such findings will pave the way towards generating targeted therapies.

Dystrophin Dp71 and the Neuropathophysiology of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by frameshift mutations in the DMD gene that prevent the body-wide translation of its protein product, dystrophin. Besides a severe muscle phenotype, cognitive impairment and neuropsychiatric symptoms are prevalent. Dystrophin protein 71 (Dp71) is the major DMD gene product expressed in the brain and mutations affecting its expression are associated with the DMD neuropsychiatric syndrome. As with dystrophin in muscle, Dp71 localises to dystrophin-associated protein complexes in the brain. However, unlike in skeletal muscle; in the brain, Dp71 is alternatively spliced to produce many isoforms with differential subcellular localisations and diverse cellular functions. These include neuronal differentiation, adhesion, cell division and excitatory synapse organisation as well as nuclear functions such as nuclear scaffolding and DNA repair. In this review, we first describe brain involvement in DMD and the abnormalities observed in the DMD brain. We then review the gene expression, RNA processing and functions of Dp71. We review genotype-phenotype correlations and discuss emerging cellular/tissue evidence for the involvement of Dp71 in the neuropathophysiology of DMD. The literature suggests changes observed in the DMD brain are neurodevelopmental in origin and that their risk and severity is associated with a cumulative loss of distal DMD gene products such as Dp71. The high risk of neuropsychiatric syndromes in Duchenne patients warrants early intervention to achieve the best possible quality of life. Unravelling the function and pathophysiological significance of dystrophin in the brain has become a high research priority to inform the development of brain-targeting treatments for Duchenne.

Developmental lag of visuospatial attention in Duchenne muscular dystrophy

Children with Duchenne muscular dystrophy (DMD) present a specific deficit of voluntary attention but to date there has been no clear characterization of their attentional skills. The present study investigated the hypothesis that DMD patients present deficits of both voluntary and automatic visuospatial attention systems and that their performance could be equivalent to that of younger healthy males. Twenty males (mean age 10 years) with diagnosis of DMD, 20 age-matched healthy males (10 years 3 months) and 20 healthy younger males (7 years 6 months) were required to perform two visuospatial attention tasks: voluntary and automatic. In the voluntary task, the performance of the DMD group was significantly worse than that of the age-matched group, and equal to that of the younger controls. In the automatic attention task also, the performance of the DMD patients was less efficient than that of the age-matched controls and equal to that of the younger children. This study supports the previous report of voluntary attention deficit in DMD and extends the evidence to include also an automatic attention system deficit. The development level of attention in DMD patients is below that expected for their age and corresponds to a delay of about three years.

Visuospatial attention disturbance in Duchenne muscular dystrophy

AIM

The cognitive deficits present in the Duchenne muscular dystrophy (DMD) are not yet well characterized. Attention, considered to be the brain mechanism responsible for the selection of sensory stimuli, could be disturbed in DMD, contributing, at least partially, to the observed global cognitive deficit. The aim of this study was to investigate attentional function in individuals with DMD.

METHOD

Twenty-five males (mean age 12y; SD 2y 2mo) with DMD and 25 healthy males (mean age 12y; SD 2y) were tested in a visuospatial task (Posner computerized test). They were instructed to respond as quickly as possible to a lateralized visual target stimulus with the ipsilateral hand. Their attention was automatically orientated by a peripheral prime stimulus or, alternatively, voluntarily orientated by a central spatially informative cue.

RESULTS

The main result obtained was that the attentional effect (sum of the benefit and the cost of attention) did not differ between the two groups in the case of automatic attention (p=0.846) but was much larger for individuals with DMD than for comparison individuals in the case of voluntary attention (p<0.001).

INTERPRETATION

The large voluntary attentional effect exhibited by the participants with DMD seems similar to that of younger children, suggesting that the disease is associated with delayed maturation of voluntary attention mechanisms.

Muscle biopsies in children--an evaluation of histopathology and clinical value during a 5-year period

Muscle biopsy is an important diagnostic tool in the investigation of children with neuromuscular disorders. This report presents the experience with paediatric muscle biopsies during a 5-year period at a routine pathology laboratory. A total number of 58 cases were included, and indications, microscopical findings, and final histopathological diagnoses were recorded. A total of 21 biopsies were from females (36%) and 37 biopsies from males (64%); 53% of the cases were from children under 2 years of age. Major pathological findings were found in 30% comprising muscular dystrophy, neurogenic atrophy, and congenital and metabolic disorders, even in cases with vague clinical manifestations. These findings confirm the high diagnostic yield of muscle biopsies, especially as new techniques have been introduced such as immunohistochemistry. Muscle pathology is difficult and emphasizes the importance of this service being undertaken by specialized laboratories with an experienced staff. Microscopical examination of muscle biopsies should be based on adequate clinical information, demonstrating the necessity of close contact between pathologists and referring physicians.

Synaptic plasticity in the dy2J mouse model of laminin alpha2-deficient congenital muscular dystrophy

Laminin alpha2-deficient congenital muscular dystrophy is a debilitating disease affecting both muscle and neural tissue as a result of mutations in the LAMA2 gene. It presents at or soon after birth with muscle weakness and is further characterised by clinical central nervous system involvement. Laminin alpha2 is part of the extracellular matrix, linked to the cellular cystoskeleton via dystroglycan which is an integral part of the dystrophin-glycoprotein complex (DGC). We examined both short- and long-term synaptic plasticity in the C57BL6J/dy(2J) mouse, an animal model of laminin alpha2 deficient congenital muscular dystrophy. Using a cerebellar slice preparation, we show that the pre-synaptically mediated paired-pulse facilitation (PPF) was no different between dy(2J) and littermate controls. Approximately half (7/12) the dy(2J) Purkinje cells displayed a blunted LTD compared to littermate controls, and one third (4/12) of dy(2J) Purkinje cells displayed LTP. This study demonstrates that a defective laminin alpha2 causes a disruption in long-term synaptic plasticity at the Purkinje cell-parallel fibre synapse.

Two children with muscular dystrophies ascertained due to referral for diagnosis of autism

We report two children who were referred for diagnostic assessment for autism and were subsequently determined to have a muscular dystrophy (MD). Each child had a history of speech delay and social impairments, but also had motor delays that had not previously been investigated. Both children met diagnostic criteria for autism spectrum disorders on standardized assessment. Each child was hypotonic and had other mild motor impairments. Serum creatine kinase (CK) activity was markedly elevated in each child, and subsequent muscle biopsy led to diagnosis of Becker's MD and congenital (autosomal recessive) MD, respectively. These cases highlight the importance of a thorough neuromotor examination for all children with suspected autism spectrum disorders.