Spinal muscular atrophy with respiratory distress type 1: Clinical phenotypes, molecular pathogenesis and therapeutic insights

Abstract Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a rare autosomal recessive neuromuscular disorder caused by mutations in the IGHMBP2 gene, which encodes immunoglobulin μ‐binding protein 2, leading to progressive spinal motor neuron degeneration. We review the data available in the literature about SMARD1. The vast majority of patients show an onset of typical symptoms in the first year of life. The main clinical features are distal muscular atrophy and diaphragmatic palsy, for which permanent supportive ventilation is required. No effective treatment is available yet, but novel therapeutic approaches, such as gene therapy, have shown encouraging results in preclinical settings and thus represent possible methods for treating SMARD1. Significant advancements in the understanding of both the SMARD1 clinical spectrum and its molecular mechanisms have allowed the rapid translation of preclinical therapeutic strategies to human patients to improve the poor prognosis of this devastating disease.


| INTRODUC TI ON
Autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a form of spinal muscular atrophy with severe diaphragmatic involvement that causes respiratory distress. This condition is due to autosomal recessive mutations in the IGHMBP2 gene, which is located on chromosome 11q13.2-q13.4. 1,2 Mellins, considering this mutation a variant of spinal muscular atrophy (SMA) 5q with respiratory onset, provided the first description of this condition in 1974, and it was not recognized as a separate clinical entity until 1996. 3,4 The actual prevalence of SMARD1 is unknown, but diaphragmatic paralysis is observed in approximately 1% of patients with an early onset of the clinical features of spinal muscle atrophy and an estimated incidence of 1/100 000. 5 The main clinical feature is the onset of respiratory distress requiring mechanical ventilation between the ages of 6 weeks and 6 months. The clinical symptoms rapidly progress in the first years of life, with distal limb muscular atrophy extending to proximal regions. The overall prognosis is poor, and progressive autonomic nervous system dysfunction also develops in association with the progressive worsening of motor functions in affected children. In fact, there are no approved treatments for SMARD1. 6

| Neonatal features
There is no specific neonatal clinical marker of this disease, although intrauterine growth retardation and premature birth are very common. 6 The majority of affected children present with nonspecific symptoms, such as weak cry, hypotonia, feeding problems, weak suckling and recurrent respiratory infections, in the first weeks of life. 5,7 Congenital foot malformations caused by distal muscle development defects and by the deposit of fatty pads in the proximal phalanges are also frequently found. 8

| Respiratory distress
Respiratory distress is usually the presenting clinical symptom and occurs between 6 weeks and 6 months of age as a consequence of the development of neurogenic diaphragmatic palsy. The presentation of respiratory distress is characterized by inspiratory stridor, weak cry, recurrent bronchopneumonia and trouble eating. This condition is almost always life-threatening in the absence of medical intervention; thus, a "pro-life decision" is often required before the diagnosis is genetically confirmed. 6,[8][9][10] Unlike SMA patients, who exhibit a bell-shaped chest and paradoxical respiration as a consequence of intercostal muscle palsy, SMARD1 patients have a normal-shaped thorax because the defect mainly involves the diaphragm. [5][6][7][8] Chest X-ray, which can show the characteristic eventration (the abnormal elevation) of the right or, less frequently, both hemidiaphragms, which is considered a highly suggestive sign of SMARD1, plays a core role in the diagnostic pathway. The confirmation of paralysis can be achieved by performing a chest ultrasound, diaphragmatic electromyography or fluoroscopy. [5][6][7][8]

| Neuromuscular features
The degeneration of the phrenic nerve is accompanied by the progressive wasting of the distal muscles of the limbs; the lower limbs are affected earlier than the upper limbs, and the proximal muscles become affected along with the progression of the disease. 6,10 The natural history of SMARD1 leads to complete paralysis of the four limbs, with an absence of deep tendon reflexes usually after the first year of life and the development of rachis malformations, such as kyphoscoliosis. The clinical features seem to progress most rapidly in the first two years of life, followed by a stabilization of the pattern and sometimes a mild improvement of some functions, such as respiratory activity and muscle strength, most likely due to the regeneration of some muscle fibres. 6,8,10 Regarding neurological assessment, motor development milestones and communication skills, in particular those specified in the semiquantitative scoring system by Eckart et al, 10

| Central and autonomic nervous system abnormalities
Cranial nerves are frequently involved in the natural course of SMARD1, although not as a presenting feature; mimicking pathologies, such as muscle weakness and tongue fasciculations, have been reported as signs of hypoglossal nerve paralysis, while the oculomotor nerves are usually spared. Epileptic seizures have also been reported, but these disorders do not seem to be aetiologically related to the disease. 8 Autonomic nervous system involvement is common in SMARD1 patients and manifests mainly as bladder incontinence, urinary retention with the need for catheterization, excessive sweating, constipation and cardiac arrhythmia. 6,8 In some cases, autonomic dysfunction can be the prominent feature of the clinical course, such as in the case of a Japanese girl affected with genetically confirmed SMARD1 who presented with catastrophic autonomic crisis with cardiac collapse, as reported by Nomura and colleagues. 11

| D IAG NOS IS
SMARD1 patients typically present with phrenic nerve palsy between the ages of six weeks and six months, and motor neuron degeneration primarily affects the distal muscles, particularly those of the lower extremities. 6 According to a cohort study enrolling 141 patients by Guenther and colleagues, the combination of these features is present in 86% of patients with IGHMBP2 mutations. Moreover, the onset of respiratory distress between six weeks to six months of age combined with preterm birth or right diaphragm eventration seems to predict the disease with a sensitivity of 98% and a specificity of 92%. 12 Conversely, the congenital onset of respiratory distress is not typical in SMARD1 and is consistent with other more common diagnoses, the most common of which are SMA1, congenital  This study demonstrated marked differences in the severity and rate of progression of symptoms in patients with improved severity scores between 1 and 3 years of age. Moreover, the authors observed that an overall higher score at the age of 3 years was usually associated with a slower progression of the disease and milder symptoms.

| NATUR AL COUR S E AND PROG NOS IS
The overall survival rate of SMARD1 without artificial ventilation, with a mean age of death of 9 months, is very poor. 17 Evidence suggests that there are three different forms of SMARD1 that vary based on the age of onset: an early-onset form, which usually appears before three months of age and is very severe, a classical onset form; and a very rare late-onset form, which is characterized by milder symptoms and a slower progression compared with that of the classical onset form and affects patients usually after the age of 12 months. 17 As of now, the oldest affected patient to be

Clinical criteria Histopathological criteria EMG criteria
Low birthweight below the 3rd centile Reduction of myelinated fibre size in sural nerve biopsies Evidence of acute or chronic distal denervation Onset of symptoms within the first 3 months Minimal evidence of ongoing myelinated fibre degeneration in biopsies taken up to 3 ± 4 months Evidence of severe slowing (<70% of lower limit of normality in one or more nerves (motor or sensory)) Diaphragmatic weakness either unilaterally or bilaterally No evidence of regeneration or of demyelination that might account for the change in fibre size.
Ventilator dependence within less than one month of onset with an inability to wean Absence of other dysmorphology or other conditions The fact that the great majority of known IGHMBP2 mutations are located in the helicase and R3H domains has led the authors to believe that these domains are essential to the function of the protein and has sparked interest in correlating the level of ATPase activity to the severity of the clinical phenotype. In Guenther et al 9 was the first group to study the protein expression levels in EBV-immortalized lymphoblastoid cell lines (LCLs) from SMARD1 patients and their parents. These authors found that patients with a milder juvenile form of SMARD1 had higher levels of protein expression than those of severe SMARD1-affected patients and that their parents expressed proteins at a level that was 75% of that exhibited by healthy controls. Moreover, these authors observed that the mRNA levels were not correlated with the amount of functional protein and thus concluded that SMARD1 pathogenesis may be related to reduced translation or protein degradation rather than diminished mRNA levels (Guenther et al 9 ). Further evidence is required to better confirm the existence of this correlation because it may be very important in the development of therapeutic strategies for SMARD1.

| IGHMBP2 MUTATI ON IN CHARCOT-MARIE-TOOTH D IS E A S E
Charcot-Marie-Tooth disease (CMT) is a spectrum of multiple hereditary syndromes causing the progressive length-dependent degeneration of peripheral sensory and/or motor fibres. CMT has an estimated prevalence of 1:2500, and its aetiology is related to more than 80 different genes. 22 The diverse phenotypic and electrophysiological symptoms lead to the classification of CMT by type (type 1 and type 2). 22

| IN VITRO AND IN VIVO D IS E A S E MODEL S
The

| The murine model
The Cox group provided the first identification of a spontaneous ighmbp2 mutation in mice in 1998. So-called neuromuscular degeneration (nmd) mice develop a disease that is similar to the human disease, with motor neuron degeneration, skeletal muscle atrophy and limb paralysis. Paralysis usually begins in the hindlimbs between the ages of 2 and 3 weeks, and then, the disease spreads to the trunk and the forelimbs. The affected mice often experience a slowdown in the progression of symptoms after that period, and most of these animals survive until the age of 14 weeks. The original phenotype of the mice was more severe, and the mice rarely survived beyond 4 weeks. 33 The main difference between murine disease and human SMARD1 is that respiratory failure occurs at later stages in mice and is not usually the cause of death, which results apparently from dilated cardiomyopathy. 16,34 Current evidence suggests that motor neuron loss begins before the onset of clinical symptoms and that it does not affect myelin sheets in the early stages of the disease. 34 The gastrocnemius has been found to be the most vulnerable muscle, while clinical evidence suggests that neural muscular junction (NMJ) denervation minimally affects the diaphragm, confirming the key difference in human SMARD1 patients. Furthermore, the fragmentation of the end motor plate has been detected in all analysed muscles and has been determined to be independent of their susceptibility to denervation, suggesting a role for the ighmbp2 protein in preventing this fragmentation. 16

| THER APEUTI C PER S PEC TIVE S
Unfortunately, there is no currently available treatment for   was not high enough to pass through the blood-brain barrier, thus limiting the efficacy of this treatment. 39 In conclusion, studies on neurotrophic factors have revealed evidence to justify future experiments to clarify whether therapeutic effects can be achieved using these substances.

| Stem cells
Thus far, the use of stem cells as a therapeutic tool for neuromuscular diseases has been widely investigated because these cells permit the replacement of degenerating motor neurons and produce cytokines and other neuroprotective factors that can help to prevent motor neuron loss. 40,41 Our group conducted various studies aiming to develop stem cell therapies for SMARD1. In the first study, the authors observed that In conclusion, stem cells represent a valid and encouraging therapeutic perspective not only for SMARD1 but also for many other neurodegenerative diseases. The next steps that will allow the progression of this field involve other in vitro studies, followed by the in vivo testing of human stem cells to better clarify the efficacy of this therapeutic tool alone and in combination with other strategies.

| Gene therapy
Gene therapy is the third possible approach to SMARD1 treatment, and its great advantage is that this therapy may have the potential to replace the deficient gene and restore a In two recent studies, Shababi and colleagues tested the ICV route (Shababi et al, 47 ) and compared this route with the IV route. 48 In the first paper, these authors treated mice by ICV injection with In conclusion, gene therapy seems to show encouraging results in laboratory and in vivo tests and thus requires great effort to reach the knowledge necessary to treat SMARD1.

| D ISCUSS I ON
Due to the great achievements provided by preclinical research, the field of neuromuscular diseases has quickly evolved in recent years, and some devastating diseases, such as SMA, 44,45 may finally be removed from the list of untreatable diseases. Unfortunately, SMARD1 remains an unsolved burden, probably due to its lower incidence and its complex and poorly understood pathogenetic mechanisms.
The main clinical features of this disease include neonatal onset (within the year of life), diaphragmatic paralysis and the wasting of distal limb muscles, which leads affected individuals to be completely dependent on ventilatory support and the daily supportive care of parents or caregivers. 6,[8][9][10] The prognosis of affected patients is currently very poor, 10 to be affected by the pathogenesis. Nevertheless, the published data and the results obtained also by our group show that gene therapy represents a real potential for the treatment of SMARD1. To verify the amplitude of this potential, further studies will be required, aiming to increase the number of analysed cases, thus permitting a wider statistical analysis of the correlation between genotype, IGHMBP2 mRNA and protein levels in animal models and human iPSCs, and the clinical phenotypes observed. Future advances in the knowledge of the pathogenetic mechanisms of this disease and in gene therapy administration will also permit the development of new experimental trials to better clarify its applicability in human SMARD1 patients.
There are still many unsolved questions, such as the best route of administration, the immunogenicity of these therapies in humans, the possible side effects and the long-term efficacy.
For the transition into the clinic, it is necessary to proceed with the validation of the method in the preclinical phase and invest in this research pathology, which, although rare, shares pathogenetic aspects with SMA and therefore also the susceptibility of treatment for SMARD1 patients.
The SMARD1 therefore represents an excellent candidate for the application of this innovative method, which has been demonstrated to be increasingly promising in the treatment of both neuromuscular and other diseases.
In conclusion, treatments for SMARD1 are not currently available, but recent preclinical therapeutic advances have laid the foundation for future solutions to this health issue, and the combination of various therapeutic possibilities that have been studied may lead to an effective therapy for SMARD1 patients.

ACK N OWLED G EM ENTS
We thank Association Centro Dino Ferrari for its support. The image was generated using images from Servier Medical Art, under a Creative Commons Attribution 3.0 Generic License. http://smart.servi er.com/

CO N FLI C T O F I NTE R E S T
The authors declare that they have no conflict of interest.