Amyotrophic lateral sclerosis (ALS) is a progressive and generally fatal motor neuron disease. Most cases are sporadic, but high familial incidence is observed occasionally. Amyotrophic lateral sclerosis 4 (ALS4) is one of many subtypes of familial ALS. It has been related to mutations in the SETX gene, a protein-coding gene whose product is possibly involved in nucleic acid processing. ALS4 is inherited in an autosomal dominant manner. Affected individuals suffer from a slowly progressive distal neuropathy and present pyramidal signs. Contrary to classical ALS, first symptoms don't manifest in adulthood but in adolescence, and affected individuals have a near-to-normal life expectancy.
In general, symptom onset occurs in patients aged less than 25 years . However, SETX mutations have also been detected in ALS patients who didn't experience any complaints until their late third or even fourth decade of life . They may seek medical advice due to progressive distal limb weakness and subsequent muscle wasting. It is important to note that symptom onset may have occurred years before they present to the physician: Because disease progression is slow, first manifestations are not necessarily understood as pathological conditions . Interestingly, ALS4 patients don't develop bulbar palsy; they don't suffer from dysarthria or dysphagia. Furthermore, respiratory muscles are spared from the disease. The patients' sensibility is usually unaltered, although minimal sensory impairment has been reported in those of advanced age . Accordingly, electrophysiological studies typically reveal a chronic motor neuronopathy characterized by reduced amplitudes of compound muscle action potentials, positive sharp waves, and fibrillations, but normal amplitudes of action potentials of sensory nerves. Nerve conduction velocities are within reference ranges . Proximal muscles are less severely affected than distal muscles.
During neurological examination, pyramidal signs can be observed. ALS4 patients show hyperreflexia, have brisk deep-tendon reflexes and the Babinski sign is present. Care should be taken when interpreting reflex tests that rely on contractions of atrophic muscles: In this scenario, upper motor neuron signs may easily be overlooked .
ALS diagnosis relies on the identification of upper motor neuron and lower motor neuron signs, to be observed in patients suffering from a progressive neurodegenerative disease that cannot be explained by other conditions. To facilitate ALS diagnosis, diagnostic criteria have been defined on various occasions   . Currently, revised El Escorial criteria are applied in most clinical trials. Those criteria are as follows :
Furthermore, the central nervous system is divided into four regions, namely the bulbar, cervical, thoracic and lumbosacral region as indicated in the previous paragraph. The presence of symptoms related to the function of any of those four regions allows for a more precise diagnosis of clinically definite, clinically probable, clinically probable if laboratory-supported, and clinically possible ALS :
A positive family history of ALS augments the certainty of diagnosis and may even justify the diagnosis of clinically definite ALS if the respective criteria are not completely fulfilled . However, SETX mutations have been detected in patients suffering from apparently sporadic ALS .
This fact highlights the importance of genetic studies. Even though molecular biological analyses are not required for the diagnosis of ALS, they are necessary to determine the subtype. In fact, the identification of sequence anomalies may accelerate the diagnostic process: If SETX mutations are known to cause ALS in a determined family, a more targeted approach to diagnosis becomes feasible. Thus, genetic analyses provide both physicians and scientists with an appropriate tool to identify carriers and family members at risk, and to promote research  .
There is no cure, and disease progression can hardly be halted. Riluzole is the only pharmacological compound approved for ALS therapy; it is assumed to reduce glutamate toxicity. It has been reported to increase survival times and to delay the onset of life-threatening symptoms such as laryngospasm and respiratory paralysis, but its efficacy is very limited . Furthermore, neither laryngospasm nor respiratory paralysis are to be expected in ALS4 patients, so decisions regarding the usefulness of riluzole have to be made on a case-by-case basis. The application of α-tocopherol has been proposed as a complementary measure to slow down disease progression in milder cases and may be considered here . Otherwise, only palliative treatment can be provided. In this context, ALS4 patients benefit from a multidisciplinary approach that aims at maintaining their mobility and their ability to cope with everyday life for as long as possible  :
ALS4-related amyotrophy may cause minor gait disorders or render people wheelchair-bound; upper-limb muscle wasting may interfere with determined task requiring dexterity or with the patient's ability to cope with everyday life. Fortunately, the disease does neither cause bulbar palsy nor respiratory paralysis, thereby reducing the likelihood of aspiration pneumonia due to dysphagia and respiratory failure - which are the most common causes of death in ALS patients. ALS4 is a slowly progressive form of ALS and while it may considerably reduce quality of life, affected individuals are assumed to have a normal or near-to-normal life expectancy  .
ALS4 is the only juvenile-onset form of ALS that is inherited in an autosomal dominant manner. It maps to chromosome 9q34 and has been related to distinct mutations of the SETX gene . In detail, it has been proposed that ALS4 may be triggered by gain-of-function mutations of SETX . SETX encodes for senataxin, a protein of as-of-yet unknown function. However, SETX is known to contain a 7-motif DNA/RNA helicase domain with strong homology to genes IGHMBP2 and RENT1. The latter encode for proteins with helicase activity and thus, a role in DNA and/or RNA processing may be assumed for senataxin. Senataxin has been referred to as "genome guardian" and studies on Saccharomyces cerevisiae cells expressing mutant yeast homologs of human senataxin indicate a role of the respective gene in managing oxidative stress, maintaining mitochondrial membrane potentials and cytosolic calcium levels  .
Of note, recessive loss-of-function mutations in the SETX gene have been associated with ataxia-oculomotor apraxia. This disease is characterized by cerebellar ataxia, oculomotor apraxia, and late peripheral neuropathy. Similar to ALS4, it follows a slowly progressive course .
The global incidence of ALS has been estimated to 1-2.6 per 100,000 people per year, and its prevalence amounts to 6 per 100,000 inhabitants . About 10% of all those cases are familial. Familial ALS is generally inherited in an autosomal dominant manner, as is the case with ALS4, a juvenile-onset form of ALS. Chen and colleagues studied four families affected by ALS4 and they calculated the patients' average at at symptom onset to be 6, 8, 17, and 21 years, respectively . Both men and women may develop ALS4. To date, SETX-related ALS has been diagnosed in patients of European and North American ancestry .
Despite extensive research, the pathophysiology of ALS remains poorly understood. The death of motor neurons is the hallmark of the disease and entails muscle weakness and atrophy, but its causes could not yet be clarified. Neuronal death has been speculated to be due to the accumulation of protein aggregates which, in turn, consist of abnormal proteins. Sequence anomalies, e.g., of genes like SETX, may be the cause of irregularities in the amino acid sequence, post-translational modification and intracellular transport of any protein, may alter their physical properties, their propensity to bind to specific targets, and their susceptibility to degradation .
No recommendations can be given to prevent the onset of sporadic ALS, other than avoiding certain risk factors . By contrast, genetic analyses may facilitate the identification of carriers and as-of-yet asymptomatic patients in families affected by familial ALS . Prenatal diagnoses may become feasible if the disease can be related to well-defined DNA sequence anomalies, but this does not yet apply to ALS4.
ALS is the most common motor neuron disease. ALS patients may be genetically predisposed to develop the disease, and distinct genes have been associated with its familial form. One of those genes is the SETX gene, which encodes for senataxin. ALS linked to mutations in the SETX gene has been designated ALS4  . Of note, the same disease may also be referred to as distal hereditary motor neuronopathy with pyramidal features .
ALS4 is a rather mild form of ALS, with juvenile-onset muscle weakness and atrophy but sparing of the bulbar and respiratory muscles. The disease follows a slowly progressive course and ALS4 patients are assumed to have an almost normal life expectancy  . Nevertheless, distal limb amyotrophy may eventually become the cause of severe disability. As of today, genetic and environmental factors affecting the severity of ALS4 have not been identified. Effective treatment cannot be provided and therapy mainly aims at maintaining the patients' mobility and quality of life.
Amyotrophic lateral sclerosis (ALS) is a progressive motor neuron disease that typically manifests in adulthood. Little is known about the causes of ALS, but at least a minor proportion of ALS patients seems to be genetically predisposed. This condition is reflected in an increased familial incidence, i.e., relatives of an ALS patient carrying certain gene defects are much more likely to develop the disease than the general population. In this context, ALS has been associated with distinct chromosome and gene anomalies. For instance, there are European and North American families whose members present mutations in a gene called SETX. SETX-related ALS has later been designated amyotrophic lateral sclerosis 4 (ALS4).
ALS4 is a relatively mild form of ALS, although symptoms manifest early: