Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an inherited condition that causes muscle weakness and respiratory failure typically beginning in infancy. Early features of this condition are difficult and noisy breathing, especially when inhaling; a weak cry; problems feeding; and recurrent episodes of pneumonia. Typically between the ages of 6 weeks and 6 months, infants with this condition will experience a sudden inability to breathe due to paralysis of the muscle that separates the abdomen from the chest cavity (the diaphragm). Normally, the diaphragm contracts and moves downward during inhalation to allow the lungs to expand. With diaphragm paralysis, affected individuals require life-long support with a machine to help them breathe (mechanical ventilation). Rarely, children with SMARD1 develop signs or symptoms of the disorder later in childhood.

Soon after respiratory failure occurs, individuals with SMARD1 develop muscle weakness in their distal muscles. These are the muscles farther from the center of the body, such as muscles in the hands and feet. The weakness soon spreads to all muscles; however, within 2 years, the muscle weakness typically stops getting worse. Some individuals may retain a low level of muscle function, while others lose all ability to move their muscles. Muscle weakness severely impairs motor development, such as sitting, standing, and walking. Some affected children develop an abnormal side-to-side and back-to-front curvature of the spine (scoliosis and kyphosis, often called kyphoscoliosis when they occur together). After approximately the first year of life, individuals with SMARD1 may lose their deep tendon reflexes, such as the reflex being tested when a doctor taps the knee with a hammer.

Other features of SMARD1 can include reduced pain sensitivity, excessive sweating (hyperhidrosis), loss of bladder and bowel control, and an irregular heartbeat (arrhythmia).

Mutations in the IGHMBP2 gene cause SMARD1. The IGHMBP2 gene provides instructions for making a protein involved in copying (replicating) DNA; producing RNA, a chemical cousin of DNA; and producing proteins. IGHMBP2 gene mutations that cause SMARD1 lead to the production of a protein with reduced ability to aid in DNA replication and the production of RNA and proteins. These problems particularly affect alpha-motor neurons, which are specialized cells in the brainstem and spinal cord that control muscle movements. Although the mechanism is unknown, altered IGHMBP2 proteins contribute to the damage of these neurons and their death over time. The cumulative death of alpha-motor neurons leads to breathing problems and progressive muscle weakness in children with SMARD1.

Research suggests that the amount of functional protein that is produced from the mutated IGHMBP2 gene may play a role in the severity of SMARD1. Individuals who have some functional protein are more likely to develop signs and symptoms later in childhood and retain a greater level of muscle function.

Normal Function

The IGHMBP2 gene provides instructions for making an enzyme called immunoglobulin mu DNA binding protein 2 (IGHMBP2). This enzyme functions as a helicase, which means that it attaches to particular regions of DNA and temporarily unwinds the two spiral strands of these molecules. When a cell prepares to divide to form two cells, the chromosomes are duplicated (replicated) so that each new cell will get a complete set of chromosomes. The replication process involves unwinding the DNA so that it can be copied. This mechanism is also involved in the production of RNA, a chemical cousin of DNA. Additionally, the IGHMBP2 protein is thought to be involved in the production of proteins from RNA through a process called translation. The IGHMBP2 protein is produced in cells throughout the body.

Health Conditions Related to Genetic Changes

Spinal muscular atrophy with respiratory distress type 1

More than 60 mutations in the IGHMBP2 gene have been found to cause spinal muscular atrophy with respiratory distress type 1 (SMARD1). SMARD1 is an inherited condition that causes muscle weakness and respiratory failure typically beginning in infancy. Most mutations that cause this condition change single protein building blocks (amino acids) in the IGHMBP2 protein and disrupt the protein's ability to unwind DNA and RNA. The loss of helicase function impedes DNA replication and the production of RNA and proteins. These problems particularly affect alpha-motor neurons, which are specialized cells in the brainstem and spinal cord that control muscle movements. Altered IGHMBP2 proteins cause these neurons to become damaged and die over time, although the exact mechanism is unknown. The cumulative death of alpha-motor neurons leads to breathing problems and progressive muscle weakness in children with SMARD1.

Charcot-Marie-Tooth disease

MedlinePlus Genetics provides information about Charcot-Marie-Tooth disease

Other Names for This Gene

• cardiac transcription factor 1
• CATF1
• FLJ34220
• FLJ41171
• HCSA
• HMN6
• immunoglobulin mu binding protein 2
• SMARD1
• SMBP2_HUMAN
• SMUBP2

Genomic Location

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

SMARD1 appears to be a rare condition, but its prevalence is unknown. More than 60 cases have been reported in the scientific literature.

What is spinal muscular atrophy?

Spinal muscular atrophy (SMA) refers to a group of hereditary diseases that can damage and kill specialized nerve cells in the brain and spinal cord (motor neurons). Motor neurons control movement in the arms, legs, face, chest, throat, and tongue, as well as skeletal muscle activity, such as speaking, walking, swallowing, and breathing.

The most common form of SMA is caused by a mutated or missing gene known as the survival motor neuron gene 1 (SMN1), which is typically responsible for the production of a protein essential to motor neurons. There are four types of this form of SMA:

1. Type l (also known as Werdnig-Hoffman disease or infantile-onset SMA) is usually evident before 6 months of age. The most severely affected children will have reduced movement and chronic shortening of muscles or tendons (contractures). Other children may have symptoms including reduced muscle tone, lack of tendon reflexes, twitching, skeletal abnormalities, and problems with swallowing and feeding. Without treatment, many affected children die before age 2.
2. SMA Type ll is usually first noticed between 6 and 18 months of age. Children can sit without support but are unable to stand or walk without help. Children may also have respiratory difficulties. Life expectancy is reduced but most individuals live into adolescence or young adulthood.
3. SMA Type lll (also known as Kugelberg-Welander disease) is seen after age 18 months. Children can walk independently but may have difficulty doing so or when running, rising from a chair, or climbing stairs. Other complications may include curvature of the spine, contractures, and respiratory infections. With treatment, most individuals can have an average lifespan.
4. SMA Type IV develops after 21 years of age, with mild to moderate leg muscle weakness and other symptoms.

Who is more likely to get spinal muscular atrophy?

The most common form of SMA is caused by a mutated or missing gene known as the survival motor neuron gene 1 (SMN1). The SMN1 gene is located on chromosome 5q and produces the survival motor neuron (SMN) protein which maintains the health and normal function of motor neurons.

People living with SMA have insufficient levels of the SMN protein, which leads to loss of motor neurons in the spinal cord and causes weakness and wasting of the skeletal muscles. The weakness is often more severe in the trunk (chest) and upper leg and arm muscles than in muscles of the hands and feet.

Many types of spinal muscular atrophy are caused by changes in the same genes. Less common SMA forms are caused by changes in other genes including the:

• VAPB gene on chromosome 20
• DYNC1H1 gene on chromosome 14
• BICD2 gene on chromosome 9
• UBA1 gene on the X chromosome

Except in rare cases, SMA is inherited in an autosomal recessive manner, meaning that the affected individual has two mutated genes, often inheriting one from each parent. Those who carry only one mutated gene are carriers of the disease who will not have any symptoms.

How is spinal muscular atrophy diagnosed and treated?

Diagnosing SMA

A blood test is available to look for mutations or deletions of the SMN1 gene. This test identifies at least 95 percent of SMA Types I, II, and III, and also may reveal if a person is a carrier. If the SMN1 gene is not found to be problematic or the individual's history and examination are not typical of SMA, other diagnostic tests may include:

• Electromyography to record the electrical activity of the muscles during contraction and at rest
• Nerve conduction velocity studies to measure the nerve's ability to send an electrical signal
• Muscle biopsy to diagnose many neuromuscular disorders
• Other blood tests

Treating SMA

There is no complete cure for SMA. Treatment consists of managing the symptoms and preventing complications.

Medications

• The U.S. Food and Drug Administration (FDA) approved nusinersen (Spinraza™) as the first drug approved to treat children and adults with SMA. The drug is designed to increase production of the SMN protein, which is critical for the maintenance of motor neurons.
• The FDA approved onasemnogene abeparovec-xioi (Zolgensma ™) gene therapy for children less than two years old who have infantile-onset SMA. A safe virus delivers a fully functional human SMN gene to the targeted motor neurons, which in turn improves muscle movement and function and survival.
• The FDA approved the orally-administered drug risdiplam (Evrysdi) to treat patients age two months of age and older with SMA.

Physical therapy, occupational therapy, and rehabilitation may help to improve posture, prevent joint immobility, and slow muscle weakness and atrophy. Stretching and strengthening exercises may help reduce contractures, increase range of motion, and keeps circulation flowing. Some individuals require additional therapy for speech and swallowing difficulties. Assistive devices such as supports or braces, orthotics, speech synthesizers, and wheelchairs may be helpful to improve functional independence.

Proper nutrition and calories are essential to maintaining weight and strength, while avoiding prolonged fasting. People who cannot chew or swallow may require insertion of a feeding tube. Non-invasive ventilation at night can improve breathing during sleep, and some individuals also may require assisted ventilation during the day due to muscle weakness in the neck, throat, and chest.

What are the latest updates on spinal muscular atrophy?

The National Institute of Neurological Disorders and Stroke (NINDS), a component of the National Institutes of Health (NIH), conducts basic, translational, and clinical research on SMA in laboratories at the NIH and also supports research through grants to major medical institutions across the country.

Scientists have developed model systems in animals and cells to study disease processes and speed up the testing of potential therapies, including the following:

• Gene therapy and specific drugs have been shown to halt motor neuron destruction and slow disease progression in mouse models and individuals with SMA. NINDS supports research to establish these methods and to provide a path toward clinical tests. Clinical trials for gene therapy in SMA are ongoing.
• Animal models of SMA represent critical tools in discovering and developing new therapies for SMA. Scientists developed zebrafish, mouse, and pig models, including models of less severe SMA types 2 and 3, which may greatly aid the identification of new therapeutic targets and candidate therapies.

Additionally, NIH-supported scientists have collected data on pre-symptomatic or recently diagnosed children with SMA types I, II, or III, and their healthy siblings. The goal of this study is to provide counselling and education to parents about possible clinical trial opportunities.

NINDS established the NeuroNext (NINDS Network for Excellence in Neuroscience Clinical Trials) clinical trials network to promote the rapid development and implementation of trials for neurological disorders that affect adults and/or children. The network is designed to develop early-phase trials aimed at identifying biomarkers—usually a physical trait or substance in the blood or other bodily fluids that can be measured to determine the presence and severity of a disease—and testing emerging therapies. One such project was to identify biomarkers for SMA and to understand the cause and mechanisms underlying the disease. Data gained through this study led to the approval decision for nusinersen (Spinraza™). Knowledge acquired from this study has also enhanced the design of additional clinical trials in SMA.