L1 syndrome

L1 syndrome is a group of mild to severe X-linked recessive disorders that share a common genetic basis. The spectrum of L1 syndrome disorders includes X-linked complicated corpus callosum dysgenesis, spastic paraplegia 1, MASA syndrome, and X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS). It is also called L1CAM syndrome (for the disorder's causative gene) and CRASH syndrome, an acronym for its primary clinical features: corpus callosum hypoplasia, retardation (intellectual disability), adducted thumbs, spasticity, and hydrocephalus. L1 syndrome can be caused by different variants in L1CAM, the gene that provides the information that allows the body to produce L1 cell adhesion molecule (sometimes called the L1 protein). The L1 cell adhesion molecule is a surface protein found on the surface of all neurons. It allows neurons to bind to one another and create synapses (connections where information is passed on from the axons of one neuron to the dendrites and cell body of another). As a result, L1 cell adhesion molecule is essential for the structural development of the brain and contributes to the ability to think, move, and develop memories. The type and severity of the L1CAM variant causing L1 syndrome in a particular person is directly related to the severity of symptoms and functional impairment that they experience. There is no cure for L1 syndrome, and prognosis is often poor. Life expectancy for people with L1 syndrome can vary dramatically depending on the severity of the condition, with some dying shortly after birth and others reaching adulthood. Treatment for people with L1 syndrome is supportive and aims to improve quality of life and minimize functional impairment.

Signs and symptoms L1 syndrome presents as a spectrum ranging from mild to severe features. A genotype–phenotype correlation exists across the L1 spectrum: the specific genetic variant that causes an L1-spectrum disorder determines the severity of the L1 syndrome in that patient. Patients with truncating (loss-of-function) variants in L1CAM (which prevent the full synthesis of L1 (protein)) experience more severe features than patients with missense variants in L1CAM (which may result in an abnormal protein but do not prevent its synthesis). Illustrating this difference in L1 syndrome severity, up to 50% of infants born with L1 syndrome caused by a truncating mutation will die before the age of 3 years despite provision of best available medical treatment. In comparison, roughly 10% of infants born with L1 syndrome caused by a missense mutation will die before the age of 3 years. Despite its presentation on a continuous spectrum, L1 syndrome is loosely divided into four discrete phenotypes.

Impacts of symptoms People diagnosed with L1 syndrome often experience issues with respect to societal roles and interactions due to the severe physical and mental disabilities associated with the disorder. These issues can vary depending on the symptoms that manifest in a specific individual and the severity of those symptoms, which is ultimately determined based on where the individual is situated on the L1 syndrome spectrum. Spasticity is one of the most common signs of L1 syndrome and is seen in all four major clinical phenotypes. It is characterized as the continuous contraction of certain muscles, leading to muscle stiffness which can interfere with normal movement and speech. In three of the major phenotypes (except HSAS) this spasticity is presented as spastic paraplegia, where the muscles of the lower limbs are stiff and continuously contracted. This spastic paraplegia often manifests itself as a gait (walking motion) disorder, specifically shuffling gait in MASA syndrome, which acts as a source of impairment and stress due to postural instability, and leads to poor quality of life and increased mortality. Aphasia is also a common disorder, especially in people with MASA syndrome (a disorder on the L1 syndrome spectrum) and describes a range of language impairments with respect to syntax (structure), semantics (meaning), phonology (sound), morphology (structure), and/or pragmatics in language comprehension or expression. People with aphasia, as well as their family members, often experience a poor quality of life due to the social isolation and depression caused by this language impairment and therefore may seek therapy to enable functional and socially relevant communication. Therapy services to address aphasia in MASA syndrome include one-on-one sessions with a clinician, group therapy, or even computer-based therapy, which is becoming more popular given its accessibility. Intellectual disabilities also contribute to the social difficulties faced by people with L1 syndrome, and can range from mild to severe depending on the person's location on the L1 syndrome spectrum. People with mild intellectual disability usually have an IQ around 50–70 (100 is the average) and are slower in all developmental areas, but they have no unusual physical characteristics and are able to blend in socially. Moderate intellectual disability characteristics include being able to maintain self care with some support from others, travel to familiar places in the community, communicate in simple ways, and having an IQ around 35–50. Severe intellectual disability is often seen in people with X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS) (on the L1 syndrome spectrum) and has several key characteristics including the ability to understand speech but otherwise having very limited communication skills; the ability to learn daily routines and simple self care, but needing direct supervision in social settings and needing family support to live in a supervised home setting. Depending on the severity of the intellectual disability, people with L1 syndrome will have varying levels of difficulty in adapting to their social environments and may need considerable support from others to complete day-to-day tasks.

X-linked hydrocephalus with stenosis of the aqueduct of Sylvius

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