SNP Shot: Genomic Insights

Sialidosis is a serious inherited condition impacting multiple organ systems, including the nervous system. It is classified into two main types based on the age of symptom onset and the severity of the condition.

Sialidosis type I, also known as cherry-red spot myoclonus syndrome, is the milder form. Symptoms usually begin in adolescence or early adulthood (teens to twenties). Initial symptoms include difficulty walking (gait disturbance) and/or vision loss (reduced visual acuity). Affected individuals also experience muscle twitching (myoclonus), coordination problems (ataxia), tremors in the legs, and seizures. The myoclonus progressively worsens, making it difficult to sit, stand, or walk, often leading to the need for a wheelchair. Vision problems worsen over time, potentially including color vision deficits or night blindness. A characteristic eye abnormality called a cherry-red spot, detectable during an eye exam, is a hallmark of this condition. Importantly, sialidosis type I does not affect intelligence or lifespan.

Sialidosis type II, the more severe form, is further subdivided into congenital, infantile, and juvenile forms. Congenital sialidosis type II features can manifest before birth. This form is associated with fluid buildup in the abdomen (ascites) or widespread swelling due to fluid accumulation (hydrops fetalis). Affected infants may also have an enlarged liver and spleen (hepatosplenomegaly), abnormal bone development (dysostosis multiplex), and distinctive facial features described as "coarse." Due to these severe health issues, infants with congenital sialidosis type II are often stillborn or die shortly after birth.

Infantile sialidosis type II shares some characteristics with the congenital form, but its symptoms are slightly less severe and emerge within the first year of life. Common features include hepatosplenomegaly, dysostosis multiplex, "coarse" facial features, short stature, and intellectual disability. As children with infantile sialidosis type II age, they may develop myoclonus and cherry-red spots. Additional symptoms may include hearing loss, gum overgrowth (gingival hyperplasia), and widely spaced teeth. Individuals with the infantile form may survive into childhood or adolescence.

The juvenile form of sialidosis type II presents with the mildest symptoms. These features typically appear in late childhood and may include subtle "coarse" facial features, minor bone abnormalities, cherry-red spots, myoclonus, intellectual disability, and dark red spots on the skin (angiokeratomas). Life expectancy for individuals with juvenile sialidosis type II varies depending on the severity of the symptoms.

Inheritance:

Sialidosis is inherited in an autosomal recessive manner. This means that both copies of the responsible gene in each cell must have mutations for the condition to develop. Parents of an individual with an autosomal recessive disorder each carry one copy of the mutated gene, but usually do not exhibit any signs or symptoms of the condition themselves.