Niemann-Pick disease is autosomal recessive, not dominant. This applies to all forms of the disease, including types A, B, and C. A child must inherit two copies of the faulty gene, one from each parent, to develop the condition. Parents who each carry one copy typically show no signs of the disease themselves.
What Autosomal Recessive Means in Practice
Every person carries two copies of each gene, one inherited from each parent. In autosomal recessive conditions like Niemann-Pick, a single working copy is enough to keep the body functioning normally. That means a person with one mutated copy and one normal copy is a “carrier” who will never develop symptoms but can pass the mutation to their children.
When both parents are carriers, each pregnancy carries a 25% chance of producing a child with the disease. There is a 50% chance the child will be a carrier like the parents, and a 25% chance the child will inherit no mutated copies at all. Because carriers are healthy, many families have no idea they carry the mutation until a child is diagnosed. The disease is not linked to the X or Y chromosome, so it affects boys and girls equally.
Three Types, Three Different Genes
Although all forms of Niemann-Pick follow the same recessive inheritance pattern, they involve different genes and cause different problems inside cells.
Types A and B are caused by mutations in the SMPD1 gene, which provides instructions for making an enzyme called acid sphingomyelinase. This enzyme’s job is to break down a fatty substance called sphingomyelin, a normal component of cell membranes. When the enzyme is missing or defective, sphingomyelin builds up inside cells, particularly in the liver, spleen, lungs, and brain. Types A and B are now collectively referred to as acid sphingomyelinase deficiency (ASMD).
Type C involves a completely different biological problem. It is caused by mutations in either the NPC1 gene (responsible for about 95% of cases) or the NPC2 gene. Instead of an enzyme deficiency, the issue is faulty cholesterol transport inside cells. The NPC1 and NPC2 proteins normally work together to move cholesterol and other fats out of a cell’s recycling compartments. When either protein is defective, lipids accumulate and eventually damage organs and the nervous system.
How the Types Differ Clinically
The distinction between types matters because the age of onset, severity, and affected organs vary significantly.
Type A is the most severe form. Infants develop an enlarged liver and spleen within the first months of life, and the disease causes profound damage to the central nervous system. Children with type A rarely survive beyond age two or three.
Type B also causes an enlarged liver and spleen, along with lung damage, but it generally spares the brain. The age of onset and rate of progression vary widely. Many people with type B live into adulthood, and some are not diagnosed until their teens or later.
Type C can appear at almost any age, from infancy through adulthood, though it most commonly shows up in childhood. Early signs often include difficulty with coordination, trouble moving the eyes up and down, slurred speech, and progressive cognitive decline. In newborns, prolonged jaundice or an enlarged liver may be the first clue. Because symptoms can be subtle and develop slowly, type C is often misdiagnosed or diagnosed years after symptoms begin.
Carrier Testing and Genetic Diagnosis
Carriers of Niemann-Pick mutations do not develop symptoms. Unlike some other recessive conditions where carriers can have mild biochemical differences detectable on blood tests, Niemann-Pick carriers generally have no clinical signs that would prompt investigation on their own.
Diagnosis typically starts when a child or adult shows suggestive symptoms. For types A and B, measuring acid sphingomyelinase enzyme activity in a blood sample can confirm the deficiency. For type C, genetic testing through a multigene panel is the most reliable approach, analyzing the NPC1 and NPC2 genes for known mutations. Doctors sometimes use a broader panel that includes genes for related conditions, since the symptoms of type C overlap with several other neurological disorders.
If a family already has one affected child, both parents can be confirmed as carriers through targeted genetic testing. Siblings each have a two-in-three chance of being carriers themselves (among those who are unaffected), which becomes relevant if they plan to have children. Genetic counseling can help families understand these probabilities and explore options like preconception carrier screening for a partner.
Available Treatments
For types A and B, an enzyme replacement therapy called olipudase alfa is now FDA-approved. It replaces the missing acid sphingomyelinase enzyme and treats the non-neurological symptoms of the disease, including organ enlargement and lung involvement. It does not cross into the brain, so it cannot address neurological damage in type A.
For type C, a medication called miglustat can help slow the progression of neurological symptoms in some patients by reducing the buildup of certain fats in the brain. It does not cure the disease, but it may stabilize or delay cognitive and motor decline. Beyond these specific therapies, treatment for all types of Niemann-Pick is largely supportive, focusing on managing symptoms like seizures, feeding difficulties, and respiratory problems as they arise.