Niemann–Pick type C (NPC) (colloquially, "Childhood Alzheimer's"^[1]) is a lysosomal storage disease associated with mutations in NPC1 and NPC2 genes. Niemann–Pick type C affects an estimated 1:150,000 people.^[2] Approximately 50% of cases present before 10 years of age, but manifestations may first be recognized as late as the sixth decade.

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Niemann–Pick disease, type C
Niemann–Pick disease, type C is associated with NPC1 mutations
Specialty	Endocrinology, neurology

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Niemann–Pick type C has a wide clinical spectrum. Affected individuals may have enlargement of the spleen (splenomegaly) and liver (hepatomegaly), or enlarged spleen or liver combined (hepatosplenomegaly), but this finding may be absent in later onset cases. Prolonged jaundice or elevated bilirubin can present at birth. In some cases, however, enlargement of the spleen or liver does not occur for months or years – or not at all. Enlargement of the spleen or liver frequently becomes less apparent with time, in contrast to the progression of other lysosomal storage diseases such as Niemann–Pick disease, Types A and B or Gaucher disease. Organ enlargement does not usually cause major complications.^{[citation needed]}

Progressive neurological disease is the hallmark of Niemann–Pick type C disease, and is responsible for disability and premature death in all cases beyond early childhood.^[3] Classically, children with NPC may initially present with delays in reaching normal developmental milestones skills before manifesting cognitive decline (dementia).^{[citation needed]}

Neurological signs and symptoms include cerebellar ataxia (unsteady walking with uncoordinated limb movements), dysarthria (slurred speech), dysphagia (difficulty in swallowing), tremor, epilepsy (both partial and generalized), vertical supranuclear palsy (upgaze palsy, downgaze palsy, saccadic palsy or paralysis), sleep inversion, gelastic cataplexy (sudden loss of muscle tone or drop attacks), dystonia (abnormal movements or postures caused by contraction of agonist and antagonist muscles across joints), most commonly begins with inturning of one foot when walking (action dystonia) and may spread to become generalized, spasticity (velocity dependent increase in muscle tone), hypotonia, ptosis (drooping of the upper eyelid), microcephaly (abnormally small head), psychosis, progressive dementia, progressive hearing loss, bipolar disorder, major and psychotic depression that can include hallucinations, delusions, mutism, or stupor.^{[citation needed]}

In the terminal stages of Niemann–Pick type C disease, the patient is bedridden, with complete ophthalmoplegia, loss of volitional movement and severe dementia.

Approximately 95% of Niemann–Pick type C cases are caused by genetic mutations in the NPC1 gene, referred to as type C1; 5% are caused by mutations in the NPC2 gene, referred to as type C2.^[4] The clinical manifestations of types Niemann–Pick types C1 and C2 are similar because the respective genes are both involved in egress of lipids, particularly cholesterol, from late endosomes or lysosomes. The NPC1 gene is located on chromosome 18 (18q11-q12) and was described by researchers at the National Institutes of Health in July 1997.^[5]

• The NPC1 gene encodes a protein that is located in membranes inside the cell and is involved in the movement of cholesterol and lipids within cells.^[6] A deficiency of this protein leads to the abnormal buildup of lipids and cholesterol within cell membranes.
• The NPC2 gene encodes a protein that binds and transports cholesterol.^[7][8] It has been shown to closely interact with NPC1.^[9][10]

Niemann–Pick type C is biochemically, genetically and clinically distinct from Niemann–Pick Types A or and B. In Types A and B, there is complete or partial deficiency of the lysosomal enzyme called acid sphingomyelinase. In Niemann–Pick type C, the protein product of the major mutated gene NPC1 is not an enzyme but appears to function as a transporter in the endosomal-lysosomal system, which moves large water-insoluble molecules through the cell. The protein coded by the NPC2 gene more closely resembles an enzyme structurally but seems to act in cooperation with the NPC1 protein in transporting molecules in the cell. The disruption of this transport system results in the accumulation of cholesterol and glycolipids in lysosomes.^[12]

Cholesterol and glycolipids have varied roles in the cell. Cholesterol is a major component of cell plasma membranes, which define the cell as a whole and its organelles. It is also the basic building block of steroid hormones, including neurosteroids. In Niemann–Pick type C, large amounts of free or unesterified cholesterol accumulate in lysosomes, and leads to relative deficiency of this molecule in multiple membranes and for steroid synthesis. The accumulation of glycosphingolipids in the nervous system has been linked to structural changes, namely ectopic dendritogenesis and meganeurite formation, and has been targeted therapeutically.^{[citation needed]}

Several theories have attempted to link the accumulation of cholesterol and glycolipids in the lysosomes with the malfunction of the NPC-1 protein.

• Neufeld et al. hypothesized that the accumulation of mannose 6-phosphate receptors (MPRs) in the late endosome signals failure of retrograde trafficking of cholesterol via the trans Golgi network.^[13]
• Another theory suggests that the blockage of retrograde cholesterol breakdown in the late endosome is due to decreased membrane elasticity and thus the return vesicles of cholesterol to the trans Golgi Network cannot bud and form.
• Iouannou, et al. have described similarities between the NPC1 protein and members of the resistance-nodulation-division (RND) family of prokaryotic permeases, suggesting a pumping function for NPC1.^[14]
• Recent 2008 evidence indicates that NPC-1 may play an important role in calcium regulation.^[15]

Niemann–Pick type C is diagnosed by assaying cultured fibroblasts for cholesterol esterification and staining for unesterified cholesterol with filipin. The fibroblasts are grown from a small skin biopsy taken from a patient with suspected NPC. The diagnosis can be confirmed by identifying mutations in the NPC1 or NPC2 genes in 80–90% of cases. This specialized testing is available at Thomas Jefferson University Lysosomal Disease Testing Lab^[16] and the Mayo Clinic.^[17]

There is no known cure for Niemann–Pick type C, nor is there any FDA-standard approved disease modifying treatment.^[18] Supportive care is essential and substantially improves the quality of life of people affected by NPC. The therapeutic team may include specialists in neurology, pulmonology, gastroenterology, psychiatrist, orthopedics, nutrition, physical therapy and occupational therapy. Standard medications used to treat symptoms can be used in NPC patients. As patients develop difficulty with swallowing, food may need to be softened or thickened, and eventually, parents will need to consider placement of a gastrostomy tube (g-tube, feeding tube).^[19]

The lifespan of patients with NPC is usually related to the age of onset. Children with antenatal or infantile onset usually succumb in the first few months or years of life, whereas adolescent and adult onset forms of Niemann–Pick type C have a more insidious onset and slower progression, and affected individuals may survive to the seventh decade. Adult cases of NPC are being recognized with increasing frequency. It is suspected that many patients affected by NPC are undiagnosed, owing to lack of awareness of the disease and the absence of readily available screening or diagnostic tests. For the same reasons the diagnosis is often delayed by many years.^{[citation needed]}

Loss of myelin in the central nervous system is considered to be a main pathogenic factor. Research uses animal models carrying the underlying mutation for Niemann–Pick disease, e.g. a mutation in the NPC1 gene Niemann–Pick type C disease. In this model the expression of Myelin gene Regulatory Factor (MRF) has been shown to be significantly decreased.^[55] MRF is a transcription factor of critical importance in the development and maintenance of myelin sheaths.^[56] A perturbation of oligodendrocyte maturation and the myelination process might therefore be an underlying mechanism of the neurological deficits.^[55]

Recent neuroimaging studies have shown patients with Niemann–Pick, type C to have a corpus callosum with microstructural abnormalities. Clear reductions in corpus callosum mean thickness and surface area have been shown when compared to age-matched controls.^[57][58] Also, studies using diffusion tensor imaging have shown marked reductions in callosal fractional anisotropy, which suggests architectural abnormalities based on the directional flow of water.^[58][59] These conclusions suggest that the corpus callosum plays an important role in the disease and should be explored for use as a biomarker of disease progression.^{[citation needed]}

Parents of children with NPC are being studied in an attempt to gain insight into the Ebola virus, which uses the protein encoded by NPC1 to enter cells. Researchers have found that mice with one normal copy of the NPC1 gene are more likely to survive Ebola infection than mice with normal two copies of the gene. Mice lacking any normal copy of NPC1 all survived. Studying cells from parents who are NPC disease carriers may allow for better understanding of how changes to the NPC1 gene affect Ebola risk.^[60]

Findings from Zhang et al.^[61] suggest that NPC is a late endocytic trafficking disease resulted, at least in part, from disruption of communication within late endocytic (LE) compartments and possibly between LE and other subcellular organelles. Crosstalk between the late endocytic compartment and other organelles such as mitochondria, endoplasmic reticulum, plasma membrane, as well as early endocytic compartments has become one of the most interesting frontiers in neurondegenerative disease research including Alzheimer's disease, Parkinson's disease, as well as lysosomal storage disorders.^[62][63]