Niemann-Pick disease, type C

Niemann-Pick type C is a lysosomal storage disease associated with mutations in NPC1 and NPC2 genes. Niemann-Pick Type C strikes an estimated 1:150,000 people. Approximately 50% of cases present before 10 years of age, but manifestations may first be recognized as late as the sixth decade.

Pathophysiology
Niemann-Pick Type C is biochemically, genetically and clinically distinct from Niemann Pick Types A or and B. In Types A & B, there is complete or partial deficiency of an 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.

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 unesterfied cholesterol accumulates 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.

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.


 * 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.


 * Recent evidence indicates that NPC-1 may play an important role in calcium regulation.

Genetics and classification
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. The clinical manifestations of types Niemann Pick Type 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.


 * 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. A deficiency of this protein leads to the abnormal build up of lipids and cholesterol within cell membranes.


 * The NPC2 gene encodes a protein that binds and transports cholesterol. It has been shown to closely interact with NPC1.

"Type D" variant
Type D Niemann-Pick has only been found in the French Canadian population of Yarmouth County, Nova Scotia, and is now known to be allelic with Niemann Pick Type C.

Genelogical research indicates that Joseph Muise (c. 1679 - 1729) and Marie Amirault (1684 - c. 1735) are common ancestors to all people with Type D. This couple is the most likely origin for the type D variant.

Symptoms
Niemann-Pick Type C has a wide clinical spectrum. Affected individuals may have enlargement of the spleen (splenomegaly) and liver (hepatomegaly), or enlarged spleen/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 and/or liver does not occur for months or years - or not at all. Enlargement of the spleen and/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.

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. Classically, children with NPC may initially present with delays in reaching normal developmental milestones skills before manifesting cognitive decline (dementia).

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 in turning 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.

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

Diagnosis
Niemann Pick Type C is diagnosed by assaying cultured fibroblasts for cholesterol esterfication 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 and the Mayo Clinic.

Treatment
There is no known cure for Niemann Pick Type C, nor is there any FDA-standard approved disease modifying treatment. 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, 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).

An observational study is underway at the National Institutes of Health to better characterize the natural history of NPC and to attempt to identify markers of disease progression.

Hydroxy-propyl-beta-cyclodextrin (HPBCD)
In April 2009, hydroxy-propyl-beta-cyclodextrin (HPBCD) was approved under compassionate use by the U.S. Food and Drug Administration (FDA) to treat Addison and Cassidy Hempel, identical twin girls suffering from Niemann Pick Type C disease. Medi-ports, similar to ports used to administer chemotherapy drugs, were surgically placed into the twins' chest walls and allow doctors to directly infuse HPBCD into their bloodstreams. Treatment with cyclodextrin has been shown to delay clinical disease onset, reduced intraneuronal storage and secondary markers of neurodegeneration, and significantly increased lifespan in both the Niemann Pick Type C mice and feline models. (This is the second time in the United States that cyclodextrin alone has been administered in an attempt treat a fatal pediatric disease. Over 20 years ago, HPBCD was used in a medical case involving a boy suffering from severe hypervitaminosis A. )

On May 17, 2010, the FDA granted hydroxy-propyl-beta-cyclodextrin orphan drug status and designated hydroxy-propyl-beta-cyclodextrin as a potential treatment for Niemann Pick Type C disease. On July 14, 2010, Dr. Caroline Hastings of Children's Hospital Research Center Oakland filed additional applications with the FDA requesting approval to deliver hydroxy-propyl-beta-cyclodextrin directly into the central nervous system of the twins in an attempt to get HPBCD across the blood-brain barrier. The request was approved by the FDA on September 23, 2010, and bi-monthly intrathecal injections of HPBCD into the spine were started in October 2010. Additional filings have been made to the FDA by Children's Hospital Research Center Oakland requesting approval to surgically install Medtronic SynchroMed pumps into the twins to deliver continuous doses of HPBCD into their brains.

Other treatments under investigation
One drug that has been tried is Miglustat. Miglustat is a glucosylceramide synthase inhibitor, which inhibits the synthesis of glycosphingolipids in cells. It has been shown to delay the onset of disease in the NPC mouse, and published data from a multi-center clinical trial of Miglustat in the United States and England and from case reports suggests that it may ameliorate the course of human NPC.

Several other treatment strategies are under investigation in cell culture and animal models of NPC. These include, cholesterol mobilization, neurosteroid (a special type of hormone that effects brain and other nerve cells) replacement using allopregnanolone, rab overexpression to bypass the trafficking block (Pagano lab) and Curcumin as an anti-inflammatory and calcium modulatory agent. The pregnane X receptor has been identified as a potential target.

Neural stem cells have also been investigated in an animal model, and clear evidence of life extension in the mouse model has been shown.

Low cholesterol diets are often used, but there is no evidence of efficacy. On March 22, 2011, scientists at the University of Notre Dame and Cornell University made a breakthrough in the cure for this disease. No known cure is known yet, but we are much closer to one.

Prognosis
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.