Browsing Pathways

Krabbe disease, also called globoid cell leukodystrophy, is an extremely rare inherited inborn error of metabolism (IEM). It is a degenerative disorder that affects the nervous system. It has an estimated prevalence of 1/100,000 in the Northern European population and a worldwide incidence of 1/100,000-1/250,000 live births. Krabbe disease is an autosomal recessive disorder that is caused by a deficiency of an enzyme called galactosylceramidase. Galactosylceramidase is a lysosomal protein that hydrolyzes the galactose ester bonds of ceramides and ceramide derivatives including galactocerebroside, galactosylsphingosine (psychosine), lactosylceramide, and monogalactosyldiglyceride. More specifically, galactosylceramidase is an enzyme that is involved in the catabolism (via the removal of galactose) of galactosylceramide, a major lipid in myelin, kidney, and epithelial cells of the small intestine and colon. Defects in galactosylceramidase lead to the accumulation of cytotoxic psychosine, which ultimately leads to apoptosis of oligodendrocytes and demyelination. As a result, this enzyme deficiency impairs the growth and maintenance of myelin, the protective sheath around nerve cell axons that ensures that electrical impulses are rapidly transmitted. Krabbe disease is part of a group of disorders known as leukodystrophies, which result from the loss of myelin (demyelination). Krabbe disease is also characterized by the abnormal presence of globoid cells, which are globe-shaped cells that often have multiple nuclei. There are three different phenotypes for Krabbe disease: infantile, juvenile, and late-onset. Neurodegeneration and early death (at age 2-3) occur in most infantile cases. In juvenile patients, the disease is often fatal 2-7 years after the symptoms begin. Adult-onset patients can survive many years after symptoms first manifest. The symptoms of infantile Krabbe disease usually begin during the first year of life. Typically, the initial signs and symptoms include feeding difficulties, episodes of fever without any sign of infection, irritability, stiff posture, muscle weakness, and slowed mental and physical development. Muscles continue to weaken as the disease progresses which decreases the infant's ability to move, chew, swallow, and breathe. It is also common for affected infants to experience vision loss and seizures. Treatment is limited to hematopoietic stem cell transplantation in pre-symptomatic infantile patients and mildly affected late-onset patients. Stem cell transplants have been shown to slow the progression of the disease.

Hyperlysinemia type I is a rare inherited inborn error of metabolism (IEM) of lysine metabolism. It is an autosomal recessive disorder that is caused by a defect in the alpha-aminoadipic semialdehyde synthase gene (AASS). The AASS gene encodes a bifunctional enzyme that contains lysine alpha-ketoglutarate reductase and saccharopine dehydrogenase. In hyperlysinemia type I, both enzymatic functions of AASS are defective. AASS is involved in the first two steps of the lysine degradation pathway. Lysine-alpha-ketoglutarate reductase catalyzes the metabolism of lysine to saccharopine, which is then cleaved to alpha-aminoadipic semialdehyde and glutamic acid by saccharopine dehydrogenase. Hyperlysinemia type I is characterized by elevated blood levels of the amino acid lysine, a building block of most proteins. Pipecolic acid can also be increased in serum and urine, while ornithine is typically decreased. Clinical symptoms of hyperlysinemia are highly variable. The descriptions range from symptom-free to severe developmental delay, spastic diplegia, seizures, rigidity, coma, episodic vomiting, and diarrhea. For the vast majority of people, hyperlysinemia typically causes no health problems, and most people with elevated lysine levels are unaware that they have this condition.

Saccharopinuria (also known as: saccharopinemia, saccharopine dehydrogenase deficiency, and alpha-aminoadipic semialdehyde synthase deficiency, hyperlysinemia type II) is an autosomal recessive disease characterized by high concentrations of saccharopine in the plasma and urine.It is caused by the deficiency of the enzyme alpha-aminoadipic semialdehyde synthase (AASS). AASS contains a lysine ketoglutarate reductase (LKR) domain which catalyzes the conversion of lysine to saccharopine, and a saccharapine dehydrogenase (SDH) domain which catalyzes the conversion of saccharopine to alpha-aminoadipic semialdehyde. Hyperlysinemia type II is categorized by the loss in SDH activity but the preservation of significant amounts of LKR activity. This leads to the accumulation of saccharopine. The loss of both enyzmatic functions is categorized as hyperlysinemia type I.

D-Glyceric aciduria is an extremely rare inherited inborn error of metabolism (IEM) of serine and fructose metabolism. It is an autosomal recessive disorder that is caused by a defect in the D-glycerate kinase (GLYCTK) gene. GLYCTK codes for D-glycerate kinase, an enzyme that is responsible for phosphorylating D-glyceric acid into phosphoglycerate. D-Glycerate kinase is an enzyme that participates in 3 metabolic pathways: (1) serine/glycine/threonine metabolism, (2) glycerolipid metabolism, and (3) glyoxylate-dicarboxylate metabolism (which is a minor pathway in fructose metabolism). Defects in the enzyme will lead to accumulations of D-glyceric acid in tissues and biofluids. D-Glyceric aciduria was first described in 1974 and is characterized by elevated levels of D-glyceric acid in the urine. Clinical symptoms of D-glyceric aciduria are highly variable. Some patients have neurological symptoms, with severe mental retardation, seizures, microcephaly, and sometimes early death, whereas others have a mild phenotype with only mild speech delay or even normal development.

Familial lipoprotein lipase deficiency (LPLD), also known as familial chylomicronemia syndrome, chylomicronemia, chylomicronemia syndrome, and hyperlipoproteinemia type Ia, is an extremely rare inherited inborn error of metabolism (IEM) of lipid metabolism. LPLD affects about 1 out of 1 000 000 people. It is an autosomal recessive disorder that is caused by a defect or deficiency in the enzyme lipoprotein lipase. Lipoprotein lipase is a water-soluble enzyme that hydrolyzes triglycerides in lipoproteins, such as those found in chylomicrons and very-low-density lipoproteins (VLDL), into two free fatty acids and one monoacylglycerol molecule. Defects in lipoprotein lipase will lead to accumulations of triglycerides and massive accumulation of fatty droplets called chylomicrons in the blood. As a result, LPLD is characterized by abnormally elevated levels of triglycerides and chylomicrons in serum and plasma (chylomicronemia). Affected individuals often experience episodes of abdominal pain, acute recurrent inflammation of the pancreas (pancreatitis), abnormal enlargement of the liver and/or spleen, and the development of skin lesions known as eruptive xanthomas. Most cases of LPLD are identified before the age of 10. In roughly one-quarter of patients, the disorder is identified during the first year of life. Some affected individuals may not be identified until adulthood. Treatment of LPLD is mainly based on medical nutrition therapy to maintain plasma triglyceride concentration below 11.3 mmol/L. Lipid-lowering agents such as fibrates and omega-3-fatty acids can be used to lower triglyceride levels in LPLD.

Glycogen storage disease, Type VII, also called GSD VII and Tarui Disease, is an inborn error of metabolism (IEM) and metabolic disorder caused by a defective 6-phosphofructokinase. 6-phosphofructokinase catalyzes the conversion of fructose 6-phosphate into fructose 1,6-bisphosphate which is the substrate of fructose-bisphosphate aldolase A. This disorder is characterized by a large accumulation of fructose 6-phosphate. Symptoms of the disorder include anemia, increased muscle glycogen content and myotonia. Currently, the major treatment for Glycogen storage disease, Type VII is associated with diet management.

Sulfite oxidase deficiency (SOD) is a disorder, an autosomal recessive disease. In classic SOD, it is usually identified a few days after the birth of an affected individual, and is recognizable through characteristic dysmorphic features, seizures, and other signs of progressive encephalopathy. Patients also have ocular lenses that are dislocated, and usually die within a few months of being born. In late- onset SOD, the disorder is identified only in the later months, usually 6-18 months, of the child’s life by a delay or regression of neurological progress. This disorder is very rare, but the actual prevalence is not known. It can be diagnosed through a sulfite test strip in urine or by a skin fibroblast culture, which will indicate an absence of sulfite oxidase.

Momoamine oxidase A (MAO-A) deficiency, or Brunner syndrome, is an X-linked recessive genetic disorder caused by a mutation in the MAOA gene that encodes for monoamine oxidase A. As such it is almost exclusively found in men. MAO-A is an enzyme that catalyzes the deamination of amines such as epinephrine, dopamine and tyramine, as part of the tyrosine metabolism pathway. In this disorder, some neurotransmitters such as serotonin and dopamine build up in the brain due to their inability to be properly metabolized. Since serotonin helps to regulate emotions and mood, with epinephrine and norepinephrine regulating stress, the unnecessary presence of the chemicals in the brain can lead to poor impulse control, aggression and other effects. The buildup of chemicals may also damage the brain, leading to a lower IQ in individuals with this disorder. In addition, foods containing the compounds that cannot be broken down, such as tyramine, can cause episodes of increased symptoms in the patients. In the subpathway that converts dopamine to homovanillic acid, there are two instances of MAO-A that are inactivated in this disorder, both in different branches. The first reaction converts dopamine to 3,4-dihydroxyphenylacetaldehyde, while the second converts 3-methoxytyramine to homovanillin. With the inactivation of MAO-A, 3-methoxytyramine builds up as there are no reactions that use it, and both of these paths lead to a decrease in the concentration of homovanillic acid, as there are no other reactions present that produce it. Another reaction, this time converting tyramine to homovanillin, is also prevented by the lack of MAO-A, which leads to an accumulation of tyramine in the body. In another branch of tyrosine metabolism, the absence of MAO-A prevents the oxidation of norepinephrine and epinephrine into 3,4-dihydroxymandelaldehyde. Its absence also prevents the oxidative deamination of metanephrine and normetanephrine into 3-methoxy-4-hydroxyphenylglycolaldehyde. As this is no longer produced, it leads to a decrease in the concentration of vanillylmandelic acid, which is produced from 3-methoxy-4-hydroxyphenylglycolaldehyde in a reaction catalyzed by aldehyde dehydrogenase.

GM2 gangliosidosis varient B or Tay-Sachs disease(TSD) is a neurodegenerative disorder which causes death in infantiles by age 5. Symptoms of TSD are present within 6 months of birth and include lack of motor development, mental retardation, seizures, and ultimately death. TSD is caused by the accumulation of GM2 gangliosides. Hexosaminadase A is the enzyme responsible for the degradation of GM2 gangliosides. It is a heterodimer made an alpha and beta subunit. The deficiency of this enzyme leads to the accumulation of GM2 gangliosides in neuronal lysosomes, eventually leading to cell death

Adenine phosphoribosyltransferase deficiency, which is also known as APRTD or APRT deficiency, is a rare inherited inborn error of metabolism (IEM) leading to the recurrent formation of kidney stones. It is an autosomal recessive disorder associated with a mutation in the enzyme adenine phosphoribosyltransferase (APRT). APRT is involved in the nucleotide salvage pathway, which provides an alternative, and energetically more efficient route to nucleotide biosynthesis in humans and most other animals. A defect in this enzyme can lead to the accumulation of the insoluble purine known as 2,8-dihydroxyadenine. In particular, when APRT has reduced or nonexistent activity, adenine accumulates which is then degraded by xanthine dehydrogenase to 2,8-dihydroxyadenine (DHA). 2,8-Dihydroxyadenine is a derivative of adenine which accumulates in 2,8 dihydroxyadenine urolithiasis (kidney stones). Kidney and urinary tract stones can obstruct the urinary tract, resulting in pain and difficulty urinating. If left untreated, the condition can eventually produce kidney failure. APRTD was first diagnosed in 1976. There are two categories of APRTD: type I involves a complete loss of the APRT function while type II involves a partial loss and is mostly found in Japan. APRT deficiency is estimated to affect 1 in 27 000 people in Japan. APRTD is rarer in Europe, where it affects 1 in 50 000 to 100 000 people. A diagnosis of APRTD can be made by analyzing kidney stones or measuring DHA concentrations in urine. APRTD is treatable with regular doses of allopurinol, which inhibits xanthine dehydrogenase activity. APRTD can also be treated with a low-purine diet and a high fluid intake.