Cystathioninuria | |
---|---|
Other names | Cystathionase deficiency |
![]() | |
Cystathionine | |
Specialty | Medical genetics |
Cystathioninuria, also called cystathionase deficiency, is an autosomal recessive [1] metabolic disorder. It is characterized by an abnormal accumulation of plasma cystathionine leading to excess cystathionine in the urine. Hereditary cystathioninuria is associated with the reduced activity of the enzyme cystathionine gamma-lyase. [2] It is considered a biochemical anomaly. This is because it associated with a wide range of diseases and its inconsistency.
Cystathionase catalyzes cystathionine to cysteine and α-ketobutyrate. [3] Cysteine is an essential amino acid and its conversion from cystathionine occurs in the trans-sulfuration pathway. The availability of cysteine is necessary for the synthesis of an important anti-oxidant, glutathione. [2] Cystathionase has a co-enzyme, pyridoxal phosphate, which is the active form the vitamin B6. This means that vitamin B6 is essential for the function of cystathionase.
Cystathioninuria can be broken down into two main categories. Primary cystathioninuria is caused by the recessive inherited deficiency of cystathionase enzyme. [4] Secondary cystathioninuria is described by non-genetic conditions of excess cystathionine. Secondary cystathioninuria includes temporary excess cystathionine of premature infants, severe generalized liver damage, thyrotoxicosis, hepatoblastoma, or neuroblastoma. [4] Cases of secondary cystathioninuria are not responsive to vitamin B6 administration. [3]
Under primary cystathioninuria, the inherited mutation of CTH gene, there are two forms. There is vitamin B6 – unresponsive and vitamin B6 – responsive cystathioninuria. [5] The vitamin B6 – unresponsive form is thought to be from a lack of the synthesis of cystathionase. This means that the mutation in CTH from this form of cystathioninuria, results in the absence of cystathionase. It could also result from the synthesis of a cystathionase that is so greatly mutated it cannot function at all. [6] On the other hand, vitamin B6 – responsive form still has synthesis of cystathionase. However, the cystathionase has an altered ability to bind to vitamin B6, its coenzyme. [6] This changed interaction lowers the efficiency of cystathionase, so it cannot convert cystathionine as well.[ citation needed ]
Cystathioninuria is inherited in an autosomal recessive manner. [1] This means the defective gene responsible for the disorder is located on an autosome, and two copies of the defective gene (one inherited from each parent) are required in order to be born with the disorder. The parents of an individual with an autosomal recessive disorder both carry one copy of the defective gene, but usually do not experience any signs or symptoms of the disorder.[ citation needed ]
Interestingly, homozygotes and heterozygotes were able to be distinguished in one study through both plasma and urinary levels of cystathionine. [3] The homozygote individuals had cystathionine levels greater than 0.5 moles per milligram of creatinine. Each of the homozygote individuals had a significant amount of cystathionine in the plasma as well. In contrast, the heterozygote individuals excreted approximately one tenth the amount of cystathionine as the homozygote individuals. The heterozygote individuals also had no detectable amounts of cystathionine in their plasma. [3]
The gene for cystathionase, CTH, has been sequenced and multiple mutations have been shown to be associated with the development of cystathioninuria. [2] Two nonsense mutations were found in exon 8 and exon 11 of CTH. Two missense mutations were also found, mainly in exon 2 and exon 7. In addition, a common non-synonymous single nucleotide polymorphism in exon 12 was also identified. The presence of various CTH mutations is consistent with the various categories associated with cystathioninuria. [2]
The main way to diagnosis cystathioninuria is simply through increased urinary excretion of cystathionine. In some cases, a genetic test is employed. [7]
The treatment, if any is available, varies depending on the category of cystathioninuria a patient has. The vitamin B6 – responsive form is best treated by an increased consumption of vitamin B6. [2] This increased consumption helps with cystathionase's altered ability to bind to the active form of vitamin B6.[ citation needed ]
Abetalipoproteinemia is a disorder characterized by abnormal absorption of fat and fat-soluble vitamins from food. It is caused by a mutation in microsomal triglyceride transfer protein resulting in deficiencies in the apolipoproteins B-48 and B-100, which are used in the synthesis and exportation of chylomicrons and VLDL respectively. It is not to be confused with familial dysbetalipoproteinemia.
Methylmalonic acidemia, also called methylmalonic aciduria, is an autosomal recessive metabolic disorder that disrupts normal amino acid metabolism. It is a classical type of organic acidemia. The result of this condition is the inability to properly digest specific fats and proteins, which in turn leads to a buildup of a toxic level of methylmalonic acid in the blood.
Adenosine deaminase deficiency is a metabolic disorder that causes immunodeficiency. It is caused by mutations in the ADA gene. It accounts for about 10–15% of all cases of autosomal recessive forms of severe combined immunodeficiency (SCID) among non-inbred populations.
Homocystinuria or HCU is an inherited disorder of the metabolism of the amino acid methionine due to a deficiency of cystathionine beta synthase or methionine synthase. It is an inherited autosomal recessive trait, which means a child needs to inherit a copy of the defective gene from both parents to be affected. Symptoms of homocystinuria can also be caused by a deficiency of vitamins B6, B12, or folate.
A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. Loci exhibiting heterozygote advantage are a small minority of loci. The specific case of heterozygote advantage due to a single locus is known as overdominance. Overdominance is a rare condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals.
Isovaleric acidemia is a rare autosomal recessive metabolic disorder which disrupts or prevents normal metabolism of the branched-chain amino acid leucine. It is a classical type of organic acidemia.
Hypoprothrombinemia is a rare blood disorder in which a deficiency in immunoreactive prothrombin, produced in the liver, results in an impaired blood clotting reaction, leading to an increased physiological risk for spontaneous bleeding. This condition can be observed in the gastrointestinal system, cranial vault, and superficial integumentary system, affecting both the male and female population. Prothrombin is a critical protein that is involved in the process of hemostasis, as well as illustrating procoagulant activities. This condition is characterized as an autosomal recessive inheritance congenital coagulation disorder affecting 1 per 2,000,000 of the population, worldwide, but is also attributed as acquired.
Tetrahydrobiopterin deficiency (THBD, BH4D) is a rare metabolic disorder that increases the blood levels of phenylalanine. Phenylalanine is an amino acid obtained normally through the diet, but can be harmful if excess levels build up, causing intellectual disability and other serious health problems. In healthy individuals, it is metabolised (hydroxylated) into tyrosine, another amino acid, by phenylalanine hydroxylase. However, this enzyme requires tetrahydrobiopterin as a cofactor and thus its deficiency slows phenylalanine metabolism.
Glutaric acidemia type 2 is an autosomal recessive metabolic disorder that is characterised by defects in the ability of the body to use proteins and fats for energy. Incompletely processed proteins and fats can build up, leading to a dangerous chemical imbalance called acidosis.
Apparent mineralocorticoid excess is an autosomal recessive disorder causing hypertension, hypernatremia and hypokalemia. It results from mutations in the HSD11B2 gene, which encodes the kidney isozyme of 11β-hydroxysteroid dehydrogenase type 2. In an unaffected individual, this isozyme inactivates circulating cortisol to the less active metabolite cortisone. The inactivating mutation leads to elevated local concentrations of cortisol in the aldosterone sensitive tissues like the kidney. Cortisol at high concentrations can cross-react and activate the mineralocorticoid receptor due to the non-selectivity of the receptor, leading to aldosterone-like effects in the kidney. This is what causes the hypokalemia, hypertension, and hypernatremia associated with the syndrome. Patients often present with severe hypertension and end-organ changes associated with it like left ventricular hypertrophy, retinal, renal and neurological vascular changes along with growth retardation and failure to thrive. In serum both aldosterone and renin levels are low.
Short-chain acyl-coenzyme A dehydrogenase deficiency (SCADD) is an autosomal recessive fatty acid oxidation disorder which affects enzymes required to break down a certain group of fats called short chain fatty acids.
Factor X deficiency is a bleeding disorder characterized by a lack in the production of factor X (FX), an enzyme protein that causes blood to clot in the coagulation cascade. Produced in the liver FX when activated cleaves prothrombin to generate thrombin in the intrinsic pathway of coagulation. This process is vitamin K dependent and enhanced by activated factor V.
Dihydropyrimidine dehydrogenase deficiency is an autosomal recessive metabolic disorder in which there is absent or significantly decreased activity of dihydropyrimidine dehydrogenase, an enzyme involved in the metabolism of uracil and thymine.
Carbamoyl phosphate synthetase I deficiency is an autosomal recessive metabolic disorder that causes ammonia to accumulate in the blood due to a lack of the enzyme carbamoyl phosphate synthetase I. Ammonia, which is formed when proteins are broken down in the body, is toxic if the levels become too high. The nervous system is especially sensitive to the effects of excess ammonia.
Sarcosinemia (SAR), also called hypersarcosinemia and SARDH deficiency, is a rare autosomal recessive metabolic disorder characterized by an increased concentration of sarcosine in blood plasma and urine ("sarcosinuria"). It can result from an inborn error of sarcosine metabolism, or from severe folate deficiency related to the folate requirement for the conversion of sarcosine to glycine. It is thought to be a relatively benign condition.
Glutathione synthetase deficiency (GSD) is a rare autosomal recessive metabolic disorder that prevents the production of glutathione. Glutathione helps prevent damage to cells by neutralizing harmful molecules generated during energy production. Glutathione also plays a role in processing medications and cancer-causing compounds (carcinogens), and building DNA, proteins, and other important cellular components.
In medical genetics, compound heterozygosity is the condition of having two or more heterogeneous recessive alleles at a particular locus that can cause genetic disease in a heterozygous state; that is, an organism is a compound heterozygote when it has two recessive alleles for the same gene, but with those two alleles being different from each other. Compound heterozygosity reflects the diversity of the mutation base for many autosomal recessive genetic disorders; mutations in most disease-causing genes have arisen many times. This means that many cases of disease arise in individuals who have two unrelated alleles, who technically are heterozygotes, but both the alleles are defective.
Cystathionine-β-synthase, also known as CBS, is an enzyme (EC 4.2.1.22) that in humans is encoded by the CBS gene. It catalyzes the first step of the transsulfuration pathway, from homocysteine to cystathionine:
Hyperprolinemia is a condition which occurs when the amino acid proline is not broken down properly by the enzymes proline oxidase or pyrroline-5-carboxylate dehydrogenase, causing a buildup of proline in the body.
Argininemia is an autosomal recessive urea cycle disorder where a deficiency of the enzyme arginase causes a buildup of arginine and ammonia in the blood. Ammonia, which is formed when proteins are broken down in the body, is toxic if levels become too high; the nervous system is especially sensitive to the effects of excess ammonia.