Laboratory Medicine - Sapienza University of Rome Inborn errors of metabolism web page in preparation THE REGISTRATION OF ATTENDANCE TO THIS ON-LINE LECTURE IS ACTIVE!       To register your attendance please type in your surname and matricola number. Surname: Matricola: Notice that your attendance will be registered only if you completed the reading, questions, and audios, and that you cannot interrupt and resume the session (but you can repeat it as many times as you like). Remember to press the [send] button before leaving this page! A confirmation message will appear at the end of this page.       A comment section has been added at the end of this lecture. Adding a comment or question does not require registration with your matricola number, feel free to comment whenever you like.       Genetic diseases constitute a major diagnostic challenge. The number of known genetic diseases is huge, several thousands of them having been identified. If we consider them one by one, they are relatively uncommon unless some special selective condition applies, as in the case of genetic defects of hemoglobin that offer (partial) protection against malaria in areas where malaria is endemic. However, as a group they are extremely frequent, their summed incidence being in the order of one case every 100 births.       Broadly speaking, inherited genetic defects can be classified in four major groups: 1) Monogenic diseases (e.g. thalassemias) 2) Chromosomal abnormalities (e.g. Down syndrome) 3) Multifactorial diseases in which affected genes and environmental factors co-operate 4) Mitochondrial genetic diseases.       This lecture is concerned only with the laboratory diagnosis of monogenic diseases affecting enzymes of common metabolic pathways. Other monogenic diseases have been considere elsewhere in this and other courses (e.g. hemoglobinopathies, thalassemias, coagulation defects, porphyrias, ereditary forms of gout, jaundice, etc.) and will be occasionally mentioned but not described here.       As to the mechanism of inheritance, monogenic defects can be: 1) Autosomal and dominant (heteroxygotes and homozygotes are both affected. One of the parents is affected, transmission can be traced vertically in the ancestry). 2) Autosomal and recessive (Only homozygotes are affected. Parents are usually heterozygotes and not affected, but brothers and sisters may be affected; the disease cannot be traced vertically but may have occurred in an apparently ranndom fashion in the family). This is the most frequent type of inheritance. 3) Sex-linked and dominant (the gene is located on the X chromosome; both sexes may be affected) 4) Sex-linked and recessive (males are affected, female cases are extremely uncommon).       Gene mutations preventing the production of the enzyme or causing loss of function are usually inherited as recessive traits, because the healthy gene can at least partially compensate the defect. Gene mutations causing gain of function are often dominant, because the healthy gene cannot compensate.       Examples of genes located on the X chromosome, and thus leading to sex-linked inheritance include those coding for coagulation enzymes (whose defect can result in hemophylias); hypoxantine-guanine phosphorybosyl transferase (whose defect can cause the Lesch-Nyhan syndrome) and Phosphoribosyl Pirophosphate Synthetase; red-green color blindness; Duchenne musular distrophy.       Biochemistry of mendelian enzyme deficiencies       In a typical autosomal recessive genetic enzyme defect, the gene presents mutations in the promoter region or in the coding region, and these cause the protein to be expressed to a reduced extent or not at all. In some cases a mutated protein is produced that has reduced stability or impaired function. The net result of this condition is that the heterozygous patient produces a lower than normal amount of the affected enzyme. However, the usual physiological concentration of substrates is around the enzyme's KM; as a consequence a defect of enzyme concentration causes an increase of substrate concentration and this, in turn, allows the available enzyme to increase its activity, thus partially compensating the defect (this is an example of a negative feedback control mechanism).       If the described compensation occurs, a heterozygous individual may appear perfectly healthy, but in some cases the clinical laboratory may reveal that the concentration of the substrate of the affected enzyme is increased (this however occurs rarely, because substrates are usually present in the cytoplasm and thus are not accessible to analysis).       In some cases the affected enzyme (or protein) is produced and is stable but has abnormal functional properties, that cannot be compensated by the normal enzyme present in the cell. When this condition occurs, the disease is either dominant or clinically manifest even though recessive. For example in methemoglobinopathies the O2 carrying capacity of blood is reduced even in the heterozygous state, and the heterozygous β-thalassemia causes a mild anemia. A more dramatic case occurs in mutations of membrane pumps or channels that, due to a mutation, may be permanently in the closed or open conformation: the first condition is usually recessive, the second is usually dominant.       When the defect is recessive, but the patient is homozygous, the affected enzyme is produced to a very little extent or not at all (at least in a functional state), and no compensation is possible. The same occurs in male subjects for recessive genetic defects of genes located on the X chromosome.       The pioneer of the study of inherited defects of metabolism is Archibald Garrod, whose essay Inborn errors of metabolism published in 1909 makes a very interesting reading even today. The student may be perplexed by the observation that the enzymes affected rarely belong to the major metabolic pathways, those he or she studied with most attention: the glycolisis, beta-oxidation of fatty acids, the Krebs cycle, and when this occurs, the mutations do not completely inactivate the enzyme. The reason is that an embrio presenting severe defects of these pathways would never develop and would encounter a very early miscarriage: diseases may be observed and diagnosed only for those defects that are compatible at least with intrauterine life. An important example of this rule is that of genetic defects of the urea cycle, a major metabolic pathway whose disruption is incompatible with extrauterine life, but compatible with the development of the fetus, given that during intrauterine life ammonia may be disposed of by the mother via placental exchange.       Laboratory diagnosis of genetic enzyme deficiencies       A genetic enzyme deficiency is suspected very early in life, because of stunted growth, or mental retardation, or other symptoms (e.g. immune deficiencies, dysfunctions of the acid-base balance often with metabolic acidosis, etc.). Even though each defect is rare or very rare, with a frequency of one in several thousands births, the sum of all genetic defects yields a frequency of over 1% of live births. If the physician suspects the disease (the suspicion will usually be a generic one) he/she may look for gross abnormalities (e.g. metabolic acidosis or aciduria; symptoms aggravated by specific foods; etc.). A systematic search for ammonia, the principal metabolytes of aminoacids, abnormal blood or urinary concentrations of metals and other substances is usually the next step. Once the possible diagnoses have been restricted to a reasonable number, gene sequencing will provide the final proof. Audio: Genetic defects of metabolism Possible reasons to suspect an inherited defect of metabolism in a baby positive familiar anamnesis stunted growth neurological symptoms (coma, seizures, unresponsivity, mental retardation etc.) repeated infections (possibly due to immunodeficiencies) abnormal pigmentation of the skin and mucosae (e.g. albinism, jaundice, etc.) abnormal smell or color of the urine acid-base and electrolyte disorders polyuria with polydipsia   Generic laboratory tests that may point to an inherited defect of metabolism hyper- or hypo-glycemia hyper-bilirubinemia and jandice hyper-ammonemia metabolic acidosis with increased anion gap abnormal density or color of the urine; excess loss of ions, bicarbonate, glucose, etc. abnormal blood cell count or hypogammaglobulinemia       For the purposes of this lecture we shall classify the genetic defects of metabolism according to the class of metabolytes affected: 1) Sugars, including favism, glycogen storage diseases and mucopolysaccaridoses 2) Lipids, including gangliosidoses 3) Aminoacids and the urea cycle 4) Nucleotides biosynthesis, salvage and degradation 5) Heme biosynthesis (porphyrias) and degradation (genetic joundice) 6) Metal accumulation diseases. 7) Lysosomal storage disorders. 8) Mitochondial diseases 9) Defects of membrane channels and pumps Diseases classified in groups 4 and 5 were treated under separate headings and will not esplicitly considered here. It is important to stress that this classification is not extensive, e.g. it does not include the defects of structural proteins (Marfan syndrome, Ehler Danlos syndrome, achondroplasia, etc.). This is because in this course we focus our attention on diseases that cause alterations in the clinical laboratory results.       The reason of the above classification is that from the laboratory diagnostic view-point the metabolic pathway affected is indicative of the biochemical alterations one may find in the analysis: e.g. defects of the heme biosynthesis pathway cause the accumulation and excretion of pyrrole and tetrapyrrole ring derivatives. However, a classification according to the metabolic pathway affected may not be clinically indicative, because diferent defects of the same metabolic pathway may lead to very different clinical syndromes. Thus the above classification hould be cross referenced to a clinical one, that takes into account at least the most frequent syndromes, that include: a) mental retardation b) morphological abnormalities (e.g. dwarfism, cardiac defects, etc.) c) stunted growth d) metabolic acidoses e) immunodeficiencies f) anemias ...       Further readings       The World Health Organization web-page on genetic diseases.       The NIH web-page on genetic diseases. Questions and exercises: 1) Which list of symptoms better describes the possible clinical presentation of an inherited defect of metabolism? stunted growth; mental retardation or other neurological symptoms; abnormal skin pigmentation or body dismorfisms. stunted growth; pre-epatic jaundice; type 1 diabetes mellitus 2) The mechanism of transmission of the vast majority of human inherited diseases is: autosomal dominant autosomal recessive sex-linked 3) The cumulative frequency of inherited diseases in humans is in the order of: 1 in 5,000 births 1 in 1,000 births 1 in 200 births 4) Some inherited diseases may be diagnosed late in life. This is because: the disease requires time to fully develop the disease is mild the disease is recessive your score: 10 Attendance not registered because of insufficient score - Retry! You can type in a comment or question below (max. length=160 chars.): All comments posted on the different subjects have been edited and moved to this web page (for optimal reading try to have at least 80 characters per line)! Thank you Professor (lecture on bilirubin and jaundice). The fourth recorded part, the one on hyper and hypoglycemias is not working. Bellelli: I checked and in my computer it seems to work. Can you better specify the problem you observe? This Presentation (electrolytes and blood pH) feels longer than previous lectures Bellelli: it is indeed. Some subjects require more information than others. I was thinking of splitting it in two nest year. Bellelli in response to a question raised by email: when we compare the blood pH with the standard pH we do not mean to compare the "normal" blood pH (7.4) with the standard pH. Rather we compare the actual blood pH of the patient, with the pH of the same blood sample equilibrated under standard conditions. Thus, if we say that standard pH is lower than pH we mean that equilibriation with 40 mmHg CO2 has caused absorption of CO2 and has lowered the pH with respect to its value before equilibration. (Lipoproteins) Is the production of leptin an indirect cause of type 2 diabetes since it works as a stimulus to have more adipose tissue that produces hormones? Bellelli: in a sense yes, sustained increase of leptin causes the hypothalamus to adapt and to stop responding. Obesity ensues and this in turn may cause an increase in the production of resistin and other insulin-suppressing protein hormones produced by the adipose tissue. However, this is quite an indirect link, and most probably other factors contribute as well. (Urea cycle) what is the meaning of "dissimilatory pathway"? Bellelli: a dissimilatory pathway is a catabolic pathway whose function is not to produce energy, but to produce some terminal metabolyte that must be excreted. Dissimilatory pathways are necessary for those metabolytes that cannot be excreted as such by the kidney or the liver because they are toxic or poorly soluble. Examples of metabolytes that require transformation before being eliminated are heme-bilirubin, ammonia, sulfur and nitrogen oxides, etc. Talking about IDDM linked neuropathy can be the C peptide absence considered a cause of it?? Bellelli: The C peptide released during the maturation of insulin, besides being an indicator of the severity of diabetes, plays some incompletely understood physiological roles. For example it has been hypothesized that it may play a role in the reparation of the atherosclerotic damage of the small arteries. Thus said, I am not aware that it plays a direct role in preventing diabetic polyneuropathy. Diabetic neuropathy has at least two causes: the microvascular damage of the arteries of the nerve (the vasa nervorum), and a direct effect of hyperglycemia and decreased and irregular insulin supply on the nerve metabolism. Diabetic neuropathy is observed in both IDDM and NIDDM, and requires several years to develop. Since the levels of the C peptide differ in IDDM and NIDDM, this would suggest that the role of the C peptide in diabetic neuropathy is not a major one. If you do have better information please share it on this site! In acute intermitted porphyria and congenital erythropoietic porphyria why do the end product of the affected enzymes accumulate instead of their substrate?? Bellelli: First of all, congratulations! This is an excellent question. Remember that a condition is which the heme is not produced is lethal in the foetus; thus the affected enzyme(s) must maintain some functionality for the patient to be born and to come to medical attention. All known genetic defects of heme biosynthesis derange but do not block this metabolic pathway. Congenital Erythropoietc Porphyria (CEP) is a genetic defect of uroporphyrinogen III cosynthase. This protein associates to uroporphyrinogen synthase (which is present and functional in CEP) and guarantees that the appropriate uroporphyrinogen isomer is produced (i.e. uroporphyrinogen III). In the absence of a functional uroporphyrinogen III cosynthase other possible isomers of uroporphyrinogen are produced together with uroporpyrinogen III, mostly uroporphyrinogen I. The isomers of uroporphyrinogen that are produced differ because of the positions of propionate and acetate side chains, and this in turn is due to the pseudo symmetric structure of porphobilinogen. Only isomer III can be further used to produce protoporphyrin IX. Thus in the case of CEP we observe accumulation of abnormal uroporphyrinogen derivatives, which, as you correctly observed are the products of the enzymatic synthesis operated by uroporphyrinogen synthase. The case of Acute Intermittent Porphyria (AIP) is similar, although there may be variants of this disease. What happens is that either the affected enzyme is a variant that does not properly associate with uroporphyrinogen III cosynthase or presents active site mutations that impair the proper alignement of the phoprphobilinogen substrates. In either case abnormal isomers of uroporphyrinogen are produced, as in CEP. Also remark that in both AIP and CEP we observe accumulation of the porphobilinogen precursor: this is because the overall efficiency of the biosynthesis of uroporphyrinogens is reduced. Thus: (i) less uroporphyrinogen is produced, and (ii) only a fraction of the uroporphyrinogen that is produced is the correct isomer (uroporphyrinogen III). is it possible to take gulonolactone oxidase to synthesize vitamin C instead of vitamin C supplement? Bellelli: no, this approach does not work. The main reason is that the biosynthesis of vitamin C, as almost all other metabolic processes, occurs intracellularly. If you administer the enzyme it will at most reach the extracellular fluid but will not be transported inside the cells to any significant extent. Besides, there are other problems in this type of therapy (e.g. the enzyme if administered orally, may be degraded by digestive proteases; if administered parenterally, may cause the immune system to react against a non-self protein). In theory one could think of a genetic modification of the inactive human gene of gulonolactone oxidase, but the risk and cost of this intervention would not be justified. In addition to these considerations, except for cases of shipwreckage or other catastrophes, a proper diet or administration of tablets of vitamin C is effective, risk-free and unexpensive, thus no alternative therapy is reasonable. However, I express my congratulations for your search on the biosynthesis pathway of ascorbic acid. Resorption and not reabsorption would lead to hypercalcemia ie bone matrix being broken down. Bellelli: I am not sure to interpret your question correctly. Resorption indicates destruction of the bone matrix and release of calcium and phosphate in the blood, thus it causes an increase of calcemia. Reabsorption usually means active transport of calcium from the renal tubuli to the blood, thus it prevents calcium loss. It prevents hypocalcemia, and thus complement bone resorption. To avoid confusion it is better use the terms "bone resorption" and " renal reabsorption of calcium". If you have a defect in renal reabsorption, parthyroid hormone will be released to maintain a normal calcium level by means of bone resorption; the drawback is osteoporosis. In Reed and Frost model: I haven't understood what is the relationship between K and R reproductive index. Thank you Professor! Bellelli: in the Reed and Frost model K is the theoretical upper limit of R0. R the reproductive index is the ratio (new cases)/(old cases) measured after one serial generation time. R0 is the value of R one measures at the beginning of the epidemics, when in principle all the population is susceptible. What is the link between nucleotide metabolism and immunodeficiencies and mental retardation? Bellelli: the links may be quite complex, but the principal ones are as follows: 1) the immune response requires a replication burst of granulocytes and lymphocytes, which in turn requires a sudden increase of nucleotide production, necessary for DNA replication. Defects of nucleotide metabolism impair this phase of the immune defense. Notice that the mechanism is similar to the one responsible of anemia which requires a sustained biosynthesis of nucleotides at a constant rate, rather than in a burst. 2) Mental retardation is mainly due to the accumulation of nulceotide precursors in the brain of the newborn, due to the incompletely competent blood-brain barrier. How can ornithine transaminase defects cause hyperammonemia? Is it due to the accumulation of ornithine that blocks the urea cycle or for other reasons? Bellelli: ornithine transaminase is required for the reversible interconversion of ornithine and proline, and thus participates to both the biosynthesis and degradation of ornithine. The enzyme is synthesized in the cytoplasm and imported in the mitochondrion. Depending on the metabolic conditions the deficiency of this enzyme may cause both excess (when degradation would be necessary) or defect (when biosynthesis would be necessary) of ornithine; in the latter case, the urea cycle slows down. Thus there is the paradoxical condition in which alternation may occur between episodes of hyperammonemia and of hyperornithinemia. When we use the Berthelot's reaction to measure BUN do we also have to measure the concentration of free ammonia before adding urease? Bellelli: yes, in principle you should. Berthelot's reaction detects ammonia, thus one should take two identical volumes of serum, use one to measure free ammonia, the other to add urease and measure free ammonia plus ammonia released by urea. BUN is obtained by difference. However, free ammonia in our blood is so much lower than urea that you may omit the first sample, if you only want to measure BUN. Why do we have abnormal electrolytes in hematological neoplasia e.g. leukemia? Bellelli: I do not have a good explanation for this effect, which may have multiple causes. However, you should consider two factors: (i) acute leukemias cause a massive proliferation of leukocytes (or lymphocytes depending on the cell type affected) with a very shortened lifetime; thus you observe an excess death rate of the neoplastic cells. The dying cells release in the bloodstream their content, which has an electrolyte composition different from that of plasma: the cell cytoplasm is rich in K and poor in Na, thus causing hyperkalemia. (ii) the kidney may be affected by the accumulation of neoplastic white cells or their lytic products. Gaussian curve: If it is bimodal is it more likely to be a "certain diagnosis" than if it is unimodal or does it only show the distinguishment from health? Bellelli an obviously bimodal Gaussian curve indicates that the disease is clearly separated from health: usually it is a matter of how precise and clear-cut is the definition of the disease. For example tuberculosis is the disease caused by M. tuberculosis, thus if the culture of the sputum is positive for this bacterium you have a "certain" diagnosis (caution: the patient may suffer of two diseases, e.g. tuberculosis and COPD diagnosis of the first does not exclude the second). However, in order to have a "certain" diagnosis it is not enough that distribution of the parameter is bimodal, it is also required that the patient's parameter is out of the range of the healthy condition: this is because a distribution can be bimodal even though it is composed by two Gaussians that present a large overlap, and the patient's parameter may fall in the overlapping region. Thus, in order to obtain a "certain" diagnosis you need to consider not only the distribution of the parameter(s) but also the patient's values and the extent of the overlapping region. Prof can you please elaborate a bit more on the interhuman variability and its difference with the interpopulation variability please? Bellelli: every individual is a unique combination of different alleles of the same genes; this is the source of interindividual variability. Every population is a group of individuals who intermarry and share the same gene pool (better: allele pool). Every allele in a population has its own frequency. Two population may differ because of the diffferent frequencies of the same alleles; in some cases one population may completely lack some alleles. The number and frequencies of alleles of each gene determine the variance. If you take two populations and calculate the cumulative interindividual variance of the population the number you obtain is the sum of two contributions: the interindividual variance within each population, plus the interpopulation variance between the means of the allele frequencies. For example, there are human population in which the frequency of blood group B is close to 0% and other populati ons in which it is 30% or more. Prof can you please explain again the graph you have showed us in class about thromboplastin? (Y axis=abs X axis= time) Bellelli: the graph that I crudely sketched in class represented the signal of the instrument (an absorbance spectrophotometer) used to record the turbidity of the sample (turbidimetry). The plasma is more or less transparent, before coagulation starts. When calcium and the tissue factor (or collagen) are added. thrombin is activated and begins digesting fibrinogen to fibrin; then fibrin aggregates. The macroscopic fibrin aggregates cause the sample to become turbid, which means it scatters the incident light. The instrument reads this as a decrease of transmitted light (i.re an increase of the apparent absorbance) and the time profile of the signal presents an initial lag phase, which is called the protrombin or thromboplastin time depending on the component which was added to start coagulation (tissue factor or collagen). Prof can you please explain the concept you have described in class about the simultaneous hypercoagulation and hemorrhagic syndrome? How can this occur? Bellelli: The condition you describe is observed only in the Disseminated Intravascular Coagulation syndrome. Suppose that the patient experiences an episode of acute pancreatitis: tripsin and chymotripsin are reabsorbed in the blood and proteolytically activate coagulation causing an extensive consumption of fibrinogen and other coagulation factors. Tripsin and chymotripsin also damage the vessel walls and may cause internal hemorrages, but at that point the consumption of fibrinogen may have been so massive that not enough is left to form the clot where the vessel has been damaged, causing an internal hemorrage. Pancreatitis is a very severe, potentially lethal condition, and DIC is only one of the reasons of its severity. You said that certain drugs (ethanol, cocaine, cannabis, opiates...) cause a necessity of higher and higher dosage, for two reasons: the enzyme in the liver is inducible and the receptors in the brain are expressed less and less. So, first, I am not sure I got it right, and second I did not understand how expressing less receptors leads to a necessity of higher dosage. Bellelli: You got it correctly, but the detailed mechanism of resistance may vary among different substances, and not all drugs cause adaptation. The reason why reducing the number of receptors may require an increased dosage of the drug is as follows: suppose that a certain cell has 10,000 receptors for a drug. When bound to its agonist/effector, each receptor produces an intracellular second messenger. Suppose that in order for the cell to respond 1,000 receptors must be activated. The concentration of the effector required is thus the concentration that produces 10% saturation. You can easily calculate that this concentration is approximately 1/10 of the equilibrium dissociation constant of the receptor-effector complex (its Kd), the law being Fraction bound = [X] / ([X]+Kd) where [X] is the concentration of the free drug. After repeated administration, the subject becomes adapted to the drug, and his/her cells express less receptors, say 5,000. The cell response will in any case require that 1,000 receptors are bound to the effector and activated, but this now represents 20% of the total receptors, instead of 10%. The drug concentration required is now 1/4 of the Kd. Continuing administration of the drug further reduces the cell receptors, but the absolute number of activated receptors required to start the response is constant; thus the fewer receptors on the cell membrane, the higher the fraction of activated receptors required. Why does hyperosmolarity happen in type 2 diabetes and not in type 1? Bellelli: Hyperosmolarity can occur also in type 1 diabetes, albeit infrequently. The approximate formula for plasma osmolarity is reported in the lecture on electrolytes: osmolarity = 2 x (Na+ + K+) + BUN/2.8 + glucose/18 this is expressed in the usual clinical laboratory units (mEq/L for electrolytes, g/dL for non- electrolytes). The normal values are: osmolarity = 2 x (135 + 5) + 15/2.8 + 100/18 = 280 + 5.4 + 5.6 = 291 mOsmol/L Let's imagine a diabetic patient having normal values for electrolytes and BUN, and glycemia=400 mg/dL: osmolarity = 280 + 5.4 + 22.4 = 307.8 mOsmol/L The hyperosmolarity in diabetes is mainly due to hyperglycemia, even though other factors may contribute (e.g. diabetic nefropathy); however the contribution of glucose to osmolarity is relatively small. As a consequence in order to observe hyperosmolarity the hyperglycemia should be extremely high; this is more often observed in type 2 than in type 1 diabetes, for several reasons, the most relevant of which is that in type 1 diabetes all cells are starved of glucose, and the global reserve of glycogen in the body is impoverished: there is too much glucose in the blood and too few everywhere else, thus reducing, but not abolishing, the risk of extreme hyperglycemia. Usually in type 2 diabetes the glycogen reserve in the organism is not impoverished, thus the risk of extreme hyperglycemia is higher. Hemostasis and Thrombosis lecture: I don't understand why is sodium citrate added to the serum solution to measure the prothrombin time. Bellelli: in order to measure PT or PTT you want to be able to start the coagulation process at an arbitrary time zero, and measure the increase in turbidity of the serum sample. To do so you need (i) to prevent spontaneous coagulation with an anticoagulant; and (ii) to be able to overcome the anticoagulant at your will. Citrate (or oxaloacetate; or EDTA) has the required characteristics: it chelates calcium, and in this way it prevents coagulation; but you can revert its effect at your will by adding CaCl2 in excess to the amount of citrate. You cannot obtain the same effect with other anticoagulants (e.g. heparin) whose action cannot be easily overcome. Dear professor I cannot do the self evaluation test because it says the the time has expired It is not possible because I havent even started them Bellelli: this is due to the fact that the program registers your name and matricola number from previous attempts. I shall fix this bug. Meanwhile try to use a fake matricola number. How is nephrotic syndrome associated hypoalbuminemia as you described in methods of analysis of protein because seems counterintuitive Bellelli: nephrotic syndrome is an autoimmune disease in which the glomerulus is damaged and the filtration barrier is disrupted; diuresis is normal but there is loss of proteins (mostly albumin) in the urine. I m sorry i confused polyurea with hypoalbuminemia but my question still stands during glomerulonephritis you mentioned something of polyurea as compensation i could not follow how this compensation mechanism works and collapse after some time in glomerulonephritis Bellelli: the condition you describe is NOT characteristic of acute glomerulonephritis. In glomerulonephritis there is damage of the glomerulus and severely impaired GFR. Thus the diuresis is severely reduced, and due to impaired filtration proteins appear in the urine. The condition you describe corresponds to the initial stage of chronic kidney failure, usually due to atherosclerosis, diabetes, hypertension or other type of damage of the kidney tissue. In this case GFR is impaired, albeit to a lesser extent than in glomerulonephritis, and the excretion of urea is reduced. This leads to increased BUN. However the increased concentration of urea reduces the ability of the tubuli to reabsorb water, because of osmotic reasons, yielding compensatory polyuria. The patient has reduced GFR but normal or increased diuresis (urine volume in 24 hours). To some extent this effect is beneficial, as it favors the elimination of urea; however it cannot completely solve the problem and in any case the progression of the disease leads to kidney insufficiency. In its essence the point is that a moderately reduced GFR can be partially compensated by reduced tubular reabsorption; a severely reduced GFR cannot.       Home of this course