Introduction
A boy, who is fifteen years old, is diagnosed with very high blood pressure (136/114 mmHg to 142/100 mmHg compared to normal 120/80 mmHg). However, there are no other symptoms. In addition, from the family history we know that his mother and two maternal uncles also have hypertension. Hypertension at young age is very unusual and taking into consideration his family history we can conclude that he possibly has a genetic inherited disorder. From the blood test results, we can see that the concentration of sodium is very high (160 mmol/L compared to normal 127.83 mmol/L), the concentration of potassium is lower than normal (3.1 mmol/ compared to 3.4 mmol/L). The concentration of CO2 is also higher than normal (32 compared to 29) and it indicates alkalosis. More than that, renin and aldosterone levels are also lower than they should be (<0.2 ng/dL compared to 1.8 ng/dLand and <1 ng/dL compared to 21 ng/dL respectively). Initial treatment with spironolactome does not improve his state. Notwithstanding, further treatment with triamterene shows good results. Taking into account all these clinical manifestations, we can diagnose the boy with Liddle syndrome. Liddle syndrome is a rare genetic inherited disorder and its main symptom is severe high blood pressure presented at young age. Liddle syndrome involves overexpression of the epithelial sodium channels, that results in increased sodium and water reabsorption and increased potassium excretion. Liddle syndrome is characterized by high concentration of sodium, low concentration of potassium, low activity of renin, low levels of aldosterone and metabolic alkalosis. The blood test results confirm the diagnosis
Explain how the defects in the underlying biological process account for the clinical symptoms
The main cause of the hypertension in Liddle syndrome is dysregulation of the epithelial sodium channels (ENaC), which are located on the luminal membrane in the collecting tubules (in the apical membranes of the renal tubule principal cells) in the kidneys, lungs, respiratory tract and some other organs. It consists of three homologous subunits: alpha, beta and gamma. The subunits are composed of two transmembrane domains between N- and C- termini. The main function of ENaC is to transport sodium ions into the epithelial cells from the extracellular fluid. When these channels open, sodium ions flow into the cells. Therefore, ENaC plays an important role in the regulation of the extracellular fluid volume and blood pressure by modulating osmolarity. When the concentration of intracellular sodium is high, PY motifs in the C-terminal of ENaC bind to WW domains of E3 ubiquitin ligase enzyme Nedd4-2. Nedd4-2 facilitates protein ubiquitination and further protein degradation by the proteasomes, therefore the expression of the ENaC on the epithelial surfaces is decreased. Ubiquitin is a 76-amino-acid polypeptide that acts as a marker that targets proteins for degradation, when binds to the amino group of the side chain of a lysine residue. After that, more ubiquitins are added so multiubiquitin chain forms. Proteasomes recognize such polyubiquitinated proteins and degrade them. The process of ubiquitination involves several steps. Firstly, ubiquitin attaches to ubiquitin-activating enzyme (E1). Binding to this enzyme activates ubiquitin. Secondly, the ubiquitin is transferred to E2, which is ubiquitin-conjugating enzyme. Finally, E3, that function is to recognize the specific substrate, mediates transfer of ubiquitin to the target protein. In Liddle syndrome, there are mutations in SCNN1B and SCNN1G genes on chromosome 16p. These genes code the beta and gamma subunits of ENaC respectively. Mutations cause deletions of proline-rich regions on these genes, which are responsible for PY motifs in the COOH-terminal of the beta and gamma subunits. PY motifs are the binding sites for Nedd4-2. As a result, Nedd4-2 cannot bind to mutated subunits so ENaC therefore channels are constantly present at the epithelium of collecting tubules, leading to increased sodium and water resorption, increased potassium excretion, which we see in the patient’s blood test results. All these factors result in increased extracellular fluid volume and, in the issue, elevated blood pressure, which we see in our patient.
Figure 1. ENaC structure (taken from http://jasn.asnjournals.org/content/19/10/1845/F1.expansion.html)
Describe the basic mechanism of action of the anti-hypertensive drugs mentioned in the case history and explain their relative effectiveness in the context of how the underlying disease
Spironolactone is the first drug that is prescribed for the patient. Notwithstanding, the there is no reduction of hypertension after 4 weeks of treatment. Spironolactone is a synthetic 17-lactone steroid in a class of potassium sparing diuretic drugs, that decreases the absorption of sodium. It is a specific antagonist of aldosterone and competes for binding to the intracellular receptors at the aldosterone-dependent sodium-potassium exchange sites, which are located in the distal tubule cells in the kidneys. Aldosterone is a hormone, that binds to the cytoplasmic mineralocorticoids receptor and elevates the reabsorption of sodium and water. Spironolactone blocks the action of aldosterone by binding to the mineralocortocids receptors, therefore increasing the excretion of sodium and water. As the patient has Liddle syndrome, that does not include any problems with aldosterone action, accordingly this drug is not effective because it has not any direct effect on ENaC. Afterwards triamterene is prescribed to the patient and it shows good results, reducing the blood pressure. Unlike spironolactone, triamterene, which is also a diuretic, has an effect directly on the ENaC and inhibit them. Thus, the absorption of sodium is decreased. The osmolarity in the lumen in the nephron is increased, but, in its turn, the osmolarity in the renal interstitial compartment is decreased. The main driving force for water absorption is the concentration of the sodium, so as the concentration decreases, the reabsorption of water is reduced due to smaller concentration gradient. Moreover, triamterene decreases the potassium excretion as this process is coupled to the reabsorption of sodium. When sodium goes into the cell, both the positive charge in the cell and the negative charge in the lumen rise, hence elevating the potassium excretion from the cell into the lumen. Since the patient has hypertension due to overexpression of ENaC, triamterene results in reduction of high blood pressure as it has direct effect on ENaC.
Describe how high blood pressure is regulated by the renin-angiotensin system and the role of the renal epithelial sodium channel in this regulation
The renin-angiotensin-aldosterone system (RAAS) is a major endocrine/paracrine system, that plays a crucial role in regulating blood pressure, by controlling blood volume and systemic vascular resistance. As can be concluded in the name, the main components that are involved are renin, angiotensin and aldosterone. The system is a cascade, where each constituent stimulates the production of the next one in the pathway. Renin, which is a protein enzyme, is synthesized by juxtaglomerular cells in the afferent arteriole entering the glomerulus in the kidney. When the pressure in the afferent arteriole decreases, renin is released into the bloodstream. Beta1-adrenoreceptors, which are located on the juxtaglomerular cells, are stimulated sympathetically and release renin. Macula dense, which are specialized cells react on changes in the sodium and chloride concentration. When the concentration is low, renin is released. Moreover, prostaglandins also promote the release of renin. Renin acts on angiotensin, which is synthesized in the liver and circulates in the bloodstream, and promotes proteolytic cleavage, forming decapeptide angiotensin I. In its turn, angiotensin I transforms into octapeptide angiotensin II. This is catalyzed by angiotensin converting enzyme (ACE), which is located in the vascular epithelium in the lungs and cleaves two amino acids from angiotensin I. Alternatively, angiotensin II is formed in other organs (for instance, liver) and can also entry the pathway. Angiotensin II has a lot of physiological effects on the body and one of them is stimulating aldosterone synthesis and release from zona glomerulosa in the cortex of the adrenal glands. Aldosterone is a steroid hormone and it binds to the nuclear receptor in the cell and targets the Na/K ATPase genes. Increased expression of these genes promotes the activation of NA/K ATPase, therefore stimulating water and sodium reabsorption. In its turn, these factors result in increased blood volume and higher blood pressure. The principal receptor of aldosterone, which is the mineralocorticoid receptor, mediates the aldosterone response, targeting alpha subunit of ENaC, that also stimulates sodium and water reabsorption. More than that, aldosterone increases the number of ENaC and their activity, therefore promoting sodium ions influx into the cells. In its turn, ENaC regulate the RAAS system via negative feedback. As channels are constantly working, the concentration of sodium is very high and the rates of sodium and water absorption are also very high. This leads to high blood pressure in the afferent arterioles and therefore inhibits the release of renin. Thus, whole RAAS system is inhibited as renin is the first component that need to be released for activation of further components.
Conclusion
15 years boy with only one symptom, which is hypertension, is diagnosed with Liddle syndrome. This diagnosis is further confirmed with blood tests results. Liddle syndrome is an uncommon genetic disorder, which is characterized by increased number and activity of epithelial sodium channels. These channels are located in many organs, such lungs and colon, but primarily in the distal tubules in the kidneys. Elevated number and activity of these channels result in increased sodium and water reabsorption, therefore promoting hypertension. Overexpression of ENaC are due to mutations in SCNN1B and SCNN1G genes, which are responsible for beta and gamma subunits of ENaC. Because of mutations, these subunits lack PY motifs, that serve as binding sites for E3 ubiquitin ligase enzyme Nedd4-2. This enzyme plays an important part in ENaC degradation. Nedd4-2 cannot bind to the ENaC, so the degradation of these protein channels is reduced. As a result, there is a more ENaC than it should be.
At first, the boy is treated with spironolactone, but after about a month of treatment, no significant reduction in blood pressure is detected. Spironolactone is a diuretic drug, which inhibits aldosterone. Aldosterone is a hormone, that is responsible for sodium reabsorption, therefore regulating the water reabsorption and, as a result, regulating blood pressure. However, in Liddle syndrome there is no dysfunction of aldosterone so spironolactone does not help the patient. So, the boy is prescribed with triamterene. Triamterene is a diuretic drug as well as spironolactone, but their mechanisms of action differ. Triamterene directly block ENaC, resulting into decreased sodium and water reabsorption and decreased potassium excretion. As this is the case in Liddle syndrome, after treating with triamterene, the reduction of blood pressure occurs.
The renin-angiotensin-aldosterone system is the main system that controls blood pressure. Renin is released from the kidneys, when the pressure in the afferent arterioles falls, therefore stimulating angiotensin from the liver. Angiotensin transform into angiotensin I, which subsequently transforms into angiotensin II by ACE. Angiotensin II has a lot of functions in the body and one of them is stimulating the production of aldosterone in the adrenal cortex. Aldosterone elevates the sodium and water reabsorption by increasing the amount of ENaC and their activity, resulting in increasing blood pressure.