Hereditary Spherocytosis, Splenomegally and the RBC cytoskeleton

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1 and 2: SSFSE MRI abdomen. Severe splenomegally(white arrows) and gallstones (red arrow). 3: Spleen after splenectomy in patient with hereditary spherocytosis. 4: Diagram of the RBC sub membrane cytoskeleton

HEREDITARY SPHEROCYTOSIS is a genetic disorder occurring in about 1 of every 5000 caucasian births. Peripheral blood smears show many microspherocytes. Removal of spherocystes by the spleen is the cause of the anemia. Heterozygosity is lethal in utero. Hereditary spherocytosis may appear early or late in life. The RBC cytoskeleton and cell membrane are abnormal in hereditary spherocytosis.The RBC sub membrane cytoskeleton consists primarily of; spectrin (the main cytoskeletal component), actin, protein 4.1, protein 4.9, and ankyrin. Spectrin dimers are bound to protein 4.1. which in turn links spectrin to actin. The spectrin /actin /protein 4.1 complex is bound to the cell membrane by ankyrin and protein 3. With defective protein 4.1, spectrin dimers bind poorly to actin and the cell membrane. The lipid bilayer over the defective cytoskeleton is abnormal, forming the spherocyte. The defective membrane allows intracellular Na+ accumulation and the influx of intracellular fluid This results in increased RBC fragility and hemolysis. When a spherocyte transverses the spleen it is removed from circulation. Sequestration of RBCs results in severe splenomegally. Clinical features of hereditary spherocytosis include; anemia varying from mild to severe, splenomegaly (seen in >50% of ), jaundice, bilirubin-type gallstones and gout.

Sources: Whitney Williams http://www.clt.astate.edu/wwilliam/unit_02.htm

Aortic Coartation and the Zebrafish Model of Developmental Biology

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TOP: Post gadolinium MRA of the thoracic aorta in a young child (left) and conventional spin echo T1 (right) show a tight area of narrowing of the thoracic aorta just past the left subclavian artery origin consistent with aortic stenosis. The preductal / transverse portion of the aortic arch also seems more narrow than expected and the patient is very young. This could be a "tubular" or infantile type coarctation of the aorta.

BOTTOM: A: Circulation of green fluorescent beads in a normal zebrafish embryo. B: Absence of trunk circulation due to gridlock mutation, which caused malformation of vessels near the heart, as indicated by arrow. C: Adult zebrafish. (Source: Mary Beth Gardiner. The Reporter. Vanderbilt University Medical Center February 08, 2002)

Zebrafish, Danio rerio, are popular small fish kept in home aquariums. In recent years they have also become a mainstay of developmental biologists. The zebrafish is ideal for study of development of the circulatory system. Zebrafish are easy and comparatively inexpensive to maintain, rapidly produce large numbers of offspring, and have easily studied genetics. Like human beings, zebrafish are vertebrates, and follow the typical vertebrate path of embryonic development. The embryos are clear and develop outside of the mother’s body, allowing scientists to watch a zebrafish embryo grow into a newly formed fish under a microscope. This transparency makes observation of anatomical defects caused by genetic mutation easier. Moreover, zebrafish are particularly useful for studies of cardiovascular defects since survival of the embryos is not dependent on circulating blood flow. At 24 hours of development, the beating zebrafish heart is clearly visible. The entire circulatory system can be visualized using micro angiogram technique, injecting fluorescent micro beads.

Two genes, notch and gridlock (grl), interact in a single pathway that determines whether precursor cells are destined to become arteries or veins. The gridlock gene is expressed in the lateral plate mesoderm prior to vessel formation. Notch gene activation amplifies the expression of gridlock, causing precursor cells to develop into arterial vessels. Notch suppression reduced gridlock expression, causing aortic disruption. In the embryos with the mutation, there is a gap at the juncture of the two arterial vessels that come together to form the aorta. Injection of the micro beads demonstrates blockage at this point and no distal blood flow. This defect is restricted to arterial vessels, venous branches are not affected. This was the first molecular evidence that defined the endothelial cells in arteries as being different from the endothelial cells in veins in the early stages of development. The gridlock gene is present in every organism studied, from the fruit fly to human. The protein encoded by the gridlock gene in zebrafish shares 86 percent of its structure with its human’s analog. Interesting note: Recent medical and surgical reviews make little mention of any underlying genetic basis for human coarctation and seem to favor a physiologic / hemodynamic cause for the stenosis. This doesn't make much sense for a congenital abnormality and I suspect it is due to a lack of familiarity with recent findings in Danio rerio.

Source:

Mary Beth Gardiner. The Reporter. Vanderbilt University Medical Center February 08, 2002

Tao P. Zhong, Sarah Childes, James P. Leu, Mark C. Fishman. The Gridlock Siganaling Pathway Fashions the First Embryonic Artery. Nature(414), 216-220, 2001

Pancreatic Adenocarcinoma and Epidermal Growth Factor Receptor

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Figure: (1) Oblique coronal CT abdomen. There is a hypodense heterogeneous mass in the head of the pancreas consistant with pancreatic adenocarcinoma.(red arrow) It is causing obstruction of the common bile duct (white arrowhead). (2) The extracellular domain of EGFR bound to EGF. (3) Domain structure of EGFR. SOURCE of (2) and (3) Wikipedia.

Pancreatic adenocarcinoma is the fifth most common human cancer. It usually presents as a focal mass (95%). It is most often in the pancreatic head (66%). With CT pancreatic adenocarcinoma is usually a small low-density mass. There is often invasion of local structures. There may be obstruction of the bile and pancreatic ducts and encasement of vessels. Distant metastases are most common in the liver and local nodes. Prognosis has been poor as the cancer is usually advanced at the time of diagnosis. New therapies based on biological response modifiers are starting to appear.Epidermal growth factor receptor (EGFR) is the receptor for epidermal growth factor (EGF). It is a member of the Erb B family of tyrosine kinase receptors. Kinases are protein switches that activate other proteins by adding a phosphate group to them via a process called phosphorylation. Mutations activating EGFR expression can result in cancer. EGFR is a transmembrane cell surface receptor. When inactive it is an isolated monomer. It is activated by binding specific ligands, including epidermal growth factor, transformation growth factor α , among others. EGFR then pairs with another EGFR or another Erb B receptor to form an active dimer. Dimerization stimulates tyrosine kinase domain activity of EGFR which starts a signal transduction, involving the mitogen activated protein kinase (MAPK) cascade, leading to DNA synthesis and cell proliferation. Dimerization can also result in autophosphorylation of tyrosine residues in the cytoplasmic domain of EGFR and activation of other proteins different from those signaled by the tyrosine kinase domain of EGFR. These proteins may be involved in such cellular functions as migration, adhesion, and proliferation. Mutations involving EGFR can lead to its constant activation resulting in uncontrolled cell division. EGFR was the first cell surface receptor linked to cancer. (Cohen et al 1976). The identification of EGFR as an oncogene has led to the development of monoclonal antibody therapy directed against EGFR, including gefitinib and erlotinib for lung cancer and pancreatic cancer, cetuximab for colon cancer, and trastuzumab for breast cancer.

Sources:

OMIM 131550

Lemoine NR, Hughes CM, Barton CM, Poulsom R, Jeffery RE, Kloppel G, Hall PA, Gullick WJ.The epidermal growth factor receptor in human pancreatic cancer. Molecular Pathology Laboratory, Hammersmith Hospital, London, U.K. J Pathol. 1992 Jan;166(1):7-12

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