Sickle Cell Anemia

Introduction

Sickle-cell disease

Before study more about Sickle Cell Anemia. We should needs to know what is the morphology of normal red blood cells & sickle blood cells.

Normal Red Blood Cell (RBC) Morphology

In order to properly understand and identify the abnormal red blood cell morphologies associated with RBC disorders, the laboratorian must first become competent in normal RBC characteristics.

Normally, RBCs will display a defined morphology in the peripheral blood. Mature RBCs, under normal circumstances, are round, biconcave disc-shaped, anuclear cells measuring approximately 7-8 microns in diameter with an internal volume of 80-100 fL. The term used to express RBCs of normal size is normocytic. Mature red blood cells, under normal circumstances, will also have an appropriate hemoglobin content (a normal MCH and MCHC), giving them a red-orange appearance on Wright-stained smears. These cells will display a central pallor (lighter area inside of the cell) no larger than 3 microns in diameter. This normal morphology is indicated by the term normochromic. It is paramount for RBCs to contain an adequate amount of hemoglobin for the purpose of transporting oxygen to the tissues and carbon dioxide back to the lungs. An example of a normocytic, normochromic peripheral blood picture is shown on the right.

In addition, the RBC membrane plays a key role in allowing the deformability of the cell to take place in order to travel through smaller vessels. Normally functioning RBCs survive for approximately 120 days in the peripheral blood circulation before being removed by the liver or spleen. Under normal circumstances, the body produces enough RBCs each day to account for the senescent (old) cells that are removed.

Certain disease states can alter the normal RBC characteristics described above. This course will illustrate and correlate various RBC morphologic changes with the specific disease states or conditions, which they are associated.

   Chemistry Of Hemglobin

The hemoglobin molecule is a tetramer consisting of 4 polypeptide chains, known as globins, which are usually:

• 2 alpha chains that are each 141 amino acids long
• 2 beta chains that are each 146 amino acids long

Hemoglobin has a quaternary structure characteristic of many multi-subunit globular proteins. Most of the amino acids in hemoglobin form alpha helices, connected by short non-helical segments. Hydrogen bonds stabilize the helical sections inside this protein, causing attractions within the molecule, folding each polypeptide chain into a specific shape.Hemoglobin's quaternary structure comes from its four subunits in roughly a tetrahedral arrangement.

Genetics Of Hemoglobin

Hemoglobin consists mostly of protein subunits (the "globin" chains), and these proteins, in turn, are folded chains of a large number of different amino acids called polypeptides. The amino acid sequence of any polypeptide created by a cell is in turn determined by the stretches of DNA called genes. In all proteins, it is the amino acid sequence which determines the protein's chemical properties and function.

There is more than one hemoglobin gene. The amino acid sequences of the globin proteins in hemoglobins usually differ between species. These differences grow with evolutionary distance between species. For example, the most common hemoglobin sequences in humans and chimpanzees are nearly identical, differing by only one amino acid in both the alpha and the beta globin protein chains. These differences grow larger between less closely related species.

Haemoglobin is a protein consisting of four polypeptide chains, two α-chains, and two β-chains folded round each other. The two α-chains of haemoglobin are identical, as are the two β-chains. In haemoglobin, an oxygen transport protein, the binding of oxygen is cooperative (as each oxygen is bound, it becomes easier for the next one to bind) and is modulated by ligands such as H⁺, CO₂ and BPG (2,3-bisphosphoglycerate, sometimes referred to as DPG).

The Haem group is a porphyrin derivative containing an iron atom, which undergoes transition from Fe²⁺ to Fe³⁺ on binding O₂, which results in allosteric quaternary structural changes in the globin chains. It is this ferrous iron atom (located centrally within each haem group) which gives Hb its red appearance. A Haem group is attached to each of the four chains.

Hb is a red-coloured protein found in erythrocytes. It is composed of a prosthetic group called haem, and a globin protein. The globin consists of four polypeptide subunits, in two pairs. Different types of Hb (all found in adult blood) contain different types of polypeptides:

Type	Peptide chains	% in adult blood	% in fetal blood
HbA	α2, β2	97	10-50
HbA2	α2, delta2	2.5	trace
HbF	α2, gamma2	0.5	50-90

During early human development, HbF is normally replaced by HbA within 6 months of birth unless the production of polypeptide chains is abnormal, e.g. in thalassaemia and other haemoglobinopathies including Sickle-cell anaemia (HbS). HbF, which consists of gamma chains, have a higher affinity for oxygen due to a less positive charge in the gamma chains. This makes BPG bind less readily and as a result stabilising the oxy form of HB.

Hb tetramer molecule can bind four O₂ molecules. The binding of one O₂ molecule increases the affinity of further binding, resulting in an oxygen-haemoglobin dissociation curve, which is sigmoidal. In deoxyhaemoglobin, a water molecule is attached to each of the four iron atoms.

On oxygenation (this effect is sometimes known as co-operative) Hb undergoes conformational changes as some of the weak non-covalent bonds, holding the deoxy (Tense form) structure together, are broken. The oxygenated form of Hb is also referred to as the Relaxed form. The co-operative effect in oxygenation is due to the binding of one oxygen molecule to one iron atom in one of the 4 chains of Hb facilitates the binding of another oxygen molecule to a second iron atom in the same molecule and so on. 

Mutations affecting the proofreading domains of POLE and POLD1 predispose.

Many individuals with multiple or large colorectal adenomas or early-onset colorectal cancer (CRC) have no detectable germline mutations in the known cancer predisposition genes. Using whole-genome sequencing, supplemented by linkage and association analysis, we identified specific heterozygous POLE or POLD1 germline variants in several multiple-adenoma and/or CRC cases but in no controls. The variants associated with susceptibility, POLE p.Leu424Val and POLD1 p.Ser478Asn, have high penetrance, and POLD1 mutation was also associated with endometrial cancer predisposition. The mutations map to equivalent sites in the proofreading (exonuclease) domain of DNA polymerases ε and δ and are predicted to cause a defect in the correction of mispaired bases inserted during DNA replication. In agreement with this prediction, the tumors from mutation carriers were microsatellite stable but tended to acquire base substitution mutations, as confirmed by yeast functional assays. Further analysis of published data showed that the recently described group of hypermutant, microsatellite-stable CRCs is likely to be caused by somatic POLEmutations affecting the exonuclease domain.

Impacts of biofuel cultivation on mortality and crop yields

Ground-level ozone is a priority air pollutant, causing ~ 22,000 excess deaths per year in Europe¹, significant reductions in crop yields² and loss of biodiversity³. It is produced in the troposphere through photochemical reactions involving oxides of nitrogen (NO_x) and volatile organic compounds (VOCs). The biosphere is the main source of VOCs, with an estimated 1,150 TgC yr⁻¹ (~ 90% of total VOC emissions) released from vegetation globally⁴. Isoprene (2-methyl-1,3-butadiene) is the most significant biogenic VOC in terms of mass (around 500 TgC yr⁻¹) and chemical reactivity⁴ and plays an important role in the mediation of ground-level ozone concentrations⁵. Concerns about climate change and energy security are driving an aggressive expansion of bioenergy crop production and many of these plant species emit more isoprene than the traditional crops they are replacing. Here we quantify the increases in isoprene emission rates caused by cultivation of 72 Mha of biofuel crops in Europe. We then estimate the resultant changes in ground-level ozone concentrations and the impacts on human mortality and crop yields that these could cause. Our study highlights the need to consider more than simple carbon budgets when considering the cultivation of biofuel feedstock crops for greenhouse-gas mitigation.