In this article we will discuss about the control of the synthesis of haemoglobin.
1. The Control of the Rate of Synthesis of the Peptide Chains.
i. Rate of synthesis of Normal peptide chains:
Approximately 95% of the red cell protein is haemoglobin. The peptide chains of haemoglobins are thus synthesized at a rate much higher than that of any other protein in the red cells and the haemoglobin synthesis is one of the main aspects of the differentiation of stem cells into erythrocytes.
Very little is known of the mechanisms controlling this differentiation. One can guess that, in the course of differentiation, the haemoglobin genes become activated in such a way that their products become the main components of the red cells; however, how the activation of a group of genes is brought about selectively in mammalian tissues is not known.
In the normal red cells only “complete” haemoglobin molecules are observed i.e., Hb-A = α2 A BA 2, Hb-F = α A 2Y F2, or Hb-A2 = α2 A δ2 A2. These haemoglobin molecules have in common α A 2 dimers. During haemoglobin synthesis the amount of βA 2Y2 F and δ2 A 2 dimers produced is equivalent to the amount of α2 dimers produced. This raises the question of how this equilibrium in the synthesis of products of independent genes is maintained.
It is known that when a defective a gene is present (as in a chain thalassemia),abnormal aggregation of β2 or Y2 dimers may be observed. It may thus be supposed that the β and Y genes have an autonomous regulation of β and Y chain production.
In pathological conditions in which an equivalent amount of a chain is not produced, the β genes during adult life or the Y genes during fetal life produce β2 and Y2 dimers in excess over the available α2 dimers. A similar situation is not observed in conditions involving a decrease in p chains production (as in β thalassemia). No abnormal aggregation of α2 dimers is observed in this disease and no evidence of an accumulation of these dimers has been reported.
The α gene seems thus to have a non-autonomous regulation. The amount of a chains produced seems to be dependent on the amount of Y or β chains available; if not enough β or Y chains are made, the production of a chains seems to be lowered in a parallel and corresponding way.
It is not known how this control is achieved; one can guess that steric hindrance factors, prohibiting the polymerisation of a chains, are of importance, if the solubility of the dimers or of the single chains is low in physiological conditions. One can visualize an equilibrium between a peptide chains sitting on or in the proximity of the ribosomal templates, dimerization of the α chains, and release of the α2 dimers upon combination with β2, Y2, or δ2 dimers.
Within the haemoglobin peptide chains there are extreme variations in the rate of synthesis. Hb-A2 (α 2 A δ2 A 2 ) represents approximately only 2.5% of the haemoglobin in normal adults. For every 40βA chains only one δ A 2 chain is synthesized. This ratio is quite constant, within the limits of random variability, and is only found to be altered in favour of the δ chains when δ chains are produced with lower efficiency, as in β thalassemia.
One wonders how this constant ratio is maintained and why the δ chains, which are extremely similar to the β chains from a chemical point of view, are produced with so much lower an efficiency. The existence of controlling genetic elements has been postulated by several authors; in their view the synthesis of the δ chains is repressed by a specific regulator gene or, vice versa, the synthesis of β chains is activated to a higher extent by a specific regulator gene. Thalassemia may then correspond to a mutation of a regulator gene, rather than of a structural gene. No clear-cut genetic or chemical evidence has so far been presented in support of this hypothesis.
ii. Rate of synthesis of Abnormal peptide chains:
The rate of synthesis of an abnormal peptide chain is, in general, lower than that of the corresponding normal chain. The Hb- A/abnormal haemoglobin ratio which is assumed to be equivalent to the ratio of normal chain/abnormal chain, is quite variable for different abnormal haemoglobins and for different individuals. Itano (1953) has reported the distribution of Hb-S and Hb-C in members of several families, pointing out the variability of the ratio Hb-AA1b-S or HbA/Hb-C, and introducing the concept of relative rates of synthesis as the determining factor in haemoglobin ratios. Itano (1953) suggests that the variability in the ratio Hb-A/abnormal haemoglobin may be dependent on the relative ability to synthesize Hb-A, rather than on the different rate of synthesis of the abnormal haemoglobin.
2. Control of the Synthesis of Fetal and Adult Haemoglobin:
Hb-F is the principal component of human fetus haemoglobin. The synthesis of Hb-A starts, however, early in fetal life; Hb-A has been detected in fetuses of 13 weeks. The level of Hb-A raises to about 10% by 22 weeks and to an extremely variable 20% at birth. The Hb-F level is fairly constant from the twenty- fourth to the thirty-second week of fetal life and then falls at a rate of 2.5 to 4% per week, Hb-F being substituted by Hb-A.
The concentration of Hb-F decreases with increasing gestational age of the new born. Babies born prematurely have higher levels of Hb-F, while babies born after term have lower levels of Hb-F; i.e., babies born 4 weeks after term seem to have Hb-F levels comparable to those of 4-week-old babies born at the normal term.
Hb-F is present for a variable time after birth; the Hb-F level decreases to about 1% or less by the end of the first year of adult life. The disappearance of Hb-F takes significantly longer than the time predicted on the basis of a complete cessation of Hb-F synthesis, the production of Hb-F decreases after birth and is very limited in normal adults.
In the course of fetal differentiation, erythropoietin activity resides in different morphological sites. Hemoglobin containing cells appear first in the mesenchyme of the embryo; the liver and the spleen are the next active erythropoietin sites until birth. The bone marrow is the main centre of red cell production around birth; after birth, active bone marrow disappears from the long bones and the erythropoietin activity is limited to the red marrow of the sternum and vertebrae.
No correlation has been established between the morphological sites of erythropoiesis and the switch from Hb-F synthesis to Hb-A synthesis. More Hb-A is synthesized in bone marrow preparations than in spleen or liver preparations; bone marrow is the latest hematopoietic site in fetuses and red cells produced in bone marrow are presumably more differentiated toward the Hb-A containing red cell type.
i. Occurrence of Hb-F in adults:
Hb-F is present in some adults affected by hereditary or acquired haematological disorders. Hereditary conditions in which Hb-F is constantly found in adults include sickle-cell anemia, thalassemia major and the anemias caused by a thalassemia gene in combination with a gene for an abnormal haemoglobin. Acquired pathological conditions in which Hb- F has occasionally been reported include pernicious anemia, hypo plastic anemia, leukemia and other types of anemia.
The level of Hb-F in anemic patients varies over a wide range; the highest levels of Hb-F have been observed in thalassemia major, where Hb-F frequently approaches 90- 95%. A slight amount of Hb-F has been reported to be present in pregnant women and in women with hydatid moles, conditions in which high amounts of chorionic gonadotropin are secreted. These observations suggested that gonadotropin may stimulate Hb-F production.
3. Hereditary Persistence of Hb-F:
The gene responsible for the persistence of Hb-F during adult life has also been called “high-F” gene. In carriers of this gene 20 to 30% of the haemoglobin is Hb-F; approximately the same amount of Hb-F is observed in individuals heterozygous for the high F gene and for the abnormal haemoglobin genes or It is quite remarkable that no Hb-A is present in these heterozygous individuals.
The high F gene thus behaves as if allelic or closely linked to the β locus. It has been suggested that the high F abnormality is the result of a mutation at the β locus and the production of Hb-F is a compensatory phenomenon. The high F gene in this view is interpreted as a thalassemic-like gene, suppressing completely the production of β peptide chains.
However, the majority of the het- erozygotes for a thalassemic gene do not show any increase of the Hb-F level over normal values. Individuals heterozygous for the high F gene and a β-thalassemic gene have been reported. The high F gene seems to interact with the βTh gene; 70%. Hb-F are normally found in thalassemic major; but the high F/thalasemic condition is of intermediate severity and quite different from thalassemic major as a pathological entity.
The hereditary persistence of Hb-F seems to be a genetic entity completely different from the hereditary anemias in which high levels of Hb-F have been reported. It has been suggested that the high F mutation may involve a regulator or operator gene, rather than a structural gene.
The function of such a regulator gene is to turn off the Y chain production and to turn on the δ and β chain production. The fact that individuals heterozygous for the high F gene and for a β s or β c gene from Hb-S or Hb-C in normal amount seems to indicate that the regulator gene exerts its control only on the structural genes in cis, located on the same chromosome.
4. Models of Red Cell Differentiation and Synthesis of Hb-F:
It may be assumed that the haemoglobin type synthesized by a red cell precursor is determined by the genetic information possessed by that cell at the moment of initiating haemoglobin synthesis and during the course of haemoglobin synthesis.
By analogy in protein synthesis in bacteria, we can speculate that the haemoglobin genes are repressed in stem cells and that when these cells differentiate into erythroblasts, specific haemoglobin genes become depressed or activated by genetically determined mechanisms.
The factor determining or activation of β rather than Y genes are unknown. It has been suggested that the change in oxygen tension from the uterine environment to the extra-uterine environment after birth may be the activating mechanism for the synthesis of β chains or that gonadotropic hormones may play a similar role.
It has been reported that stem cells may sometimes differentiate into red cells without intervening division. Similarly, in the fetus the red cells, are released in circulation after a number of divisions of the erythroid precursors, limited by the age of the fetus itself and possibly by the accelerated fetal erythropoietin activity.
A correlation may thus exist either between the number of divisions that the progenitors of a red cell have undergone in the absence of morphological differentiation or the time spent in the bone marrow by these cells as erythroblasts and the haemoglobin type contained by the daughter red cell.
Red cells produced after few cell divisions from stem cells may possibly contain more Hb-F than red cells produced after several divisions of the erythroblasts. In this view the information directed to the haemoglobin chain synthesis changes with the aging and/or the replication of the erythroid precursors.
The Hb-F/Hb-A system may be useful in the study of mechanisms of differentiation, because of the well-defined chemical nature of the proteins involved. In the switch from Hb-F to Hb-A production, only the synthesis of one type of peptide chain, the Y-chain, seems to be discontinued, while the synthesis of a new type of peptide chain, the β chain is initiated. Only two genes, the β and Y genes, are apparently involved in switching their activity; this fact may provide a unique opportunity for the study of the differentiation of a simplified genetic and biochemical system.