The three examples of autosome-linked genetic disease are: 1. Sickle Cell Anemia 2. Alkaptonuria (Black Urine Disease) 3. Phenylketonuria.
Example # 1. Sickle Cell Anemia:
Sickle cell anemia is an inherited abnormal disease caused by mutation of autosomal gene. The red blood cells of certain individuals have peculiar property of undergoing reversible alterations in shape when subjected to changes in the partial pressure of oxygen .
Sometimes RBC are sickle-shaped instead of being flat discs and are very inefficient oxygen- carriers. The people with such red cells are called sickle cell trait but they are apparently healthy. The result is a progressive haemolytic anemia, usually resulting in early death. This condition is caused by a mutant gene Hbs1.
Linus Pauling showed (1945) first that the individual with this disease (genotype Hbs1 Hbs1) have no normal hemoglobin A in their red blood-cells and instead have an abnormal hemoglobin (Hbs1) which has a lower negative charge than HbA1; and secondly that the HbA1; Hbs1 heterozygous carriers of the disease have both HbA1; and HbS1 in their red blood cells, leading to mild sickling but not to anemia. Pauling considered this to be a gene- controlled molecular disease.
Symptoms of Sickle Cell Anemia:
The sickling leads to increased fragility of the red blood cell causing hemolytic anemia and increased viscosity of the blood, causing the red cells to stagnate in small blood vessels and consequently form thrombi and infarcts. Regular phenomenon in sickle cell anemia is autospleenectomy guided by repeated spleenic infarcts and that is responsible for the susceptibility of these patients to pneumococcal meningitis and other infections.
The sickling phenomenon is common in Central Africa where in many areas 20% or more of the population may have the sickle cell trait and a significant fraction of individuals (1-2%) may be expected to die of sickle cell anemia in early life. It is also found in USA. In irregular fashion Hbs has also been found in certain racial groups in Arab, India and among American Negros.
Effect of Sickle Cell Anemia on Man:
People with sickle cell trait heterozygous (SiS/SiA) are usually symptomless except under certain circumstances, such as exposure to high altitudes and infections, particularly of the respiratory tract and bleed from the renal papillae.
Those homozygous for sickle cells are resistant to the malarial protozoon. Those homozygous for sickle cell trait (SiS/SiS)usually die early from anemia, those homozygous for normal blood Hb (SiA/SiA) suffer from malaria, and those heterozygous (SiS/SiA) live reasonably healthy lives. Their blood is resistant to the malarial parasite and the anemia is not severe, but they perpetuate the disadvantageous gene in their progeny.
Genetic Alteration in the Hemoglobin Molecule:
Several different forms of hemoglobin are known to occur in man. Most of them were detected initially by altered electrophoretic mobility; some have been distinguished chemically by finger-printing and by other procedures. Each is controlled by a particular gene.
The human adult hemoglobin “A” molecule is composed of four polypeptide chains, two identical alpha (α) chains and two identical beta (β) chains. The α chain has 141 amino acids and the beta chain has 146 — making a total of 574.
Rapid advances are being made in the study of differences in the chemical nature of hemoglobin. This material consists of complex molecules that are composed of colored, iron containing — heme and a colorless protein — globin.
In many populations, particularly those of African stock, there are numerous individuals whose red blood cells take on a sickle-like shape when they are exposed to low oxygen tensions outside the body. Such persons fall into two classes.
The majority are perfectly healthy and inside their bodies less than one per cent of the blood cells are abnormally shaped, they are said to possess the “sickle cell trait”. A small minority, however, has a severe anemia which often becomes fatal before those afflicted reach the age of reproduction, and more than 1/3rd of their blood cells have abnormal shapes. They are called “sickle-cell anemics”.
In 1949, Neel and Beet independently demonstrated that the sickle cell trait was the heterozygous manifestation of a gene which, when homozygous, resulted in sickle cell anemia. But in the same year Pauling and his colleagues reported their discovery that hemoglobins from persons with sickle cell anemia, those with sickle cell trait, and normal individuals behaved differently on electrophoresis. Individuals whose cells are not sickle- shaped are homozygous for an allele HbA1 (HbA1 HbA1); possessors of the trait are heterozygous for the allele HbS1 (HbA1 HbS1); and sickle- cell anemics are homozygous HbS1 HbS1.
Molecular Basis of Sickle Cell Anemia:
The molecular basis of the sickle cell character began to be understood when it was discovered that a special kind of hemoglobin (S) different from that of normal individual (A) is present in the blood cells of persons with the sickle trait or the sickle cell anemia. The discovery that there are different molecular forms of hemoglobin stimulated research to determine the amino acid sequences of the different forms.
This work was greatly aided by a technique described by Ingram in 1956 and called “finger-printing”. When the beta chains of normal human hemoglobin were broken down to piece by piece, the amino acid in the peptide chain designated as part 4 were found to be arranged in the following order:
Hemoglobin S of sickle cell patients was found to have all eight amino acids in same order except for number six glutamic acid which is replaced by valine.
Hemoglobin C Anemia:
In 1950 Itano and Neel, when studying the parents of some children who exhibited an atypical form of sickle cell anemia, found that only one of the parents had blood that could be induced to sickle. The erythrocytes of the other parent appeared to contain normal hemoglobin.
When the hemoglobin of the normal parent was subjected to electrophoresis and amino acid analysis by Hunt and Ingram, 1958, it was found that peptide fragment 4 contained lysine as its 6th amino acid, but was identical with normal and sickle type hemoglobin. The new type of hemoglobin was lebelled ‘C and has the polypeptide chain composition of αA2 βC2.
Hemoglobin C Anemia:
So the genes determining C, A and S type hemoglobins are alleles since their amino acid differences occur within the same cistron. Hemoglobin C heterozygotes appear to be unaffected by the gene. However, homozygotes suffer from a mild anemia that is accentuated by infection.
Individuals who have compounds of both thalassemia (insufficient number of hemoglobin per erythrocyte) and hemoglobin C suffer from a severe anemia, so much as do the people with sickle cell anemia and hemoglobin C compounds.
Formulae of the hemoglobins are:
Even more specific chemical formulae for the hemoglobins stating the specific amino acid change are possible on the basis of information from finger-printing and related analyses: e.g.
Detection of HbS1:
Hemoglobin S can be distinguished from the hemoglobin of people not suffering from the sickle cell disease by electrophoretic mobility. When placed in an electric field under appropriate conditions, normal hemoglobin with glutamic and in position six has a negative charge and migrates toward the positive pole, whereas hemoglobin S with valine replacing glutamic acid has no net charge and does not migrate in the electric field.
It is interesting to note that a normal HbA1 HbA1 individual form only normal adult hemoglobin, than an anemic HbS1 HbS1 individual has none of it and that the heterozygote HbA1 HbS1 with the sickle cell trait, produces in co-dominant manner, both types of haemoglobin, the abnormal one making up from a quarter to nearly half of the mixture.
The One Cistron: One Polypeptide Hypothesis:
It is evident from several observations that α and β chains are determined by different genes, one synthesized separately and are then assembled to give hemoglobin by random association of α and β subunits.
Explanation of Sickle Cell Anemia from Genetic Code:
Data from studies of cell-free systems indicate that one of the DNA triplets that codes for glutamic acid is CTC. This triplet would code for the complimentary GAG in the mRNA transcript substitution of an A for a T (GAG) in DNA would result in GUG as the following tables and mRNA triplet that codes valine. Substitution of a T for a C (TTG) would produce AAG, an m-RNA triplet that yields lysine.
A single base change in the DNA sequence specifying an alternation of one amino acid at a specific point in a particular protein molecule could account for the disease sickle cell anemia.
Sickle cell anemia may be described as recessive, dominant or co-dominant, depending on how it is defined and tested. If the clinical picture is taken as a distinguishing criterion, then the sickle cell gene has a recessive effect, since only the homozygotes has the disease.
If the presence of positive sickling test is used as a criterion, then the sickle cell gene has a dominant effect, since both the heterozygote and the abnormal homozygote have a positive test result.
But if one looks for the production of ar abnormal hemoglobin by electrophoresis then the two genes producing normal and sickle cell Hb have a codominant effect because each produces normal and sickle cel Hb.
A Pedigree of Causes of Sick Le Cell Anemia (See Table 11.2):
Example # 2. Alkaptonuria (Black Urine Disease):
Alkaptonuria is a useful model for discussion of inborn errors of metabolism and has historical precedence, since it was the condition that was the basis for Garrod’s concept.
One of the end points of phenylalanine and tyrosine metabolisnn is the breakdown of homogentisic acid to CO2 and H2O. This reaction is accomplished under the influence of an enzyme that is present in the liver, named homogentisic acid oxidase. When the enzyme is defective, large amount of homogentisic acid are excreted in the urine, which turns black upon exposure to the air.
In addition, quantities of homogentisic acid accumulate in the body and become attached to the collagen of cartilage and other connective tissues. The metabolic pathways of phenyanine and tyrosine are shown in the Table 11.3.
The cartilage of the ears and the sclera, which is collagenous in nature are stained black. These manifestations are called ochronosis. In the joints such as those of the spine, the accumulation leads to arthritis. When alkaptonurics are fed increased quantities of phenylalanine or tyrosine, there is corresponding increased amount of homogentisic acid excreted in their urine.
Garrod interpreted alkaptonuria as being caused by the congenital deficiency of a particular enzyme due to the presence in double dose of an abnormal Mendelian factor or gene. An important implication of the idea was that the normal allele of this gene must in some way be necessary for the formation of the enzyme in the normal organism.
This was the first clue to the now well-established generalisation that genes exert their effects in the organism by directing the synthesis of enzymes and other proteins.
Frequency of Individual with Alcaptonuria:
Approximately one person in every million is homozygous for the mutant gene and suffers from disorder.
The condition is present from birth and can be recognized because of the dark color appearing on wet diapers.
An increase of phenylalanine or tyrosine in the diet of normal persons is not followed by the appearance of homogentisic acid in their urine.
Alkaptonuria is inherited as an autosomal recessive trait and is an example of a genetic enzyme block in which the phenotypic features are due to accumulation of a substance just proximal to the block. The pedigrees were quite characteristic, and Garrod had little hesitation in concluding that they implied an hereditary or genetical basis for the condition.
He consulted Bateson, one of the earliest geneticists, who pointed out that the situation could be readily explained in terms of laws of Mendel. The pedigrees were exactly those to be expected if alkaptonuria was determined by a rare recessive Mendelian factor, or as we should now say, gene.
The affected individuals could be presumed to be homozygous for the abnormal gene. According to Stern, most pedigrees fit recessive inheritance but at least one in which affected individuals descended from affected parents for three generations, has been frequently claimed to be evidence for dominant inheritance.
However, recent genealogical studies show connections between members of this kindred with others in which the trait is undoubtedly recessive. This suggests that the same recessive allele is responsible for alkaptonuria in all these kindreds.
Garrod inferred that homogentisic acid, although it had never been detected in tissues, was itself a normal intermediate in the catabolism of Phenylalanine and tyrosine, and that in alkaptonuric subjects the essential defect was a failure in this degradation, due to the lack of a necessary enzyme.
In a alkaptonuric homogentisic acid cannot be broken down further, so it tends to accumulate in the cells of the liver where this metabolic process mainly occurs, leaks into the circulation and is excreted into the urine in large quantities. Alkaptonuria is relatively a benign disease.
The amino acids phenylalanine and tyrosine are normal dietary constituents. However, the diet supplies much more than the body can use as such and the excess is metabolized via homogentisic acid. No essential metabolites are formed subsequent to homogentisic acid.
The fact that the kidney cannot retain excess homogentisic acid prevents major deviation from normal metabolism. The only pathological consequence of alkaptonuria is a slow deposition of pigment in some of the joints, leading ultimately to a form of arthritis.
Example # 3. Phenylketonuria:
A mutation in the gene that determines a given enzyme may produce a disorder of the type, Garrod called, “inborn errors of metabolism”. Much has been learned about the genetic control of enzymes and about intermediary metabolism by a study of mutant forms in the human species as well as in the micro-organism.
Phenylketonuria (PKU) is a such genetic defect in aromatic acid metabolism. The defect is in the enzyme involved in the conversion of phenylalanine to tyrosine (Table-11.3 and 11.4). The deficient enzyme is phenylanine -4- hydroxylase which in the normal individual occurs in the liver and catalyses parahydroxylation of the amino acid phenylalanine to give tyrosine.
The gene involved is an autosomal recessive and in homozygotes, the phenylalanine accumulates and is converted to phenylpyruvic acid. Phenylalanine is continuously being produced from the normal breakdown of tissue protein, and from the digestion of dietary protein.
Its conversion to tyrosine in the liver is the first step in its catabolism and if this is blocked it accumulates intracellularly and appears in high concentrations in the body fluids. The level of phenylalanine in blood serum in phenylketonuria is generally more than thirty times normal and there is an increased excretion of the amino acid in urine.
Some of the phenylpyruvic acid becomes concentrated in the cerebrospinal fluid while the rest is excreted in the urine. Excess accumulation of phenylalanine causes the production of several products.
i) Phenylpyruvic acid,
ii) Phenyl lactic acid,
iii) Phenyl acetic acid in addition to phenyl acetyl glutamic acid,
iv) Phenyl-hydroxyphenyl acetic acid and several other products.
These chemicals accumulate in blood due to low renal threshold excretion in urine. In plasma of normal individual phenylalanine level is one milligram per cent which increases to 20-60 mg per cent in PKU’s but, when detected early, this could be improved.
Frequency of Individual with PKU:
1 in 15,000 child birth.
The PKU individuals are also known as phenylpyruvic idiots and have very little IQ (less than 20) in 2/3rd and moderate IQ (between 21 and 50) in 1/3rd cases with severe mental retardation, certain physiological and physical disabilities. The PKU individuals are feeble minded and have light pigmentation.
The feeble mindedness is thought to be due to an impairment of the brain tissue by the phenylpyruvic acid in the cerebro-spinal fluid. The light pigmentation is due to a decreased formation of tyrosine which is one of the precursors of melanin.
Identification of PKU Children:
If we applied ferric chloride (FeCl3) to the diapers of newborn babies it remains yellowish in normal individual but becomes green in PKUs suggesting the presence of phenyl pyruvic acid in the PKUs.
The clinical disorder is present only in the homozygous but the heterozygote does not manifest the disorder. In several conditions, however, the heterozygous carrier can be identified by special means, one of which involves stressing the particular enzymatic step.
In PKU, the phenotypically normal but genetically heterozygous parents of affected persons tend to show blood levels of phenylalanine that are higher and last longer than normal when a standard dose of this amino acid is administered. This is the so-called phenylalanine tolerance.
PKU is an autosomal recessive condition. Patients, therefore, inherited an abnormal gene from each parent. Parents who are heterozygotes for the diseases although having a higher testing blood level of phenylalanine, yet are clinically normal. Two heterozygotes have a 25% chance of having one normal child, a 50% chance of carriers and a 25% of child with PKU.
Advice for PKU’s Individuals:
Removal of phenylalanine from diet and supplement of all the products of phenylalanine, so that the normal metabolism remains unaltered. Some also used activated charcoal for the phenylalanine removal. Lofenalac is the synthetic diet made for these babies.
High proteins, e.g. fish, meat, eggs must be eliminated and more of fruits and vegetables should be introduced although little protein may interpretably be administered. All children necessitating early diagnosis and therapy, it will be treated by use of artificial enzymes.
From the above discussion, it is clearly understood that from phenylalanine to tyrosine or phenylalanine to hydroxyphenyl pyruvic acid, this step is under gene control, since it does not occur in the absence of normal allele.
Autosomal Recessive Disease:
Growth hormone deficiency in man or ateliotic dwarfism:
A deficiency of human growth hormone not associated with other pituitary deficiencies was observed as a autosomal recessive inheritance of sexual ateliotic dwarfism. This disease is due to an autosomal recessive trait.
Major clinical features of this syndrome:
(a) Life span is about 39-77 yrs.
(b) Height 123-139.5 cm.
(c) Weight 30.6-42 kg.
(d) Secondary sexual character normal.
(e) Voices were high-pitched with a “small” timbre.
(f) Normal secretions of TSH, FSH and ACTH.
(g) Low concentration of growth hormone in serum.
(h) Puberty is often delayed.
Similar other genetic diseases of hormonal deficiencies have also been recorded in man e.g. deficiency of pituitary gonadotropin, adrenocorticotropin and thyrotropin. All these diseases are due to the mutations of autosomal gene.