SELECTED STUDIES

Section Editor: Prof. Talaat I. Farag

ConsanguinitY:
a special issue in inheritance of hereditary anemias in Saudi Arabia

By Prof. Mohsen El-Hazmi and Prof. Arjumand Warsy

Summary

Hereditary hemolytic anemia constitute a major problem in several populations of the World due to their high frequency of occurrence and chronic nature. In Saudi Arabia, many of the hereditary hemolytic anemia (sickle cell anemia, α- and β- thalassaemia; glucose-6-phosphate deficiency) occur at a high but variable frequency in the different regions, with the highest frequency in the malaria endemic regions. Due to high consanguinity rates, the number of homozygous (affected) births is high in some regions. The major consequence of these defects is mild to severe anemia and several associated complications. Regular blood transfusion and chelation therapy are essential for the survival of many of the patients, and this result in severe organ damage and psychosocial problems. Life long care and support must be provided not only to the patients but also to their families, who require psychological support and constant counseling. Considerable financial burden and strain is faced by the family and community.

Consanguinity prevalence is high is Saudi population and has been regarded as the major cause for the occurrence of several rare autosomal recessive disorders. However, investigation into the frequency of some of the hereditary hemolytic anemia reveals that consanguinity does influence the number of homozygous births in areas where the disorder is rare and occurs at a frequency of less than 1%, however, area with already a high frequency of the disorders fail to show any further detrimental influence on the number of homozygous births.

This paper presents a brief overview of the nature of hereditary hemolytic anemia occurring in Saudi population and discusses these in the light of the high prevalence of consanguinity.

Introduction

Hereditary hemolytic anemia result from a number of different inherited abnorma­lities which affect the stability and integrity of the red cells. The major defects are those that affect the hemoglobin structure or synthesis and those that affect the red cell metabolism. The abnorma­lities in the structure or rate of biosynthesis of the globin chains of hemoglobin (Hb) and deficiencies of red cell metabolic enzymes generally result in excessive hemolysis. The defects in the globin chain structure or synthesis are referred to as "hemoglobinopathies". Types of hemoglobinopathies include: (a) those due to structural defects in the globin chains e.g. sickle cell hemoglobin (HbS); (b) those due to decreased rate of globin chain synthesis i.e. the thalassemias; (c) those involving both structural changes and rate of synthesis i.e. the Hb E/thalasse­mias; and (d) the hereditary persistence of total hemoglobin, which are characterized by persistence of Hb F synthesis during adult life. Enzymopathies result from deficiency of an enzyme and affect the rate of metabolic pathways necessary for the stability and integrity of the red cells.

The hemoglobinopathies, thalassaemias and enzymopathies together constitute the major causes of inherited anemias in human populations, particularly in areas that have a past or present history of malaria endemicity. According to a World Health Organization report of 1987, approximately 7% of the world populations are carriers for one or more of these abnormal genes.

The hemoglobin structural disorders

The hemoglobin structural disorders (e.g. HbS, HbC, HbE etc) result from mutations in the exons of the structural genes of the globin chains. The mutation may occur in the a-,b-,g-,d- globin genes and results in structural changes in hemoglobin. Some of these structural alterations have no clinical consequence while others produce hemoglobin with altered structure, functions and stability. Various degrees of clinical consequences have been reported associated with these abnormal hemoglobins. These includes hemoglobins with altered oxygen affinity (increased or decreased), decrease stability and excessive hemolysis.

The thalassaemias

The thalassaemias constitute a diverse heterogenous group of inherited defects which are generally associated with a state of hypochromic-microcytic anaemia of varying degree depending on the amount of available globin chains. The thalassaemias are classified depending upon the gene in which the mutation has taken place and whose synthesis is decreased, i.e. a-thalassaemia result from decreased synthesis of a-chains and b-thalassaemias result from decreased synthesis of b-chains. In addition a- and b-thalassaemias are further subgrouped as a+ or b⁺ thalassaemia, in which some a or b-chain synthesis occurs, and a° or b° thalassaemias in which no a- or b-globin chain is synthesized. The thalssaemias result from either deletion or point mutations. Deletion of the whole gene, part of the gene and different types of point mutations occurring within the intervening sequences (introns), the promoter region, the poly­adenylation signals, exons, flanking regions, regulatory regions, in remote DNA sequences and in the Locus Control Region result in decreased expression or complete absence of the globin gene expression and hence lead to the thalassaemias. Some of these mutations result in the complete absence of the globin gene expression and hence no globin chain synthesis and are referred to as the α⁰- and β⁰-thalassaemias, while others result in decrease synthesis of the globin chains and are referred to as the α⁺- and β⁺-thalassaemias. Hence the molecular pathology of the thalassaemias is exceptionally diverse compared to other genetic disorders. All mutations are cis mutations i.e. they affect only the gene on the chromosome in which the mutation.

World distribution of hemoglobinopathies

The most frequently identified hemoglobinopathies are those resulting from inheritance of Hb S. For a long time, there existed a misconception that Hb S is limited only to the Negro race, however, later extensive studies revealed the occurrence of Hb S in non-Negro population in India, Southern Italy, Northern Greece, Sicily, Southern Turkey, Saudi Arabia and North Africa. In addition, as a result of population admixtures and movements the Hb S is also identified, though at lower frequencies in the European countries, the Americas and in Canada. Within each country differences in the gene frequency are reported in different areas and in different racial groups and tribes. The best investigated are the tribes in Africa, living in the same locality but with significant difference in the gene frequency of Hb S gene. The Bantu speaking tribes, for example, have a signifi­cantly higher Hb S gene frequency compared to the Hamitic speaking tribes in Uganda and Tanzania.

The maintenance of high frequency of Hb S in certain areas of the world has led to the suggestion that the heterozygous for the Hb S gene have some advantage over normal homozygous (Hb AA) and sickle cell disease patients and are, therefore, able to survive and reproduce. Thus as a result of "balanced polymorphism" the Hb S gene has been maintained at relatively constant levels in populations. It was as early as in 1952 when Mackey and Vivarelli suggested that the environmental factors maintaining Hb S gene is malaria.

Several in vivo and in vitro studies have since confirmed the "malaria hypothesis" and a close correlation is demonstrated between malaria endemicity and the Hb S gene. There is evidence that the Hb S heterozygotes have a natural resistance against malaria and are therefore able to survive during the childhood when malaria caused by Falciparum malaria is most critical and fatal. The other frequently identified Hb structural variants include Hb C, Hb E, Hb D Punjab and Hb O Arab. Each of these variants occurs at a high frequency in some populations, where polymorphic levels have been reached, while in other areas sporadic cases have been reported. In addition, unlike the correlation between Hb S gene frequency and malaria, no other similar environmental factor has yet been identified that could explain the high frequency of these hemoglobins in some areas.

Hereditary anemia due to Hemoglobinopathies in Saudi Arabia

The presence of hereditary anemia caused by hemoglobinopathies due to structural abnormalities of hemoglobin, were first recognized in Saudi Arabia in 1963 by Lehmann and co-workers during a screening program in the eastern province of Saudi Arabia. Thereafter, Gelpi reported the presence of sickle cell gene in Saudis living in the oasis population of Al-Qateef and Al-Hasa. The author also observed that the sickle cell anemia was mild in this population and suggested that other coexisting genetic abnormalities, such as G-6-PD deficiency or the thalassaemias or other abnormal hemoglobin variants, ameliorate the clinical presentation of sickle cell anemia, thus producing a benign or mild form of the disease. At about the same time, Weatherall and co-workers described a mild form of sickle cell disease in a patient with sickle cell homozygosity and a-thalassaemia gene and termed the condition as "a new sickling disorder". Subsequently, the benign nature of sickle cell disease in the Saudis from the Al-Qateef and Al-Hasa oases was confirmed and the presence of a high percentage of HbF was considered as an ameliora­ting factor. The homozygous sickle cell anemia patients in the eastern province were easily distinguishable from those of African origin by the mildness of clinical manifestations and the associated high Hb F level. The lower incidence of vaso-occlusive complications, persistence of splenic functions, lower morbidity due to other complications and lower risk during pregnancy were all attributed to elevated Hb F in the Saudi patients. However, later studies revealed mild sickle cell anemia even in the absence of elevated levels of Hb F.

Sickle cell gene in Saudi Arabia

Studies conducted in different regions of Saudi Arabia during the late 1970s and early 1980s revealed the presence of Hb S and other red cell genetic defects in several regions of the country. Significant differences in the frequency of HbS gene were reported in the different areas of the country, and even within the same areas Hb S gene frequency was found to be significantly variable.

In 1980, King Abdulaziz City for Science and Technology financed a project on the study of Natural History of Sickle Cell Disease in the Eastern Province population. This study was based on the screening of neonates and follow-up of the sickle cell disease patients during childhood. This study reported a gene frequency of Hb S of 5% to 27% of newborn in Al-Khober, Dammam and Qateef.

In 1982, another detailed screening of the Saudi population in the different regions of the country, was initiated. Several abnormal hemoglobins were identified (HbC, D, E, O-Arab, G, F Dammam etc). Only sickle cell hemoglobin (Hb S) was found to be polymorphic while all others occurred in a few cases and were rare variants (frequency <0.01). Figure 1 presents the gene frequency of HbS in different areas and cities of Saudi Arabia. The Hb S gene was encountered in all areas screened in the Kingdom. The Hb S gene frequency was the highest in Al-Qateef and Al-Hafouf in the Eastern Province of Saudi Arabia, followed by Mahayel, Abha, Al-Qunfuda and Jaizan in South-western province. In the North-western province, the frequency of Hb S gene was around 0.08 in Khaiber and Al-Ula. Generally, the non-malaria endemic regions such as the Central Province and the Northern Province had a very low frequency of Hb S gene.

Figure 1: HbS gene frequency in different areas and cities of Saudi Arabia

Past or present malaria endemic regions have a high frequency of Hb S gene. However, in the malaria endemic regions there were a few exceptions where lower Hb S gene frequencies were encountered in Al-Baha, Najran and Yanbu which have been malaria endemic areas in the past.

The frequency of a-Thalassaemias in Saudi Arabia

a-thalassaemias was identified in all regions of Saudi Arabia though at a variable frequency. The frequency of a -thalassaemia in the different regions of the country are presented in Table 1. High frequency of a- thala­ssaemia was identified both in the eastern and western provinces of the country. Interes­tingly, the frequency in the different regions in Saudi Arabia correlates positively with malaria endemicity. In the central province where malaria is non-endemic the a -thalassae­mia occurs at a lower frequency, while in areas such as those in the eastern and western provinces of the country which has been endemic to malaria since the past, a-thalassaemia occurs at a high frequency and seems to go hand-in-hand with the sickle cell gene. Comparing the frequency with those reported in literature for other populations reveals that Saudi Arabia stands among the countries which have the highest a-thalassaemia frequency.

The frequency of ß-thalassaemias in Saudi Arabia

The presence of ß‑thalassaemia was first recognized in the eastern province of Saudi Arabia, but the frequency was believed to be lower than the frequency of Hb S or a‑thalassaemia genes. Based on the elevated level of Hb A₂ only 1.5% of  unrelated individuals were found to have ß‑thalassaemia. It was suggested that the predominant form of ß‑thalassaemia in the population was ß°‑thalassaemia through a few cases of ß⁺‑thalassaemia were also identified. Screening studies initiated during the 1970's showed the presence of ß‑thalassaemia in several regions of Saudi Arabia and in 1982 the gene frequency of ß⁺‑thalassaemia was reported in the north‑western province, central pro­vince, Najd, Tehamat Aseer and the eastern province of Saudi Arabia. Within each area, a wide range of gene frequency was identified.

In a later study, the different areas of Saudi Arabia were screened to deter­mine the frequency of ß-thalassaemias. The frequency of ß-thalassaemia in different regions of Saudi Arabia is presented in Table 2. The highest frequency of ß-thalassaemia was encountered in the western province while the lowest was in central province.

Table 1: Frequency of a-thalassaemia in different areas of Saudi Arabia

Frequency of glucose‑6‑phosphate dehydrogenase deficiency in Saudi Arabia

Glucose-6-phosphate dehydrogenase deficiency was first reported in the Saudi population in the eastern province in the mid-sixtees, and it was shown that the frequency of G‑6‑PD deficiency varied significantly in the different villages in the Eastern Province. In the oasis populations, the G‑6‑PD deficiency was significantly higher compared to the frequency in the non‑oasis population. Later, G‑6‑PD deficiency was reported in the western province of the country, though the frequency was found to be considerably lower compared to the frequency in the eastern population. Screening studies were initiated in several regions of the country and the G‑6‑PD deficiency gene was found to be widely distributed in the different provinces of the country, though at a variable frequency. These investigations also showed that the G‑6‑PD deficiency gene occurs at a high frequency in the areas where Hb S and the thalassaemia genes are also present at high frequencies.

Table 2: Frequency of b-thalassaemia in different areas of Saudi Arabia

In a National Screening program, initiated in 1982 several areas of Saudi Arabia in the Eastern, Western, Northern, and Central Provinces of the country were screened and G‑6‑PD deficiency was identified in all regions of the country, though in the central province the frequency was significantly lower compared to the other provinces. The frequency of G-6-PD deficiency in the Saudi males and females in different regions are presented in Table 3. In each area, the actual number of G‑6‑PD deficient females was significantly higher than expected number calculated applying the Hardy‑Weinburg Equilibrium. There could be due to several factors contributing to disturb the Hardy-Weinburg equilibrium in this population. These include:

            (a)       High rate of consanguinity in the Saudis. The rate of consanguineous marriages was over 50% in the Saudi population. In addition, customs prevail whereby marriages generally occur between members of the same tribes, and    

            (b)       High frequency of inactivation of the normal X‑chromosome in the female heterozygotes, thus presenting a higher percentage of females as deficient.

Table 3: Frequency of G-6-PD deficiency in different regions of Saudi Arabia

Hereditary hemolytic anemia and consanguinity

There is a misconception that consanguinity increases the allele frequency of commonly encountered disorders and hence the frequency of these disorders is elevated in regions where there is a higher prevalence of cousin marriages. In theoretical terms consanguinity does not alter the allele frequencies, but increases the probability of increased mating between heterozygous individuals who are carrying the same recessive mutant allele, thus increasing the risk of homozygous births. Hence, the risk of genetic diseases which are expressed in homozygous are higher in consanguineous marriages compared to non-consanguineous marriages especially for the autosomal disorders.

Since the majority of the hereditary hemolytic anemia prevalent in Saudi Arabia are inherited as autosomal recessive disorders and a few such as G-6-PD deficiency as X-linked recessive disorders, the special role played by consanguinity needs to be evaluated.   

Consanguinity and types of counsaguinity in the Saudis

Consanguinity (Kon’san’-guin’it E), is a word derived from two Latin word: con meaning 'shared' and sanguis meaning' blood', and describes relationship between two people who are related as they share common ancestors (a shared blood relationship) [cognatio naturalis]. Consanguineous marriages refer to marriages between two people, genetically related, by descent, from a common ancestor (F ≥ 0.0156). In clinical genetics, a consanguineous marriage refers to union between individuals who are related as second cousins or closer. Consanguinity is of different degrees depending on how close, generationally, the common ancestor is to an individual. Various types of 'Consanguinity Tables' have been constructed and are helpful in identifying the degree of cousin relationship between two individuals using their most recent common ancestor as the reference point. Figure 2 presents a simplified version of such tables and shows the different types of consanguinities. Lineal consanguinity i.e. in line of decent was practiced in some ancient civilizations, but in the civilized World, in almost all religions and cultures it is prohibited as it is incestuous. Collateral consanguinity is of different types and the degree of shared genes decrease as the relations are removed further. Offspring's of siblings are 1^st cousins; their children are 2^nd cousins and their children are 3^rd cousins for each other. These are some of the common forms in cultures where consanguinity exists at high prevalence.

Figure 2: A schematic presentation of collateral and lineal consanguinity

Degree of Relation and Proportion of Shared genes

First degree relations of an individual are his/her parents, children, dizygotic twins and sibs. They share half their genes and the chance of homozygosity by decent is 1/4 (i.e. 25%). Second degree relations are individual's half sibs, uncles, aunts, nephews, nieces and double first cousins. They share 1/8 i.e. 12.5% genes and the chance of homozygosity by decent is 1/4 (i.e. 25%). As the degree of relation increases, the proportion of shared genes and chance of homozygosity by decent, decreases. Table 4 presents a simple tabulation of the types of relations, proportion of shared genes and chance of homozygosity by decent. 

Table 4: A summary of the degree of relation with examples and the proportion of shared genes and chance of homozygosity by decent.
 

Consanguinity in Saudi Arabia

Several studies have reported the prevalence of consanguinity in the different regions of Saudi Arabia. Among Saudi Arabs, cousin marriages include double first cousins, first cousins, first cousins once removed, second cousins and some distant cousins. Table 4 presents the prevalence of first and second cousin marriages as reported in different studies in Saudi Arabia. As can be seen the prevalence varies from 24 to 55%, in different area. However, if marriages with other distant relations are also included then the prevalence rises in all areas to above 50%. i.e. every 1 in 2 people is married to a relative.

It is obvious that Saudi Arabia can be ranked among those countries of the World that have high consanguinity prevalence. These include majority of the Arab countries, Afghanistan (55.4%), some regions of India (0.5-55.1%), Iran (2.8-52.1%), Pakistan (31.1-61.2%), and West Africa (25.9-65.5%). The low prevalence countries include USA (0.2%), Canada (1.5%), the Caribbean (0.8-3.3%), South America (0.2%), Singapore (5.0%), Philippines (0.4%) and South Africa (0.4-6.1%). Indonesia (17.8%) and Bangladesh (6.7-17.6%) have slightly higher prevalence.

Table 5: Prevalence of consanguinity (1^st and 2^nd cousin marriages) in Saudi population

Thus all rare disorders are expected to increase significantly in areas where there is a high rate of consanguinity. While in areas with already a high frequency of the recessive disorders, there is always a high chance that unrelated spouses are carriers of the same abnormal gene and hence express it to there offsprings at almost the same rate as first cousins.

Conclusion

Saudi Arabia presents a unique opportunity to study the effect of cousin marriages on various health problems. Premarital screening programs have been initiated in an attempt to prevent the birth of affected children and being applied very successfully for hereditary hemolytic diseases.

By Prof. Mohsen El-Hazmi is a Professor of Medical Biochemistry at the College of Medicine, King Saud University, Riyadh. He is also a member of Saudi Arabia's Al-Shourah Council. His email is Hazmigene1@hotmail.com

Prof. Arjumand Warsy is a Professor of Biochemistry at the College of Science, King Saud University, Riyadh, Saudi Arabia. Her email is aswarsy@ksu.edu.sa.