Prevalence and molecular characterization of Glucose-6-Phosphate dehydrogenase deficient variants among the Kurdish population of Northern Iraq

Glucose-6-Phosphate Dehydrogenase (G6PD) is a key enzyme in the pentose monophosphate pathway and provides the NADPH essential for a number of biosynthetic and detoxifying reactions. The gene encoding G6PD is located at Xq28, and its deficiency is the most common enzymopathy in humans. As an X-linked recessive trait, it is predominantly a disease of males affecting an estimated 400 million people worldwide [1]. This deficiency is associated with a variable spectrum of clinical manifestations including the following: favism, neonatal jaundice, drug induced hemolysis, chronic non-spherocytic hemolytic anemia and infection induced hemolysis [2]. The latter variability is suggestive of biochemical heterogeneity. More than 400 G6PD biochemical variants have already been described [1]. More recently cloning and sequencing of the G6PD gene allowed researchers to characterize more than 140 molecularly distinct variants [3].

G6PD deficiency is prevalent in many countries in the Eastern Mediterranean Region including Iraq [4–8]. Some studies have addressed the prevalence of G6PD deficiency in central and southern Iraq [8–10], but none are available from the northern region. The population of northern Iraq consists of a majority of ethnic Kurds unlike other parts of the country where Arabs predominate. Kurds constitute the fourth largest ethnic group in the Eastern Mediterranean after Arabs, Persians and Turks. They inhabit a region called Kurdistan covering adjacent parts of Iraq, Iran, Turkey and Syria. The origin of Kurds has been a subject of speculation. Based on cultural and linguistic evidence, some historians believe that Kurds are predominantly an Indo-European ethnic group who had migrated and settled in Iran, Iraq and Turkey more than 2000 years ago [11]. However the region has been inhabited by earlier populations since prehistoric times and has been ruled throughout its long history by several major powers including the Assyrians, Persians, Greeks, Romans, Arabs and Turks [12]. All these factors may have contributed to the genetic makeup of the present-day Kurds.

Genetic studies including those of inherited blood disorders are very scarce on Kurds, and such studies may help explain the origin and spread of these disorders and may also help shed some light on the history of this ethnic group. No study of significance has addressed the frequency of G6PD deficiency in the Kurdish Iraqi population, and none has defined the deficient molecular variants among any group of Iraqis, despite the fact that favism is a commonly encountered health problem throughout the country.

This study aims to: 1) determine the prevalence of G6PD deficiency in a random sample of healthy Iraqi Kurdish males attending a regional premarital screening center, and 2) define the molecular basis of G6PD deficiency among a sample of G6PD deficient males in the same region.

The study was conducted in the period between July and December 2009. The subjects enrolled were males attending the Central laboratory in Dohuk, northern Iraq for routine premarital health screening. This laboratory serves a population of around one million of mainly ethnic Kurds, and is the only facility in this province that is authorized to provide government mandated premarital investigations. The latter currently include investigation for blood groups, HBsAg, HIV and carrier screening for specific hemoglobinopathies. The center receives on average around 30-40 couples per day. The study subjects were the male partners of the first 10 couples attending the center on every other working day throughout the study period.

A 6 ml blood sample was collected in K2-EDTA tubes from each enrollee. The sample was used to screen for G6PD deficiency by methemoglobin reduction test [13]. All those identified as G6PD deficient and an equal number of age matched non-deficient individuals were further tested by quantitative enzyme assay according to the manufacturer instructions (Biolabo - France). Confirmed deficient individuals were recalled for molecular testing.

Additionally, 65 unrelated male patients who had documented hemolytic episodes associated with G6PD deficiency, were also recalled and had their G6PD deficient status reaffirmed by quantitative enzyme assay.

Informed consent was taken form all enrollees or their guardians and the study was approved by the appropriate ethical committee at the Dohuk college of Medicine, Iraq.

Statistical analysis utilized the Mann Whitney U test and Chi Squared test wherever appropriate. A p < 0.05 was considered significant.

The silent mutation (1311 C→T) was documented in 93/95 (97.8%) of the Mediterranean deficient variants, in 1/10 of the Chatham cases and in one of the three uncharacterized variants.

G6PD deficiency has long been recognized as a common inherited hematological disorder in Iraqis including Kurds [7, 8]. Kurds make up about 20% of the population of Iraq, and they live in Northern and Northeastern Iraq. The area covered by the current study in northern Iraq lies at the center of the Kurdish inhabited areas of the Eastern Mediterranean Region.

The second most frequent variant detected by the current study is G6PD Chatham (1003, G→A) seen in 8.7% of deficient individuals. G6PD Chatham, although first described in a patient of Asian Indian origin, is now recognized as one of the common variants worldwide [15]. Table 5 outlines the frequencies of the latter mutation in various Eastern Mediterranean countries and shows rather comparable figures among Iranian Kurds in western Iran [15]. However, figures for its frequency vary in other parts of the latter country reaching 27% in Northern Iran [16, 17]. This variant may have been introduced to northern Iraq through gene flow from neighboring Iran.

G6PD Cosenza (1376 G→C) was first described in Southern Italy [43] and has been reported in variable frequencies by some Iranian studies including the one on Kurds [15, 16]. However, this variant was not detected in any deficient individual in the current study. G6PD Aures (143 T→C) was first described in Algeria [22]. Since then it has also been reported in variable frequencies in Jordan, Kuwait and western Saudi Arabia [18, 19, 44]. However it was not reported among Iranian Kurds or in neighboring Turkey [15, 26] as is the case in the current study.

The African A-variant (202 G→A) was not detected among our sample. This is in contrast to an earlier report from central Iraq where it was reported in 4.3% of G6PD deficient subjects [9] and in some Arab Eastern Mediterranean countries where rates between 5.8-14.2% were given (table 5). However, and similar to the current study, absence of the variant has also been described in reports on Iranian Kurds and Northern Iranians [15, 16]. Absence of the variant in these areas is most likely related to the fact that the Kurdish inhabited areas of Iraq and neighbouring Iran were, unlike Arabia, a much less likely destination of African gene flow prior to or during the reign of the prosperous Islamic empire.

This study has documented that the frequency of G6PD deficiency among the predominantly Kurdish population in Northern Iraq is 10.9%, which is higher than that reported from the neighboring Iranian Kurdish population. The study which is the first molecular study on G6PD deficient variants from Iraq documented that G6PD Mediterranean and Chatham constitute the large majority of the deficient variants which is similar to findings reported in surrounding populations. Further studies including larger number of patients, more diverse ethnic backgrounds and including screening for other deficient variants and DNA sequencing are needed to give a more comprehensive view of G6PD variants in Iraqis.