Hemoglobin E Levels in Double Heterozygotes of Hemoglobin E and Sea-type Α-thalassemia

Coinheritance of α-thalassemia and hemoglobin E (Hb E) is prevalent in Thailand, where the gene frequencies of thalassemia and hemoglobinopathies are high. Hb E carriers with, concomitant inheritance of α-thalassemia 1 are known to have a lower level of Hb E. In this study, we reviewed the Hb E levels in Hb E carriers, who either had or did not have Southeast Asian (SEA)-type α-thalassemia, in order to seek a Hb E level that may be used as a predictor for concomitant αthalassemia carrier status. The Hb E levels as measured by microcolumn chromatography in 844 Hb E carriers detected during a prenatal screening program for severe thalassemia at Chiang Mai University Hospital were reviewed. Hb E levels ranged from 12.3-35.0% (23.3±3.1%) in 751 Hb E carriers without SEA-type α-thalassemia and from 11.6-32.0% (17.0±3.7%) in 93 concomitant Hb E and SEA-type α-thalassemia carriers. The difference was significant (p<0.01). However, the absence of SEA-type α-thalassemia could not be predicted by the higher Hb E level alone, as 3% of double heterozygotes demonstrated a level of more than 25%. Our study confirms a lower Hb E level in double heterozygotes with Hb E and SEA-type α-thalassemia. Nevertheless, the difference does not provide sufficient discriminatory power for the reliable exclusion of α-thalassemia status. semia (Fucharoen, 2001). Therefore, prenatal screening for Hb E carriers and a fetal diagnosis program would provide an alternative for couples at risk. Hb E carriers are initially screened by the Hb E Screen or DCIP test. The diagnosis is confirmed by Hb E fractionation, which is conventionally done by microcolumn chromatography (Sanguansermsri et al, 1998, Tongsong et al, 2000). α-Thalassemia 1 is characterized by an in cis deletion of two α-globin genes. Screening for α-thalassemia 1 carrier status is included in prenatal screening since when both parents are α-thalassemia 1 carriers, the chance of having an offspring with Hb Bart’s hydrops fetalis will be 1 in 4 for each pregnancy. An α-thalassemia 1 carrier status is usually screened for either by a low MCV (mean corpuscular volume) of less than 80 fl, or increased red blood cell osmotic resistance. Further testings by DNA-based techniques are required to provide a definite diagnosis for carriers of α-thalassemia. The laboratory part of the prenatal thalassemia screening program at our institute is comprised of an osmotic fragility test (OFT) as a SOUTHEAST ASIAN J TROP MED PUBLIC HEALTH 468 Vol 36 No. 2 March 2005 screen for α-thalassemia 1 and β-thalassemia carriers. The OFT detects only 70% of Hb E carriers, therefore the Hb E Screen is used for screening Hb E carriers. Individuals with positive results on the OFT are further tested by microcolumn chromatography to determine the Hb A2 and Hb E levels. They are also tested with concurrently a PCR-based test designed SEAtype α-thalassemia, which is the most common cause of α-thalassemia 1 in Thailand. Individuals with a positive Hb E Screen are further tested by microcolumn chromatography to determine the Hb E level (Tongsong et al, 2000). Hb E makes up approximately 25-30% of total hemoglobin in heterozygotes (Fucharoen, 2001). The percentage of Hb E is influenced by various factors, such as iron status and the number of α-globin genes (Wasi et al, 1967, 1968; Huisman, 1997). In Northern Thailand, the prevalence of SEA-type α-thalassemia heterozygotes is 14% (Lemmens-Zygulska et al, 1996). The prevalence of coexisting SEA-type α-thalassemia is not uncommonly seen at 10.3% among Hb E carriers (Sanguansermsri et al, 1998). As has been shown in previous studies that double heterozygosity of Hb E and α-thalassemia results in a decrease in the quantity of Hb. We retrospectively reviewed the percentages of Hb E in Hb E carriers to compare the levels between the two groups, with and without concurrent SEAtype α-thalassemia, and to seek a specific Hb E level that may be used as a predictor for a concomitant SEA-type α-thalassemia carrier status, which may help to decrease the need for PCR tests in thalassemia prenatal screening. PATIENTS AND METHODS After approval was received from the institutional Research Ethics Committee, prenatal thalassemia screening results from pregnant women and/or spouses from the Thalassemia Unit, Department of Pediatrics, Faculty of Medicine, Chiang Mai University, Chiang Mai, Thailand between January 2000 and May 2002 were reviewed. A Hb E carrier was defined as a case with a Hb E level of between 10-35% by modified microcolumn DEAE Sephadex A50 chromatography (Sanguansermsri et al, 1998). A SEAtype α-thalassemia carrier was defined by a positive result on a SEA-PCR test (Sanguansermsri et al, 1999). HbE levels and PCR results from Hb E carriers were recorded. Means, standard deviations and ranges of Hb E levels in the Hb E carriers with and without SEA-type α-thalassemia were calculated. Means were compared by the two-tailed Student’s t-test. A Receiver Operator Characteristics (ROC) curve was created to determine the sensitivities and specificities of the Hb E levels in predicting a negative result for SEA-type α-thalassemia.