Human Leukocyte Antigen -DQA1 Gene Allelic Distribution: Experience of a Major Tertiary Care Center in Lebanon

Abstract

Aims: DQA1 is a human leukocyte antigen (HLA) class II molecule that is similar to the other class II molecules DR and DP. This study is the first of its kind to describe the distribution of HLA-DQA1 alleles in Lebanon. Methods: HLA-DQA1 typing was detected using the polymerase chain reaction/sequence-specific priming method in 111 Lebanese individuals referred for HLA typing and possible bone marrow donation. Results: Our data was compared to that of several populations. Some similarities were found between the Lebanese, Tunisian, Spanish, and Kuwaiti populations. Conclusion: This very first report from Lebanon will be of great help for later research to study the association of DQA1 alleles with major diseases in the Lebanese population and will add to the published international literature related to this important histocompatibility locus.

Introduction

Polymorphism of the major histocompatibility complex genes and their allelic distribution in various ethnic groups has been the focus of several immunogenetic studies. The human leukocyte antigen (HLA) system is located on the short arm of chromosome 6 and is a very dense and polymorphic region of the human genome (Price et al., 1999). It is divided into two groups: class I and class II. Among the HLA class II genes are DR, DQ, and DP. They are heterodimeric membrane glycoproteins that are expressed by several antigen-presenting cells such as B-lymphocytes, macrophages, dendritic cells, activated T-lymphocytes, and thymic epithelial cells (Kaufman et al., 1984). HLA-DQ is expressed at a lower level on the surface of the cell (Linch et al., 1984). The α and β chains of HLA-DQ are encoded by DQA1 and DQB1 genes, respectively. Both chains reveal amino acid polymorphism that is not restricted to the peptide-binding groove (Kwok et al., 1993). Strong linkage disequilibrium exists between DR and DQ and also between DQA1 and DQB1 loci (Begovich et al., 1992).

The extensive genetic information provided by the HLA-DQA1 locus makes it a powerful marker to study the resistance and vulnerability to several diseases such as multiple sclerosis (Marrosu et al., 1998), insulin-dependent diabetes mellitus (Moustakas et al., 2002), Graves' disease (Maciel et al., 2001), and celiac disease (Zubillaga et al., 2002). HLA-DQA1 typing has also been a useful tool for bone marrow transplantation and forensic studies (Chakraborty and Kidd, 1991).

Unique distributions of the DQA1 alleles have been demonstrated in different ethnic groups. Therefore, interest in the DQA1 gene has increased because of its vital use in anthropological studies. Data on DQA1 polymorphic genes aided in understanding the genetic diversity of different populations as well as migration patterns and evolutionary relationships of human populations (Bowcock et al., 1991).

Our study focused on the characterization of DQA1 alleles in healthy Lebanese individuals and its comparison to existing studies on different populations such as Tunisian (Ayed et al., 2004), Kuwaiti (Alsaeid et al., 2006), Iranian (Amirzargar et al., 2001) Greek (Papassavas et al., 2000), Russian (Kapustin et al., 1999), Mexican (Cortes et al., 2004), Spanish (Planelles et al., 2006), and Chinese (Yang et al., 2005). To our knowledge, this study is the first of its kind in the Lebanese population. It can be later elaborated, through clinical research, into an improved understanding of the association of the DQA1 alleles signature with various diseases, specifically in the Lebanese community.

Materials and Methods

Samples

We analyzed the reported results of 111 samples from individuals (nonpatients) referred for HLA typing at the Molecular Diagnostics Laboratory of the American University of Beirut Medical Center. As they are potential bone marrow or kidney donors (screened by their referring attending physicians and transplant specialists), these individuals highly represented healthy Lebanese individuals originating from diverse regions and religious communities of the country.

DNA extraction

DNA was extracted using the PEL-FREEZ extraction kit (PEL-FREEZ, DYNAL), labeled, and stored at −80°C.

HLA-DQA1 genotyping

The commercially available DQA1 SSP UniTray Kit (Dynal Biotech, Invitrogen Corporation, Brown Deer, WI) was used to determine the HLA-DQA1 genotype. It utilizes the polymerase chain reaction/sequence-specific priming (PCR-SSP) method. Primer mixes are available in the kit, which can be used to detect the specific HLA-DQA1 alleles. PCR amplification was performed following the manufacturer's instructions. It includes initial denaturation at 96°C for 1 min, followed by five cycles of 96°C for 25 s, 70°C for 50 s, and 72°C for 45 s. Another step includes 21 cycles of 96°C for 25 s (denaturation), 65°C for 50 s (annealing), and 72°C for 45 s (extension). The final step includes four cycles of 96°C for 25 s (denaturation), 55°C for 60 s (annealing), and 72°C for 120 s (extension). After the cycle is complete, PCR products were loaded onto a 2% agarose gel stained with ethidium bromide for electrophoresis. The gel was later photographed and interpreted.

Results

Listed in Table 1 are the DQA1 allele frequencies in the Lebanese population compared with the frequencies reported in several populations such as Tunisian (Ayed et al., 2004), Kuwaiti (Alsaeid et al., 2006), Iranian (Amirzargar et al., 2001), Greek (Papassavas et al., 2000), Russian (Kapustin et al., 1999), Mexican (Cortes et al., 2004), Spanish (Planelles et al., 2006), and Chinese (Yang et al., 2005). The most common alleles in the Lebanese population with their corresponding allele frequencies are DQA1*0505 (33.18%), DQA1*0301 (9.55%), DQA1*0102 (9.09%), and DQA1*0501 (9.09%). The allele frequencies in the Lebanese population were calculated by direct counting method.

Table 1.

Distribution of DQA1 Alleles in the Lebanese Population and Other Ethnicities

Allele	Lebanese	Tunisian	Kuwaiti	Russian	Greek	Spanish	Mexican	Chinese	Iranian
DQA1*0505	0.3318	0.2717	NR	NR	NR	0.279	NR	NR	NR
DQA1*0301	0.0955	0.1956	0.2600	0.1600	NR	0.166	0.296	0.1905	0.13
DQA1*0102	0.0909	0.2282	0.2100	0.1850	0.4390	0.289	0.028	0.0733	0.145
DQA1*0501	0.0909	0.3152	0.3500	0.2650	0.6190	0.2230	0.222	0.152	0.39
DQA1*0201	0.0727	0.3152	0.2600	0.1050	0.1350	0.299	0.102	0.0897	0.075
DQA1*0303	0.0682	0.0434	0.1140	NR	0.0330	0.0900	NR	NR	NR
DQA1*0101	0.0591	0.1847	0.0000	0.1550	0.1800	0.2260	0.083	0.0916	0.055
DQA1*0103	0.0500	0.0869	0.0700	0.0850	0.1270	0.1400	0.037	0.1007	0.105
DQA1*0104	0.0455	0.0326	0.0500	0.1550	0.0700	0.0470	0.102	0.0952	0.095
DQA1*0105	0.0455	0.0652	0.0100	NR	NR	0.0270	NR	NR	NR
DQA1*0401	0.0182	0.0326	0.0800	0.0450	0.0004	0.0560	0.13	0	0.005
DQA1*0509	0.0182	NR	NR	NR	NR	NR	NR	NR	NR
DQA1*0302	0.0136	0.0217	0.2200	NR	0.0290	0.0230	NR	0.1484	NR
Total	111	100	88	76	246	301	54	273	100

The grey-colored values refer to the most frequent alleles compared in the text.

NR, not reported.

Discussion

Among HLA class II molecules are the DR, DQ, and DP, which are structurally similar to each other. The α-chain of HLA-DQ is encoded by the DQA1 gene, which is located on the short arm at chromosome 6 and is a highly polymorphic region of the human genome (Price et al., 1999). HLA DQA1 typing has been a valuable tool for many means including bone marrow transplantation, disease association studies, and anthropological reports worldwide. As a result, DQA1 typing, by both serological and molecular means, has always been performed in diverse populations.

In the present study, 13 different DQA1 allelic frequencies were detected by PCR-SSP method in a total of 111 Lebanese individuals referred to our tertiary care center's Histocompatibility Laboratory for HLA typing, as part of their routine work-up as possible bone marrow or kidney donors. Thus, our study is the first to describe HLA-DQA1 typing among healthy individuals in Lebanon. The Lebanese population has long been described to be genetically diverse because of its geographical location between the East and the West and also because of a history of political situation that has witnessed major invasions by various nations. The frequencies of the DQA1 alleles were compared with those available from eight healthy population groups: Tunisian, Kuwaiti, Iranian, Greek, Russian, Mexican, Spanish, and Chinese (Table 1). Our findings were that the most prevalent alleles in the Lebanese population were DQA1*0505 (33.18%), DQA1*0301 (9.55%), and DQA1*0102 and DQA1*0501 (both with the same frequency of 9.09%).

The DQA1*0505 allele with the highest frequency (33.18%) in the Lebanese population is not highest in any other population mentioned in Table 1. However, it is common in the Tunisian and Spanish population, with similar frequencies of 27.17% and 27.90%, respectively. It is noteworthy to mention that the DQA1*0505 allele was not reported in the Kuwaiti, Iranian, Greek, Russian, Mexican, and Chinese populations. Moreover, the DQA1*0509 allele was only reported in our study. Interestingly, the second most common allele in our population DQA1*0301 is also the second most common allele in the Kuwaiti population. DQA1*0102, which is the third most prevalent allele in Lebanon, is also the third in Tunisia. In addition, DQA1*0501 is the predominant allele in the Tunisian, Kuwaiti, Russian, Iranian, and Greek populations, but it comes in third place in the Lebanese population.

In summary, the aim of our study was to investigate the distribution of the HLA-DQA1 alleles for the first time in the Lebanese population. Some similarities exist between the Lebanese, Tunisian, Spanish, and Kuwaiti populations. This in turn ensures the genetic diversity of our population. Our data form the foundation for later studies that can be conducted to asses the association of DQA1 alleles with major diseases in the Lebanese population.

Footnotes

Disclosure Statement

No competing financial interests exist.

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