Exploring the association between glucose-6-phosphate dehydrogenase deficiency and color blindness in Southeast Asia
Article Category: Brief communication
Published Online: Mar 21, 2018
Page range: 365 - 370
DOI: https://doi.org/10.1515/abm-2018-0009
Keywords
© 2017 Ingfar Soontarawirat, Mallika Imwong, Charles J. Woodrow, Chalisa Louicharoen Cheepsunthorn, Nicholas P.J. Day, Richard Paul, Pratap Singhasivanon
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.
Glucose-6-phosphate dehydrogenase (G6PD) is an essential enzyme that plays a key role in protecting cells from oxidative stress, which is particularly important for red blood cells. Approximately 140 mutations in the gene for G6PD (
G6PD deficiency poses particular problems for eradication of
X-linked red–green color blindness (or red–green dichromacy) is the most common type of inherited color blindness, occurring in approximately 8% of men and 0.5% of women globally. It is caused by an absence of the photopigments in red or green cones (protanopia or deuteranopia, respectively) or a shift in photopigment response of the red or green cones (anomalous trichromacy). These abnormalities are because of complex mutations in opsin genes on the Xq28 chromosome coding for the long-wavelength and middle-wavelength sensitive cone photopigments (
The telomeric region of the X chromosome within band Xq28 consists of about 3 Mb and contains the genes coding for G6PD, the opsin red or green color pigments, and coagulation factor VIIIc. The distance between the opsin genes and
The present study aimed to examine the association between G6PD deficiency and color blindness in a Karen population that lives in an area endemic for
Extent of homozygosity flanking the G6PD487A (Mahidol) allele over a 2.4-Mb region (based on Louicharoen et al. [7]). Positions of the 30 genotyped SNPs used to assess homozygosity are tagged by red arrows, and the positions of the Mahidol variant SNP and the SNP within the opsin locus are shown with orange arrowsFigure 1
The study was approved by the Ethics Committee of the Faculty of Tropical Medicine, Mahidol University of Thailand (approval No. TMEC10-030). The procedures followed were in accordance with the contemporary revision of the Declaration of Helsinki.
The study was conducted in Suanphung District of Thailand in a population that has been involved in malaria epidemiological studies since 1995. The characteristics of this population have been previously described [14], with people identifying themselves as Karen (85%), Thai (14%), and the remaining Mon and Burmese (1%). Family structures, G6PD phenotype, and G6PD genotype, along with nearby markers on the X chromosome, were previously ascertained in a study examining selection of G6PD deficiency on
The project protocol and objectives were explained to potential participants from the described population, and documented informed consent was individually obtained from all study participants (or their parents in the case of children).Individuals who agreed to participate in the study were tested with Ishihara plates (38 numbered figures with background color combinations) following the Ishihara test instruction [15]. In this method, color vision is initially assessed by reading plates 1–21; 17 or more correctly read plates indicates normal color vision, while correct reading of 13 or less plates indicates deficient red–green color vision, which is further confirmed using plates 22–25 (no individuals scoring 14–16 correct plates in the first 21 plates were found). For participants who were illiterate, plates 26–38 were used, and the winding line on the plate was traced with a brush by the participant. The test takes about 15–20 min per case and can be undertaken in a single visit. Results were evaluated and confirmed by an ophthalmologist at Phramongkutklao Hospital, Bangkok, Thailand.
Statistical analysis was conducted using GraphPad Prism (with a Fisher exact test for difference in proportions).
Of the 2,428 individuals, G6PD phenotypes (fluorescent spot test) and genotypes for the 3 most common
Eleven of the 186 males who could be assessed were color blind (5.9%). Ten of these had red–green color blindness, and 1 participant was totally color blind (
Distribution of 186 male participants in terms of color blindness and G6PD deficiency genotypeG6PD deficient (n = 37) G6PD normal (n = 149) Normal color vision 36 (97.3%) 139 (93.3%) (34 Mahidol, 2 Viengchan) Red-green color blind 1 (2.7%) 9 (6.0%) (Mahidol) Total color blindness 0 (0%) 1 (0.7%)
The frequency of the
Previously, 60 of the 186 tested individuals had also been typed for 30 single-nucleotide polymorphisms (SNPs) dispersed along a 2.4 Mb region encompassing
Haplotypes for 60 male participants rs1573656 (G<A) N Normal color vision G G 20 G A 22 A G 14 Red–green color blind G G 3 Total color blindness G A 1
We examined the relationship between color blindness and G6PD deficiency in a population in which the Mahidol variant of G6PD deficiency is known to have undergone recent positive selection. Under such circumstances, there is strong linkage disequilibrium and hence the possibility that phenotypic characteristics that are determined by closely neighboring genes are positively associated because the respective mutations are physically associated on the same chromosomes in the population (“coupling”) or disassociated on “opposite” chromosomes (“repulsion”).
Evidence for linkage between G6PD deficiency and color blindness has been found in a number of locations, including Israel [16], the United States of America [17], and Sardinia [12, 18, 19]. However, many studies have revealed no association (positive or negative), presumably because the linkage disequilibrium is weak (so recombination breaks down any relationships) or because the phenotypes have more than one common cause in the studied population. For the same reason, relationships observed in one population are unlikely to apply to others. For example, the 697 bp deletion in
In our population, G6PD deficiency and red–green color blindness were broadly negatively associated (in repulsion); 34 of 35 males with the Mahidol variant, and both individuals with the Viengchan variant had normal color vision, while 9 of the 10 red–green color blind individuals were G6PD normal. However, a single individual had both the Mahidol variant of G6PD deficiency and red–green color blindness, as a result of which, the negative association between the 2 conditions did not reach statistical significance. A lack of power may also explain this. Incidentally the finding excludes the possibility of using red–green color vision status as a means of assessing G6PD deficiency.
The lack of complete negative association (repulsion) between the 2 conditions could reflect crossing-over between the 2 deficiency alleles (bringing them into physical association); in our previous genetic analysis, linkage disequilibrium between the Mahidol deficiency variant and the silent rs1573656 SNP (located in the region of the red–green color genes) was not 100% [7], so this is a possible explanation. A more likely explanation is the presence of more than 1 genetic cause for red–green color blindness in this population. Our present study was only partially able to explore the underlying reason; haplotypes that consisted of the G6PD genotype and rs1573656 located in the region of the red–green color sensing genes were only available for around one-third of individuals. Consistent with the strong linkage disequilibrium already reported for this region in this population, only 3 haplotypes were found, with the Mahidol variant exclusive to the G allele at rs1573656. Interestingly, the individual with total color blindness had the A allele at rs1573656 consistent with this condition being genetically determined independently.
Despite the occurrence of a long-range haplotype covering genes involved in G6PD deficiency and color blindness, we did not find a significant phenotypic association between the 2 conditions, a finding which is likely to reflect multiple causes of color blindness in this population.