Dichromacy: Color Vision Impairment and Consanguinity in Heterogenous Population of Pakistan

Abstract: 
Background and Objectives: Dichromacy, an X-linked recessive disorder is identified worldwide, more in males than females. In European Caucasians, its incidence is 8% in males and 0.5% in females. In India, it is 8.73% in males and 1.69% in females, and in Iran, it is 8.18% in males and 0.43% in females. Population based epidemiological data about dichromacy in different ethnic groups in Pakistan is not available. The aim of this study was to find out the population prevalence of inherited red-green dichromacy in a heterogenous population of the district of Chiniot, Punjab, Pakistan, and to determine the impact of consanguinity and ethnicity. 
Methods: In this cross-sectional study, boys and girls of the higher secondary schools were examined in the three tehsils of district Chiniot. Pseudoisochromatic Ishihara Test has been employed for detection of dichromacy in the study population. The sample size was calculated statistically as 260, which was expanded to 705 and divided by population density of the three tehsils. 
Results: Screening of 359 males and 346 females revealed 19 (5.29%) dichromat males and only 2 (0.58%) females. The study population belonged to 23 castes / isonym groups. The consanguinity found in the district of Chiniot is 84.82% and in the dichromat families, it is 85.71%, of which 52.37% are first cousin. 
Interpretation & Conclusion: The study has shown that the incidence of dichromacy could be reduced through genetic counseling


Introduction:
Dichromacy is an X-linked genetic trait presenting red-green colour-blindness clinically. Human eye has two types of photoreceptor cells: rods and cones located in retina. Rod cells are responsible for discrimination of black and white light, while cone cells have three types of photoreceptors i.e., short, medium and long wavelength photoreceptors. Cone cells are responsible for color discrimination. (1) Human eye detects and responds to light through a series of events called phototransduction. (2) Dichromacy may be deuteranopia, protanopia or tritanopia. Deuteranopia is the lack of function of middle wavelength photoreceptors, thus, do not discriminate redyellow-green range of colour. Less severe form of deuteranopia is termed as deuteranomaly. Protanopia is the deficiency in discrimination of red colour due to lack of function of long wavelength photoreceptor cells. Protanomaly is less severe form of protanopia like deuteranomaly. (1) A trichromat person (with normal colour vision) has three types of photoreceptors: blue, green and red. Each photoreceptor has capability to phototransduce about 100 gradations of colour and exponentially a trichromat can see up to 2.3 million colours. But a dichromat can only see about 10,000 colours (3,4). Genetics: Dichromacy, being an X-linked recessive disorder, affects males more than females because females have two gene alleles in contrast to males having only one. X and Y are sex chromosomes that make male and female karyotypes as XY and XX. In sex linked inheritance, both alleles are expressed in contrast to autosomal inheritance in which only dominant allele is expressed. Dichromat fathers inherit his trait to their daughters who also have trichromat allele (inherited by mother) and expressed in 50% of cone cells, a so-called carrier condition. Carrier females (of dichromacy) after marrying a trichromat man may inherit their dichromat X chromosome to her son (XY). Her son will be dichromat, but her daughters will again be carriers of the trait. A female can only be dichromat if both of her parents inherit their dichromat X chromosomes. This phenomenon is more prevalent in populations with higher consanguinity rates. (5) OPN1MW and OPN1LW are the genes responsible for deuteranopia (also deuteranomaly) and protanopia (also protanomaly) respectively. Both genes are located on the long arm of X chromosome in band 8 of region 2. Sixteen mutations are reported for the OPN1MW gene in Human Gene Mutations Database (HGMD) that are of different types including missense, nonsense, insertions and deletions. Of these sixteen, only seven mutations cause deuteranopia. According to HGMD, fiftyfive mutations of OPN1LW gene are reported and only 11 mutations are causing protanopia or protanomaly. Mutations in OPN1MW and OPN1LW other than mutations causing deuteranopia and protanopia are cause of altered absorption spectrum, blue cone monochromatism, Bornholm eye disease, cone-rod dystrophy, and X-linked cone degeneration.

Worldwide
Prevalence: Worldwide dichromacy is present in approximately 7% of males and 0.5% of females, however, prevalence rates vary in different populations: China -6.9% and 1.7% (6), France -8.95% and 0.50% (7), Germany -7.75% and 0.36% (7), Greece -7.95% and 0.42% (7), India -8.73% and 1.69% (8), Iran -8.18% and 0.43% (9), Iraq -8.47% and 1.37% (10), Israel -10% in Ashkenazi Jews and 4% in Yemenite Jews (11), Korea -5.9% and 0.4% (7), Nepal -3.8% among males (12), Netherlands -7.95% and 0.45% (7) Norway -8.01% and 0.44% (13), Saudi Arabia -5.85% and 0.75% (14), Switzerland -7.95% and 0.43% (7), and Taiwan -5.3% and 0.2% (7) among males and females respectively. This study aims to find prevalence of dichromacy in a multi-ethnic heterogenous population in Pakistan. History of consanguinity is also recorded and reported in association with dichromacy. Inclusion Criteria: Both male and female students from secondary classes lying in age group of 14-18 years without any vision impairment were included. Exclusion Criteria: Individuals with congenital vision defects other than dichromacy were excluded. Statistical Analysis: The data, recorded in SPSS (Version 20.0) for analysis purpose, were analysed for descriptive statistics. Sampling distribution frequencies, gender distribution among studied population and frequency of dichromacy among different castes were calculated. The percentage of dichromacy among males and females is calculated separately. In addition, percentage of dichromacy in each tehsil and its distribution by gender were also calculated. Chi Square Test was used to measure the association between dichromacy and consanguinity. This (chi square test) resulted in reporting the associations in terms of Odds Ratios and their 95% confidence intervals giving the Chi-square with its p-value.

Figure 13: Distribution of different castes in study population
Frequency plot of participating castes is shown using the grouped bar chart. Ethical Consideration: The study was approved by Ethical Review Committee of University of Health Sciences, Lahore-54600, Pakistan. Participating volunteers took an informed consent form.  Figure 1, while distribution of dichromacy among different castes is demonstrated in Table 1. A total of 21 cases of dichromacy were found in three tehsils: 9 from Bhowana (7 males and 2 females), 6 from Chiniot (males) and 6 from Lalian (males). The total percentage of dichromacy of the study population of the District Chiniot is found 2.98%. The prevalence (percent) of dichromacy among males is 5.29% and that of females is 0.58%. Tehsil wise prevalence of dichromacy in Bhowana, Chiniot and Lalian is 3.76%, 2.29% and 2.92% respectively. The percentage among males is 6.19%, 4.58% and 5.21% in Bhowana, Chiniot and Lalian respectively. Percentage among females of Bhowana, Chiniot and Lalian is 1.58%, 0.00% and 0.00% respectively. Of these dichromats, protanomaly was found in 1 individual, protanopia was found in 6 individuals, deuteranomaly was found in 2 individuals and deuteranopia was found in 6 individuals. There were six dichromats that couldn't be classified into protanopia/protanomaly and/or deuteranopia/deuteranomaly using Pseudoisochromatic Ishihara Test. The prevalence of dichromacy types among males and females is described in Table 2. The consanguineous history of the parents of study subjects revealed 84.82%, which was slightly higher in the parents of dichromats (85.71%), of whom 52.37% were first cousins. Interestingly, patrilineal dichromats were 33.33% and matrilineal were 19.04%. Second cousin marriages were 19.04%, and marriages in Bradri (same clan) were 19.04%. Inter-caste marriages were only 9.52%. Table 3. Pearson Chi Square Test reported association between dichromacy and consanguinity (p-value is 0.003). Pedigrees of the dichromat individuals were drawn to understand the pattern of inheritance. However, extended family exploration and identification of dichromats were not possible because of far-off residences of relatives and other social issues in local context of the Pakistani population. Two pedigrees are part of this article as figure 2 and figure 3. Figure 2 is male proband and figure 3 is female proband.

Discussion:
In Pakistan, the known percentages of dichromacy among males are 3.59% (15)   population is representing different races and ethnic origins, for which specific caste information is not available. While our study in Chiniot represents local urban and rural inhabitants who share close ethnicity and nearby geographic / climatic factors except Baloach, Memon, and Pathan. Baloach, Memon and the Pathan populations are far in ethnicity but live in the same geographical area. But these ethnic groups are also inhabitants of the district for generations, as reported in population censuses by national census bureau. The third study, conducted in medical colleges of Faisalabad, Punjab, recorded 2.4% of male and 4.48% of female dichromats. (18) This study represents the sample of students from different ethnic and geographical backgrounds of the Punjab. On the other hand, the figure of 4.48% for dichromat females seems not only quite high but erroneous, because it gives the figure of 2.2% for male dichromats, which is too low. If the figure for females is taken as correct, then the figure for males should have been at least 10 times higher than females. Dichromacy is an X-linked trait and its incidence is always higher in the hemizygous male sex. The fourth study, conducted in a specific group of professionals, i.e., aesthetic dentistry practitioners of Karachi, gives dichromacy in 10.0% of males and 1.60% of females. (19) Karachi is the business hub of Pakistan. Thus, population of Karachi is a conglomeration of different ethnic, cultural and geographic origins. Only 100 males and 183 females were included in this study among which 13 (4.59%) were dichromats. However, presence of male (10.00%) and female (1.64%) dichromats was higher than expected. The information about the ethnicity and consanguinity is not available of the specialized group, which may have some sampling bias. Another similar study (Khalid et. al) reported prevalence of dichromacy as 7.95% and 1.39% among male and female dentistry students in Peshawar (20). Peshawar is the capital of Khyber Pakhtunkhwa Province. Khyber Pakhtunkhwa population is of Pashtun origin whereas in our study, any Pashtun (Pathan is not found to be dichromat, probably due to a limited sample. This study provides population-based prevalence of dichromacy and is compared with prevalence of dichromacy in different populations of the world in Table 5. Our estimated percentage of dichromacy among males is 5.29%, while percentage of dichromacy in India is 8.73% (8) and in Iran 8.18% (9). The percentage of dichromacy in our study among females is 0.58% comparable to incidence of dichromacy among females in Iran (0.43%) (9). Indian Muslims reported dichromacy among females is 1.69% which is too high probably due to higher consanguinity rates. ( (7) partner be performed for dichromacy and other genetic diseases.

Conclusion:
Current study is significant for reporting the prevalence of dichromacy in local population consisting multiple isonyms. This study also showed association of dichromacy with consanguinity in studies population. In this study, dichromacy is identified first time in the Central Punjab district, Chiniot and reported prevalence of dichromacy as 5.29% in males and 0.58% in females. The study showed the association of dichromacy with consanguinity. Suggestion: It is suggested that children be tested for dichromacy at the time of admission to school and should be provided with customized teaching to manage learning barrier of dichromacy. It is known that dichromacy has a strong impact on the professional career of individuals. The dichromats should be counseled for their careers and adaptations to live a happy and peaceful life. Future Prospect: Because of this pioneer study, we plan to investigate the prevalence of dichromacy with larger sample and in other ethnic groups in other provinces of Pakistan which may include molecular based genetic study of dichromacy.