|Year : 1978 | Volume
| Issue : 1 | Page : 1-5
Genetic heterogeneity in cataracts
T Padma1, JS Murthy1, P Siva Reddy2
1 Department of Genetics, Osmania University, Hyderabad, India
2 Director of Ophthalmology, Sarojini Devi Eye Hospital and Institute of Ophthalmology, Hyderabad, India
Department of Genetics, Osmania University, Hyderabad
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Padma T, Murthy J S, Reddy P S. Genetic heterogeneity in cataracts. Indian J Ophthalmol 1978;26:1-5
Clinical heterogeneity arises mainly due to differences in the genetic constitution of persons and their response to the environmental and exogenous factors. There are a number of inherited diseases exhibiting clinical variations of which cataract is one.
The concept of genetic basis in cataracts was put forward as early as 1720 by Janin. In the following years the modes of inheritance (autosomal recessive, dominant or sex linked) of different types of cataracts was described by many investigators,,,. Further variations were observed in the expressions of cataracts right from the age of their onset, based on which congenital, juvenile and senile types have emerged. These conditions may occur isolated or in association with other anomalies affecting ectodermal (nervous system and skin), mesodermal (skeleton) and endodermal (metabolic) tissues. Factors like pleiotropism, anticipation in the following generations, penetrance and expressivity of the genes are also known to cause genetic variability.
Present attempt was made with a view to understand the genetic heterogenicity in the inherited cataracts, as exists in the local population.
| Materials and Methods|| |
A total of 247 patients suffering from congenital (87), juvenile (34) and senile (126), cataracts were examined at Sarojini Devi Eye Hospital, Hyderabad. In 40 of these cases, the family history was positive for the condition and in 31 of them the cataract could be classified as zonular (5), total (3), lamellar (2), complicated (3), nuclear (1) and cortical (17) types. Detailed pedigrees were obtained in each of these cases by questioning the patients or persons accompanying the patients (in case of congenital cataract) on the familial incidence of the disease, age at onset of the condition, and incidence of consanguinity among the parents. In all possible cases the affected relatives of the patient were also examined.
In congenital cataracts, cases with suspected malnutrition, vitamin-A deficiency, mental retardation, microphthalmos, nystagmus etc. were avoided. All the pedigrees were analysed for the mode of inheritance and heterogeneity in their expression.
| Results and Discussion|| |
Genetic heterogenicity may be resorted to in various ways such as mode of inheritance, chromosomal variations or linkage and association. This paper essentially deals with the heterogenicity arising out of differences in modes of inheritance in cataract. Though the existence of heterogenicity is already suggested in cataracts, the actual extent of such heterogeneity is not known for any population. In view of the fact that there are geographical variations in the incidence of the cataracts, a knowledge of the extent of heterogeneity may be helpful in understanding the factors responsible for the incidence of various types of cataracts.
[Table - 1] gives the possible modes of inheritance of the 31 cases of cataracts studied, on the basis of their pedigree information.
Among the two cases of congenital total cataracts, one with both normal parents and consanguinity can be regarded as a definite recessive type while the other could be either dominant or recessive, but more probably a dominant one. Of the five zonular cataracts, four may be of recessive type and one may probably be a dominant one. The inheritance of lamellar type is indefinite but there is possibility of it being a dominant.
In general, out of the 8 congenital cataracts studied, 5 (62.5%) may be regarded as recessive and the rest 3 (37.5%) of indefinite inheritance with the possibility of both dominant and recessive modes. In view of the known fact that many of the congenital cataracts are dominantly inherited an incidence of 62.5% of recessive congenital cataracts seems to be a significant characteristic feature of this population. Further in 80% of these recessive cases there was incidence of parental consanguinity supporting the recessive mode of inheritance. [Figure - 1],[Figure - 2],[Figure - 3],[Figure - 4],[Figure - 5] illustrate the nature of heterogeneity as observed in the congenital cataracts.
Considering the juvenile cataracts 4 out of 5 cases showed a dominant inheritance [Figure - 6] and parental consanguinity was not reported in any of these cases. The remaining one case was of indefinite inheritance. There was also a possibility of anticipation operating in case of juvenile cataract [Figure - 6], where a senile cataract in the earlier generation, manifested as juvenile cataract in subsequent generations.
Of the 17 senile cortical cataracts 10 cases, with normal parents could be regarded as due to a recessive gene and the other 7 with one affected parent and absence of parental consanguinity could be due to dominant gene. The single case of nuclear cataract is likely to be a recessive one.
When all the cataracts were considered, in general there were 16 (51.6%) cases due to recessive 4 (12.9%) due to dominant and 11 (38.7% due to indefinite mode of inheritance. The indefinite types could be either dominant or recessive since one or both parents and their offsprings are found to be affected in these cases.
The pattern of heterogenicity in the present cataract cases where a large proportion of them are recessives with a relatively small proportion of dominants is interesting. Statistical information on the incidence and inheritance patterns in different cataracts is not available for any other population for purposes of comparison. But it is known that the incidence of cataracts is very high in Indian population as compared to others. The higher incidence of recessive cataracts could be the result of high rate of consanguinity prevailing in this population. The incidence of cataracts is likely to be maintained or even increase if inbreeding persists in families with positive history for the condition especially of the congenital type. One possibility of reducing the morbidity rate due to cataracts is to discourage consanguinity in families with positive history for the condition.
| Summary and Conclusions|| |
1. Of 31 pedigrees of various types of cataracts, 16 (51.6%) were found to be recessively inherited, 4 (12.9%) were dominantly inherited and the rest 11 (38.7%) were of indefinite inheritance.
2. Figures for such an extent of genetic heterogeneity in modes of inheritance for any other population are not available.
3. The higher incidence of recessive cataracts, in this population may be due to higher incidence of parental consanguinity and the incidence of such inherited cataract may be reduced by discouraging parental consanguinity in the families with positive history of the condition.
| Acknowledgements|| |
The authors are grateful to Professor O.S. Reddi, Head, Department of Genetics, Hyderabad, for providing facilities and to the Staff of Sarojini Devi Eye Hospital, Hyderabad for extending their kind cooperation during the course of present investigations.
| References|| |
Duke Elder, S., 1969, System of Ophthalmology
Vol. XI, 63,
289, Henry Kimpton, London.
Francois, J., 1961, Heredity in Ophthalmology
355, Mosby St. Louis.
Janin, 1720, Observations Sur. loeil.,
Murty, J.S. and T. Jamil, 1972, Acta. Genet. Med. Gamaellol, 4, 327.
Sorsby, A., 1970, Ophthalmic Genetics, 97,
Waardenberg, P.J., Franceschetti, A. and Klein, D., 1961, Genetics and Ophthalmology,
851, Black Well scientific publications Ltd., Oxford.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9]
[Table - 1]