Indian Journal of Human Genetics Medknow Publications on behalf of Indian Society of Human Genetics ISSN: 0971-6866 EISSN: 1998-362x Vol. 11, Num. 3, 2005, pp. 135-139

Indian Journal of Human Genetics, Vol. 11, No. 3, September-December, 2005, pp. 135-139

Original Communication

Modes of genetic transmission of dyslexia in south Indian families

Saviour P, Ramachandra NB

Human Genetics Laboratory, Department of Studies in Zoology, University of Mysore, Manasagangothri, Mysore-570006, India
Correspondence Address:N. B. Ramachandra, Human genetics Laboratory, Department of Studies in Zoology, University of Mysore, Manasagangothri, Mysore-570006, India. E-mail: rnallur@sancharnet.in

Code Number: hg05026

Abstract

Keywords: Dyslexia, Reading-spelling deficit, Spelling deficit, Phenotype, Inheritance

Acquisition of language has become ever more important in this information age. Most children acquire it naturally in a sequence of listening, speaking, reading and writing.[1] Failure in any of these processes may lead to life long socioeconomic and mental health consequences.[2] There are many causes of language disability; the most common is developmental dyslexia, which is defined as difficulty in learning to read and spell despite adequate education, intelligence, sociocultural opportunities and without any obvious sensory deficits.[3] It accounts for 80% of learning disabilities. The phenotype of dyslexia is complex and different phenotype dimensions can be distinguished.[4] Although multiple etiologies are proposed for the this complex trait exact cause still remains unknown but substantial evidence from genetic and neurobiological studies suggest that dyslexia is a disorder which is influenced by genetic factors and the underlying deficit is in the language areas of the brain.

Though dyslexia is a major educational problem, no much study has been done on the family patterns of dyslexia in India. There is a great dearth of proper statistical data to show the incidence of dyslexia in Indian population. Here we report the patterns of genetic transmission of dyslexia in 23 Indian families.

Materials and Methods

Results

Out of 23 families, 19 were extended families and four were nuclear families. All the families were Dravidians and their native language was Kannada. [Table - 1] provides comprehensive data on the frequency of individuals affected with different dyslexia phenotypes and compensation rate of those deficits. Out of 392 individuals of 23 families screened; about 53% and 47% were males and females respectively. Of the 109 individuals affected, 62.39% and 38.53% were males and females respectively [Table - 1]. c[2] test reveals that there is significant difference between affected males and females. Affected individuals in the families ranged from 1 to 26. Based on the affectedness of the trait, these individuals were classified into severe dyslexics and mild dyslexics. Both severe and mild dyslexics were shown two phenotypes, reading-spelling deficit as well as only spelling deficit.[Figure - 1] illustrates high frequency of severe reading-spelling deficit. Individuals who have severe spelling deficit was better compensated the difficulty than severe reading-spelling deficit [Figure - 2]. Mild spelling deficit was better compensated than that of other deficits.

Three generation pedigrees of 23 families revealed three types of inheritance patterns of dyslexia, viz, autosomal dominant inheritance, autosomal recessive inheritance and sporadic cases [Table - 2]. Of these, autosomal dominant inheritance was more prevalent (65.22%) than other types. Reading-spelling deficit had shown all the three types of inheritance patterns however, autosomal dominant pattern was seen in most of the cases. Spelling deficit had shown more of sporadic pattern than autosomal dominant inheritance pattern, while autosomal recessive inheritance of spelling deficit was not found in any of the pedigrees. Pedigrees are presented in [Figure - 3] showing each category of inheritance pattern. Out of 23 families, 8 families showed consanguineous marriage where three families showed sporadic pattern of spelling deficit, three families were consistent with autosomal dominant pattern of reading-spelling deficit and two families had shown autosomal dominant inheritance of both reading-spelling deficit and spelling deficit.

Out of 109 dyslexics, it was found that 21 Dyslexic individuals were allergic to dust, milk products and to certain vegetables like radish and citrus fruits. It was also found 9 dyslexic individuals were left handed and 33 individuals had short sight which is corrected using appropriate lens. Eighteen subjects had speech delay in the early childhood.

Discussion

The prevalence of dyslexia ranges from 3-17.5% of the school age children.[5],[6],[7] To our knowledge such prevalence studies has not been done in India. In the present study, among the selected families, the prevalence of dyslexia is found to be 27.8%. This indicates that occurrence of dyslexia is much more (~1/3) among the individuals of the family. As in other studies[8],[9] males are more affected than females (1.6:1). It is found that reading-spelling deficit is the common phenotype of dyslexia. It is interesting to note that reading difficulty is always associated with spelling deficit and not vice versa. Compensation rate is more among individuals affected with mild reading-spelling deficit and it is least in severe reading-spelling deficit. Individuals who have severe spelling deficit are better compensated than those with severe reading-spelling deficit. Number of individuals compensated mild spelling deficit out numbers rest of the individuals with other deficits. This suggests that individuals with mild dyslexia phenotypes (>50%) show higher compensation rate than that of individuals with severe dyslexia phenotypes. Compensation of the deficits to a maximum extent is possible by providing special education; otherwise the deficit will remain as such.

In this era of genomics, families are still the starting point of genetic investigations. Familial aggregation of dyslexia was first reported in the turn of the last century.[10] Many studies have been done on familiality, heritability and modes of transmission of dyslexia which suggest genetic factors have significant role in predisposition of dyslexia.[11],[12],[13]Different genetic models of dyslexia have been postulated and there is evidence for both polygenic and monogenic inheritance.[14] Hallgren[15] was noted autosomal dominant transmission of dyslexia in 80% of the families studied. Pennington et al[16] also found that in 20-30% of families with dyslexic children, the inheritance pattern is autosomal dominant. Contrary to this, Lewitter et al [17] and Grigorenko et al [18] reported that there was no evidence for a single major locus, autosomal dominant, autosomal recessive or codominant transmission. On the other hand, inheritance of dyslexia in Indian families is poorly understood. In the present study, autosomal dominant, autosomal recessive and sporadic patterns of dyslexia are recorded, of which, autosomal dominant pattern is seen in majority of the cases. Autosomal dominant inheritance is more frequent in the dyslexic families as it is not fatal for the individual.

With the advancement of human genome research geneticists have made substantial progress in identifying the genetic basis and also there have been numerous reports of loci, DYX1 at 15q21, DYX2 at 6p21.3, DYX3 at 2p16-p15, DYX5 at 3p12-q13, and DYX6 at 18p11.2 which are involved in genetic predisposition of dyslexia.[4],[19],[20],[21],[22] These genetic loci may harbor many genes which are responsible for this complex trait. Each phenotype of this complex trait may be controlled by a gene(s). Therefore, isolation of the defective genes will initiate a new phase of research which will provide a more fundamental understanding of the nature of dyslexia, eventually leading to early diagnosis, risk estimation, better methods of treatment and prevention.

Dyslexia is found to be associated with retarded speech development,[4], [23] left handedness and autoimmune disorders.[17] Present study also recorded a few cases of speech delay in the early childhood, left handedness and auto immune diseases associated with dyslexia. It is well known that consanguineous marriages increase the inheritance of defective genes in the family. In the present study about 30% of the families are with consanguineous marriages which could be a factor for the high prevalence of dyslexia in these families.

A family history of dyslexia is a consistent risk factor and this knowledge can be applied to the prevention and remediation of dyslexia. Without specialized knowledge of familial dyslexia patterns it is often difficult to exclude familial clustering and respond adequately to the concerns of the affected individuals. The pupils with dyslexia are generally ignored and promoted to higher grades until they become dropouts or delinquents. High prevalence of dyslexia could be one of the factors for the high illiterate population in India. Once children are diagnosed as dyslexic, they would require specific intervention and ongoing support if they are not to be further handicapped in their learning. Therefore, more studies on dyslexia are required in India and we feel that there should be implementation of legislation in the country to take utmost care of these children because dyslexia is an invisible handicap.

Acknowledgements

References

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