INTRODUCTION
Speech is a unique motor function and when affected causes both receptive and expressive communication disorders, significantly reducing the quality of life. Stuttering is an expressive fluency disorder, characterized by repetitions, prolongations, blocks, along with secondary behaviors (head jerks, lip tremors and eye blinks) and often lead to psychological problems such as increasing anxiety1.
Developmental stuttering arises in children of 2-5 years age group, but most of them (80%) recover spontaneously. But there is greater chance of recovery (male female ratio ~5:1) among females2–4. Prevalence of stuttering ranges from 0.3% to 5.6% and the average prevalence over the lifespan may be lower than the commonly cited 1%5. In a recent study based on 75000 school children in India, we reported a prevalence of 0.46%6.
Research studies on the etiology of stuttering focused mainly on neuroimaging and genetics. Most of brain imaging methods have consistently reported structural or functional differences contributing to inefficient communication in stuttering. It includes over activity of the dopamine neurotransmitter7, abnormal functional lateralization of cortical connections8, deficits in white matter tract that connects motor and auditory structures, corpus callosum as well as cortical and subcortical areas9. There is also growing consensus about the genetic origin of Central Nervous System dysfunctions 10.
Genetic dissection is challenging due to gene-gene/gene-environment interactions, genetic heterogeneity, gender bias, incomplete penetrance and phenocopies 11. Initial linkage studies found suggestive evidence for chromosome regions (1, 2q, 3q, 5q, 7q, 9p, 9q, 13q, 15q, 18p, 18q, 20p) implicated in stuttering but with little overlap across studies 12,13,14. However definitive evidence for linkage was identified on chromosomes 3, 12 and 16 in highly consanguineous Pakistani families15,16,17 and on chromosomes 2, 3, 14 and 15 in a large Cameroon family18. Although linkage studies are spread across Hutterite, European and American population, the four genes,GNPTAB, GNPTG , NAGPA and AP4E1, identified are restricted to two regions [Pakistan19; Cameroon20] with distinct ethnicities. The combined contribution of these genes were estimated to be 20%21. All these genes point to intracellular trafficking deficits22. Indeed genomics of stuttering is an emerging field where the involvement of new genes are yet to be identified.
GWAS study suggested ten candidate genes (FADS2, PLXNA4, CTNNA3, ARNT2, EYA2, PCSK5, SLC24A3, FMN1, ADARB2 and non-coding RNARNU6-259P ) involved in neural pathways23. Mutations so far identified implicate lysosomal dysfunction but the biological mechanisms that affect speech are under investigation. Mice models also point to deficits in inter-hemispheric connectivity in astrocytes of corpus callosum 24,25, thus linking genes to brain activity. Stuttering genes GNPTAB and GNPTGwere also previously implicated in a rare lysosomal storage disorder - mucolipidosis.
From the genetic perspective, genes identified play role in targeting enzymes to lysosomes that is crucial for biogenesis and also in the maintenance of myelin sheaths. From neurological perspective hyperactivity of dopamine and the white matter abnormalities observed in stuttering, provide a possible neurochemical basis but the effect of the mutations in neural cell biology is still unexplored. Owing to significant plasticity of brain it was unable to account for the observed differences among PWS and control, as to whether they are cause or result of stuttering26. Thus the connecting dots between dopamine, neural circuits and cellular waste disposal is yet to be connected.
No studies from India are available till date that implicate any genes for stuttering. This gap motivated us to first ideally ensure the frequency of the previously implicated genes for stuttering in our population, before initiating advanced approaches. We evaluate the recurrence of the reported mutations among the three reported (GNPTAB, GNPTG, NAGPA) stuttering candidate genes in PWS from south India. Attempts to employ identical experimental design to concurrently verify and replicate the findings independently would on one hand help understand ethnicity specific variations and on another hand would also enable reproducibility of results and facilitates pooling of data during meta-analysis.