(ii) STU 63 family with c.802A>C variant inGNPTG gene
Since this variant was found only in the proband, it may be a de novo variant. This heterozygous variant was not observed so far in stuttering population, but reported in ExAC database.
In general, de novo being a rare genetic variant, may be more deleterious than an inherited variant since they are less subjected to evolutionary selection. They may be prime candidates when genetic diseases occur sporadically30. In our study the role of this de novo variation in stuttering is ostensibly supported by high conservation score and its absence in unaffected. Recurrence of this mutation in unrelated PWS may provide further evidence for its role.
Since most of the mutations so far reported in stuttering are by and large heterozygous, similar to that observed in our study, we wanted to comprehend how heterozygous mutations are involved in stuttering. Hence to study the impact of the de novo heterozygous missense variant identified in GNPTG , (i) quantification of mRNA by RT-PCR (ii) activity of lysosomal enzymes in plasma was carried out. We assessed if there were any differences in the expression of targeting genes and also targeting function of the lysosomal enzymes between the affected and unaffected members.
If the variation affects the targeting function, the enzyme will not be targeted to lysosomes but will be secreted in plasma. Thus the enzyme deficiency can be demonstrated by elevated enzyme activity in plasma31. Nevertheless, in our study the activity of lysosomal enzymes were not elevated in plasma, indicating that the enzyme might be successfully targeted to lysosomes. We propose that, since the variation observed is in heterozygous condition, either the normal copy is sufficient or this variation does not affect the function of the enzyme. Similarly, there was no fold change in the mRNA level of the three genes between the affected (proband) and unaffected members (father, mother and sister) of the family. Hence it was difficult to conclusively demonstrate the pathogenicity of this de novomutation in stuttering.