The influence of ZDHHC9 mutation on structural brain organisation shows both similarities and differences when compared with other groups with a similar phenotype or genetic mechanisms. Like ZDHHC9 mutation \cite{Raymond_2007}, Fragile-X syndrome is a cause of X-linked intellectual disability. Leow et al. investigated local and global properties of the white matter connectome in Fragile-X syndrome (FXS) \cite{Leow_2014}. FXS is caused by CGG trinucleotide repeats in the Fragile-X mental retardiation 1 (FMR1) gene on the X chromosome \cite{Belmonte_2006}. Leow and colleagues reported an association between the number of trinucleotide repeats in the FMR1 gene and global network efficiency in male premutation carriers as well as local differences in efficiency and clustering coefficient in left temporal nodes (also see \cite{Bruno_2016}). Our results for ZDHHC9 also indicated a reduction in global efficiency of the structural network similar to that reported for FXS, suggesting that this observation relates non-specifically to low IQ. However, topographical analysis of clustering coefficient and local efficiency indicated reductions in the parietal and frontal lobe in the ZDHHC9 group, whereas reductions in temporal areas were less pronounced or were statistically indistinguishable from the control group. In other words, mutations in ZDHHC9 and FXS show a convergent reduction in global network efficiency, but different local patterns of efficiency and clustering coefficient that distinguish the groups.

Rolandic epilepsy is another relevant neurodevelopmental condition for comparison due to the overlapping phenotype of expressive language deficits and epilepsy with centro-temporal spikes that were also observed in the carriers of ZDHHC9 mutation \cite{Baker_2015}. A study by Besseling and colleagues identified a reduction in structural white matter connectivity of the Perisylvian system, including the left inferior frontal, supramarginal, and postcentral gyrus \cite{Besseling_2013}. Studies of functional connectivity indicated reduced integration of these areas and delayed convergence of structural and functional connectivity in RE \cite{Besseling_2013a, Besseling_2014}. Further, graph theoretical analysis of the functional connectome indicated reduced clustering coefficient and local efficiency in areas of the parietal and frontal lobe in RE similar to the findings of structural connectivity differences in the current study \cite{Xia_2013}. In summary, studies of functional and structural connectivity in a neurodevelopmental condition of mixed aetiology with a similar phenotype to ZDHHC9 mutation showed reduced connectivity in areas of the parietal and frontal lobe akin to the structural connectivity changes observed in the current investigation. We are not aware of another connectome analysis of a developmental language disorder (either of known or unknown origin) against which to compare the results of our study.