Suzanne de Bruijn

91 Homozygous KIAA1549 variants are associated with retinitis pigmentosa In addition, we showed localization of KIAA1549 at the connecting cilium of mouse photoreceptor cells, providing the first information on KIAA1549 function in photoreceptors. Moreover, KIAA1549 localization was also noted at the outer plexiform layer of themouse retina. Proteins localized at the ribbon synapses of the outer plexiform layer are often structural or synaptic vesicle proteins or are involved synaptic vesicle trafficking. 26,27 The KIAA1549 antibody will recognize both isoforms and thus does not provide additional knowledge on alternative localization of the isoforms. Hypothetically, the long and short isoforms may harbor a unique function at either one of the identified locations. Additional research is required to unravel the functional differences between the short isoform and the ubiquitously expressed long isoform of KIAA1549. Besides localization in the photoreceptor, there is additional evidence for ciliary function is at the molecular level. A recent study based on proximity-dependent biotinylation revealed an interaction between KIAA1549 and TMEM17. 28 TMEM17 is a part of the Meckel syndrome (MKS) protein complex located in the ciliary transition zone, inwhich it facilitates cilium formation. Also, pathogenic variants in genes encoding proteins in this complex are known to cause (severe) ciliopathies. 29 TMEM17 pathogenic variants have been reported to cause oral-facial-digital syndrome type 6. 29 The MKS complex contains both cytoplasmic and transmembrane proteins, and functions as a barrier preventing rapid diffusion of transmembrane proteins between cilia and plasma membranes. 30 The interaction between KIAA1549 and TMEM17 was only observed in cells in non-ciliated conditions, which suggests that the interaction is involved in a cilium-related process. 21 We have studied the role of KIAA1549 in ciliogenesis, by knocking down the expression of the gene in hTERT-RPE1 cells using siRNAs. siRNA-transfected cells did not show a difference in percentage of ciliated cells or ciliumlength, suggesting that KIAA1549 does not have a direct role in the cilium formation explaining the nonsyndromic phenotype observed in the patients of Family A and B, as well as the family of Abu-Safieh et al., which is restricted to the retina. Pathogenic variants that do affect genes essential for ciliogenesis, such as TMEM17 , would give rise to a phenotype likely affecting multiple organs as in ciliopathies. Transmembrane proteins present at the transition zone are often involved in the sensing and transducing of extracellular signals. Like TMEM17, KIAA1549 is a transmembrane protein, therefore it is plausible that KIAA1549 may be involved in these processes at the primary cilium of the photoreceptors specifically. 30 In conclusion, by employing WES we have identified that homozygous frameshift or missense variants in KIAA1549 are associated with RP in two families. We demonstrated retina-specific expression of the short isoform of KIAA1549 and provide evidence that damaging variants targeting the long transcript may cause RP by reducing the