Suzanne de Bruijn

90 Chapter 2 MAPK pathway which is involved in the development of cancer. For this reason, these fusion genes are the major cause (66%) for pilocytic astrocytomas, the most frequently occurring central nervous system tumor in children and young adolescents. Besides this role in oncogenesis, knowledge about the function of KIAA1549 is limited. Recently, a homozygous truncating variant in KIAA1549 was found in an arRP family with two affected siblings in a study performed by Abu-Safieh et al. 5 Involvement of KIAA1549 in photoreceptor function was suggested, however no functional data was provided. 5 Nevertheless, KIAA1549 is reported to be among the top 4% of genes being enriched for binding sites for the photoreceptor specific transcription factor CRX. 24 Kiaa1549 expression was evaluated in a Nrl -/- knockout mouse that is characterized by degenerated rod photoreceptors. In this mouse, Kiaa1549 expression was found to be reduced ~88% (wildtype: 106 reads, knockout: 13 reads) when compared to the wildtype mouse, based on number of sequencing reads. 24 In this study, expression levels of the major short and long transcripts of KIAA1549 have been evaluated in a set of human tissues, which demonstrated that both isoforms are present in the retina, of which the expression of the transcript encoding the short isoform is significantly higher in retina compared to other tissues. We hypothesize that both isoforms are required for the correct function of the protein in the retina, as the homozygous frameshift variant affectingthe long isoformhasdetrimental consequences as observed in Family A and the family previously described in the study of Abu-Safieh et al. 5 By performing an in vitro experiment in which HEK293T cells were transfected with KIAA1549 -targeting siRNAs that specifically recognize the long transcript of KIAA1549 ( Table S1 ), also a significant decrease in expression of the short transcript was observed ( Figure S2 ) which suggests a functional dependency between the two transcripts. Hence, observed variants in the long transcript likely cause a decrease in the abundant retinal expression of the short transcript and thereby could lead to retinal degeneration. The fact that the identified variants have different consequences on the two KIAA1549 transcripts could explain the phenotypic differences observed among the affected individuals. The phenotype of the family described by Abu-Safieh et al. (a non-recordable ERG at age 35) (personal communication Prof. F.S. Alkuraya and N. Patel, PhD) is more comparable with Family A (non-recordable ERG at age 32 in patient A-II:2) than Family B (severely reduced photopic and moderately reduced scotopic ERG at age 54), which may be in line with the genotype having a damaging variant in the long transcript. Identification of additional families with KIAA1549 -associated RP are required to provide deeper insight into a possible phenotype-genotype correlation. 25