Schizophrenia (SCZ) is a devastating psychiatric disease with a worldwide prevalence of approximately 1%. It is therefore one of the leading causes of public health burden. Twin studies suggest that the impact of heritable factors causing SCZ is very high. Thus, in the recent past huge efforts were made to identify the genetic factors responsible for the disease. Many genes and genomic variations have already been associated with schizophrenia, but the interplay of these genes, as well as the precise mechanism of how they are involved in the development of schizophrenia is still not fully understood.
To get a deeper insight into the role of SCZ associated genes I used protein-protein interaction analyses combined with bioinformatical methods. My goal was to answer three mayor questions:
The first question was, if schizophrenia associated proteins form clusters within protein-protein interaction networks and how these clusters are involved in functional processes. For that reason, a propagation-based algorithm was invented that identified five clusters with high potential for SCZ relevance. The two highest scoring clusters represented known synaptic complexes and were validated with LuTHy assays.
The second question was, if there is a potential SCZ relevance of a set of 39 protein coding candidate genes of a small exome sequencing study and if their importance could be prioritized. Therefore, a protein-protein interaction network was created, using the HIPPIE database, including all medium high confident interactions of these genes. In a next step the density of SCZ associated proteins within the created network were compared to all HIPPIE proteins, not already included in the created network and their connectivity to SCZ related proteins. Chi-squared tests revealed indeed a significant enrichment of schizophrenia associated proteins within the created candidate protein-protein interaction network. In order to rank candidate genes, the browser based ToppNet tool was used.
The third question should shed light on the functional role of ZNF804A. This protein had repeatably been associated with schizophrenia before, but its functional role remained unclear. By following the hypothesis “guilt by association”, a proteome scaled Y2H screen was preformed and 18 new ZNF804A interacting proteins had been been identified with functional enrichment for RNA binding, the circadian clock and inflammation pathways. By using DULIP and LuTHy assays, 67% of identified ZNF804A interactions were validated. The functional implications of ZNF804A with the most promising interaction partner STAT2 were further analyzed. STAT2 is a key protein of the intracellular interferon response and ZNF804A was identified to co-translocate with STAT2 into the nucleus upon interferon induction. Overexpression, as well as CRISPR/Cas9 induced knock down of ZNF804A indicated a potential modulating role of ZNF804A in STAT2 mediated interferon response.
The results of my work help to better understand the role of SCZ related genes and their interplay. Additionally, my studies demonstrate that protein-protein interaction analyses are able to gather information on different levels and are a key tool set to reveal the molecular implications of genes associated with schizophrenia
