Abstracts

Rationale: Hypothalamic hamartoma (HH) with gelastic epilepsy is a well-recognized benign congenital tumour of the hypothalamus that generally results in a refractory drug-resistant epilepsy syndrome of early life. Surgical resection allows limited access to small deep-seated lesions that cause disease. It has been demonstrated that germline variants in GLI3, a component of the sonic hedgehog pathway, cause syndromic forms of HH (e.g., Pallister-Hall syndrome). Subsequently, we and others have shown that somatic variants in GLI3 and other sonic hedgehog pathway genes in hamartoma tissue can cause the sporadic disease. Methods: In this study we utilized exome sequencing on whole-blood and HH tissue-derived DNA to implicate somatic and germline variants within genes involved in ciliogenesis and intraflagellar transport of the primary cilium, an organelle essential for normal sonic hedgehog signalling during development. Results: In eight unsolved sporadic HH cases negative for sonic hedgehog variants, we identified candidate variants in three ciliopathy genes: DYNC2H1, WDR60 and CPLANE1. DYNC2H1 and WDR60 respectively encode the heavy and intermediate chains of the ciliary IFT motor protein dynein 2 whilst CPLANE1 encodes a protein involved in ciliogenesis that has already been implicated in syndromic forms of HH (e.g. Orofaciodigital VI and Joubert syndromes). Three of these individuals carry somatic frameshift, missense or nonsense variants in DYNC2H1. Interrogation of the germline revealed secondary DYNC2H1 nonsense and missense variants in two of these cases, and a second hit CPLANE1 missense variant in the other. The fourth individual identified carries novel somatic and germline frameshift variants in WDR60. Interrogation of four additional unsolved cases has revealed novel germline nonsense, missense and splicing variants in DYNC2H1. The absence of HH tissue in three of these cases has prohibited the interrogation of secondary somatic hits. Conclusions: We have identified three sporadic HH individuals that carry two-hits within single cilia genes, namely DYNC2H1 or WDR60. Additionally, we have identified one individual with two-hits in multiple cilia genes DYNC2H1 and CPLANE1, and four individuals with germline variants in DYNC2H1. These genes encode functional components of primary cilia, the cell organelle in which GLI3 and other sonic hedgehog pathway proteins localise in order to transduce signalling. We provide evidence for the contribution of germline and somatic variation in cilia genes to sporadic HH disease. Funding: National Health and Medical Research Council (NHMRC) Grant ID:1079058National Health and Medical Research Council (NHMRC) Grant ID:1129054