Biallelic TMEM72 variants in patients with a nephronophthisis-like phenotype

INTRODUCTION: Nephronophthisis (NPHP) is an autosomal recessive kidney disease resulting mainly from primary cilium defects, with unspecific and variable symptoms that can progress to kidney failure needing replacement therapy at a young age. Currently, up to 64% of likely NPHP cases can be diagnosed by assessing known genes. Therefore, there is a need to gain more insight in what genes can cause this disease. METHODS: In a diagnostic setting, we performed broad genetic testing in patients with advanced kidney disease. We carried out in silico and in vitro analyses for TMEM72, including immunohistochemistry and affinity proteomics, and in vivo experiments to further interpret our findings. RESULTS: We identified biallelic TMEM72 variants in nine patients from six families with a phenotype suggestive for NPHP. Five families presented with kidney failure at a (young) adult age. One family had a different phenotype with prenatal onset of kidney failure and neurological symptoms. The phenotypes of the patients correspond to TMEM72 expression mainly in the kidney. In silico analyses indicate that homozygous loss-of-function variants are likely not tolerated in TMEM72. Immunohistochemistry staining of kidney biopsies revealed altered localization and expression of TMEM72 in cases compared to controls. In human-derived tubuloids, we showed that TMEM72 localizes to the cilium. Furthermore, using an affinity proteomics approach, we found an association of TMEM72 and ciliary function, more specifically in selective ciliary cholesterol transport. CONCLUSION: We present the first genetic evidence, underlined by immunohistochemistry and protein binding assays, linking TMEM72 variants to kidney disease and ciliary function. We conclude that TMEM72 is a candidate gene for NPHP. Future work is needed to further characterize TMEM72 variants and unravel its disease mechanism.