Gudrun Schottmann ¹ Carmen Martínez Almudéver ² Julia C. Knop ² Eun Kyung Suk ³ Zianka Meyer ⁴ Jürgen Kohlhase ⁵ Nastassja Himmelreich ⁶ Jirko Kühnisch ⁷ Claus-Eric Ott ⁸ Wenke Seifert ^2*

• ¹ Zentrum für Sozial- und Neuropädiatrie (DBZ), Vivantes Klinikum Neukölln, Berlin, Berlin, Germany
• ² Institute of Cell Biology and Neurobiology, Charité University Medicine Berlin, Berlin, Germany
• ³ Praxis für Humangenetik, Berlin, Germany
• ⁴ Diagenom GmbH, Rostock, Germany
• ⁵ SYNLAB MVZ Humangenetik Freiburg GmbH, Center for Human Genetics Freiburg, Freiburg, Germany
• ⁶ Zentrum für Humangenetik, Tübingen, Germany
• ⁷ Experimental and Clinical Research Center, a cooperation between the Max Delbrück Center for Molecular Medicine in the Helmholtz Association and Charité - Universitätsmedizin Berlin, Berlin, Germany
• ⁸ Institute for Medical Genetics and Human Genetics, Charité University Medicine Berlin, Berlin, Germany

Cohen syndrome (CS) is an early-onset pediatric neurodevelopmental disorder characterized by postnatal microcephaly and intellectual disability. An accurate diagnosis for individuals with CS is crucial, especially for their caretakers and future prospects. CS is predominantly caused by rare homozygous or compound heterozygous pathogenic variants in the VPS13B gene that disrupt protein translation and lead to loss of function of the encoded VPS13B protein. The widespread incorporation of next-generation sequencing approaches in genetic diagnostics increases the number of individuals carrying VPS13B mutant alleles. In parallel, it raises the detection of variants of unknown clinical significance, necessitating further functional pathogenicity validation. Here, we report a family with two CS patients. Within this family four rare VPS13B variants have been detected: c.710G>C, p.Arg237Pro; c.6804delT, p.Phe2268Leufs*24; c.7304C>T, p.Ala2435Val; and c.10302T>A, p.Tyr3434* challenging the interpretation of their disease causing character. The variants c.6804delT, p.Phe2268Leufs*24 as well as c.710G>C, p.Arg237Pro were detected in trans and are genetically considered as CS disease causing. Functional characterization of the missense variant c.710G>C, p.Arg237Pro shows diminished localization at the Golgi complex, underlining its clinical relevance and ACMG classification as likely pathogenic, class 4. Taken together, we emphasize combined genetic and functional testing of VPS13B missense variants to guarantee precise molecular diagnosis and personalized medical care for CS patients.