Bi-allelic JAM2 Variants Lead to Early-Onset Recessive Primary Familial Brain Calcification
Lucía Schottlaender, Rosella Abeti, Zane Jaunmuktane, Carol Macmillan, Viorica Chelban, Benjamin O’Callaghan, John McKinley, Reza Maroofian, Stéphanie Efthymiou, Alkyoni Athanasiou‐Fragkouli, Raeburn Forbes, Marc P. M. Soutar, John H. Livingston, Bernardett Kalmar, Orlando Swayne, Gary Hotton, Stanislav Groppa, Blagovesta Marinova Karashova, Wolfgang Nachbauer, Sylvia Boesch, Larissa Arning, Dagmar Timmann, Bru Cormand, Belén Pérez‐Dueñas, Gabriella Di Rosa, Jatinder S. Goraya, Tipu Sultan, Jun Mine, Daniela Avdjieva, Hadil Kathom, Radka Tincheva, Selina Banu, Mercedes Pineda-Marfa, Pierangelo Veggiotti, Michel D. Ferrari, Alberto Verrottı, Gian Luigi Marseglia, Salvatore Savasta, Mayte García-Silva, Alfons Macaya Ruiz, Barbara Garavaglia, Eugenia Borgione, Simona Portaro, Benigno Monteagudo Sanchez, Richard G. Boles, Savvas Papacostas, Michail Vikelis, Eleni Zamba Papanicolaou, Efthymios Dardiotis, Shazia Maqbool, Shahnaz Ibrahim, Salman Kirmani, Nuzhat Rana, Osama Atawneh, George Koutsis, Marianthi Breza, Salvatore Mangano, Carmela Scuderi, Eugenia Borgione, Giovanna Morello, Tanya Stojkovic, Massimi Zollo, Gali Heimer, Yves Dauvilliers, Pasquale Striano, Issam Al-Khawaja, Fuad Al-Mutairi, Sherifa A. Hamed, Alan Pittman, João Ricardo Mendes de Oliveira, Maria De Grandis, Angela Richard-Loendt, Francesca Launchbury, Juri Althonayan, Gavin McDonnell, Aisling Carr, Suliman Khan, Christian Beetz, Atıl Bişgin, Sevcan Tuğ Bozdoğan, Amber Begtrup, Erin Torti, Linda Greensmith, Paola Giunti, Patrick J. Morrison, Sebastian Brandner, Michel Aurrand‐Lions, Henry Houlden
Abstract
Primary familial brain calcification (PFBC) is a rare neurodegenerative disorder characterized by a combination of neurological, psychiatric, and cognitive decline associated with calcium deposition on brain imaging. To date, mutations in five genes have been linked to PFBC. However, more than 50% of individuals affected by PFBC have no molecular diagnosis. We report four unrelated families presenting with initial learning difficulties and seizures and later psychiatric symptoms, cerebellar ataxia, extrapyramidal signs, and extensive calcifications on brain imaging. Through a combination of homozygosity mapping and exome sequencing, we mapped this phenotype to chromosome 21q21.3 and identified bi-allelic variants in JAM2. JAM2 encodes for the junctional-adhesion-molecule-2, a key tight-junction protein in blood-brain-barrier permeability. We show that JAM2 variants lead to reduction of JAM2 mRNA expression and absence of JAM2 protein in patient's fibroblasts, consistent with a loss-of-function mechanism. We show that the human phenotype is replicated in the jam2 complete knockout mouse (jam2 KO). Furthermore, neuropathology of jam2 KO mouse showed prominent vacuolation in the cerebral cortex, thalamus, and cerebellum and particularly widespread vacuolation in the midbrain with reactive astrogliosis and neuronal density reduction. The regions of the human brain affected on neuroimaging are similar to the affected brain areas in the myorg PFBC null mouse. Along with JAM3 and OCLN, JAM2 is the third tight-junction gene in which bi-allelic variants are associated with brain calcification, suggesting that defective cell-to-cell adhesion and dysfunction of the movement of solutes through the paracellular spaces in the neurovascular unit is a key mechanism in CNS calcification.