Biallelic truncating variants in MAPKAPK5 cause a new developmental disorder involving neurological, cardiac, and facial anomalies combined with synpolydactyly

[Purpose]: This study aimed to identify the genetic cause of a new multiple congenital anomalies syndrome observed in three individuals from two unrelated families.[Methods]: Clinical assessment was conducted prenatally and at different postnatal stages. Genetic studies included exome sequencing (ES) combined with single-nucleotide polymorphism (SNP) array based homozygosity mapping and trio ES. Dermal fibroblasts were used for functional assays.[Results]: A clinically recognizable syndrome characterized by severe developmental delay, variable brain anomalies, congenital heart defects, dysmorphic facial features, and a distinctive type of synpolydactyly with an additional hypoplastic digit between the fourth and fifth digits of hands and/or feet was identified. Additional features included eye abnormalities, hearing impairment, and electroencephalogram anomalies. ES detected different homozygous truncating variants in MAPKAPK5 in both families. Patient-derived cells showed no expression of MAPKAPK5 protein isoforms and reduced levels of the MAPKAPK5-interacting protein ERK3. F-actin recovery after latrunculin B treatment was found to be less efficient in patient-derived fibroblasts than in control cells, supporting a role of MAPKAPK5 in F-actin polymerization.[Conclusion]: Our data indicate that loss-of-function variants in MAPKAPK5 result in a severe developmental disorder and reveal a major role of this gene in human brain, heart, and limb development.We are grateful to patients and their parents for their participation in this study. The work at IIB was financially supported by the Spanish Ministry of Science, Innovation and Universities (PID2019-105620RB-I00/AEI/10.13039/501100011033 and SAF2016‐75434‐R (AEI/FEDER, UE)