The genetic bases of Alzheimer’s disease (AD) and frontotemporal dementia (FTD) have been comprehensively studied, which is not the case for atypical cases not classified into these diagnoses. In the present study, we aim to contribute to the molecular understanding of the development of non-AD and non-FTD dementia due to hyperammonemia caused by mutations in urea cycle genes. The analysis was performed by pooled whole-exome sequencing (WES) of 90 patients and by searching for rare pathogenic variants in autosomal genes for enzymes or transporters of the urea cycle pathway. The survey returned two rare pathogenic coding mutations leading to citrullinemia type I: rs148918985, p.Arg265Cys, C>T; and rs121908641, p.Gly390Arg, G>A in the argininosuccinate synthase 1 (ASS1) gene. The p.Arg265Cys variant leads to enzyme deficiency, whereas p.Gly390Arg renders the enzyme inactive. These variants found in simple or compound heterozygosity can lead to the late-onset form of citrullinemia type I, associated with high ammonia levels, which can lead to cerebral dysfunction and thus to the development of dementia. The presence of urea cycle disorder-causing mutations can be used for the early initiation of antihyperammonemia therapy in order to prevent the neurotoxic effects.

The genetic bases of Alzheimer's disease (AD) and frontotemporal dementia (FTD) have been comprehensively studied, which is not the case for atypical cases not classified into these diagnoses. In the present study, we aim to contribute to the molecular understanding of the development of non-AD and non-FTD dementia by surveying rare pathogenic variants. The analysis was performed on 91 patients, the DNA samples of which were pooled and then subjected to whole exome sequencing. Among detected variants two are rare pathogenic and show significant difference in frequency between the analyzed pool and gnomAD Bulgarian control exomes. The rs373478202, G>T, B9D1 variant, most likely present in heterozygous state; is causative for the autosomal recessive ciliopathy- Joubert syndrome, which involves specific CNS malformation. The rs148918985, C>T, ASS1 variant leads to arginosuccinate enzyme deficiency and in compound heterozygosity with other rare pathogenic variant detected in the same gene causes the late-onset form of citrullinemia type I. This disorder causes high ammonia levels, which can lead to cerebral dysfunction. In conclusion, the detected rare pathogenic variants suggest that, in certain cases, the development of non-AD and non-FTD dementia can be mediated by structural alterations in neuronal cilia or by hyperammoniemia.