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Introduction: Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) is a cerebral small-vessel disease (CSVD). Mutations in the high-temperature requirement serine peptidase A1 gene (HTRA1) cause CARASIL via a decrease in protease activity of HTRA1. Although most of the heterozygotes with the HTRA1 mutation are healthy, manifesting heterozygotes have been reported. We have elucidated that the mutant HTRA1s that develops CSVD in a heterozygote state have a distinct molecular mechanism, resulting in the dominant negative effect. These individuals showed mild phenocopy of CARASIL. However, it is not clear whether brain MRI findings in manifesting heterozygotes are different from those of CARASIL. In this study, we aimed to clarify the characteristic brain MRI features in manifesting heterozygotes by comparing them to those in CARASIL.Methods: We have evaluated 19 MRIs in eight manifesting heterozygotes and 21 MRIs in seven CARASIL patients and scored the MRIs by using a semi-quantitative scale for CARASIL, which scored white matter lesions (WMLs) (signal score) and atrophy (atrophy score) (Nozaki et al. Neurology 2015). Statistical analysis was conducted using software R 3.2.2. We obtained written informed consent from all individuals.Results: Signal score in manifesting heterozygotes was significantly lower than that in CARASIL (Mean ± SD; 14.6 ± 1.9 vs. 23.1 ± 5.0, p < 0.0001), however, there was no difference in atrophy score between the two groups (Mean ± SD; 5.5 ± 2.2 vs. 7.5 ± 5.5, p = 0.20). Atrophy score showed positive correlation with the disease duration in both groups (r2 = 0.48, p = 0.0014 vs r2 = 0.41, p = 0.0041), however signal score showed no correlation with the disease duration.Conclusion: WMLs is milder in manifesting heterozygote as compared with CARASIL. In contrast, the brain atrophy is not influenced by the HTRA1 mutation status but positively correlated with the disease duration. The rate of carriers for pathogenic HTRA1 mutations are higher than expected. These characteristic findings of brain MRIs might be useful to pick up the candidate for the genetic screening for HTRA1.