The mutant HTRA1s observed in manifesting heterozygotes might result in an impaired HTRA1 activation cascade of HTRA1 or be unable to form stable trimers.

Cerebral small-vessel disease is a common disorder in elderly populations; however, its molecular basis is not well understood. We recently demonstrated that mutations in the high-temperature requirement A (HTRA) serine peptidase 1 (HTRA1) gene cause a hereditary cerebral small-vessel disease, cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL). HTRA1 belongs to the HTRA protein family, whose members have dual activities as chaperones and serine proteases and also repress transforming growth factor-β (TGF-β) family signaling. We demonstrated that CARASIL-associated mutant HTRA1s decrease protease activity and fail to decrease TGF-β family signaling. However, the precise molecular mechanism for decreasing the signaling remains unknown. Here we show that increased expression of ED-A fibronectin is limited to cerebral small arteries and is not observed in coronary, renal arterial or aortic walls in patients with CARASIL. Using a cell-mixing assay, we found that HTRA1 decreases TGF-β1 signaling triggered by proTGF-β1 in the intracellular space. HTRA1 binds and cleaves the pro-domain of proTGF-β1 in the endoplasmic reticulum (ER), and cleaved proTGF-β1 is degraded by ER-associated degradation. Consequently, the amount of mature TGF-β1 is reduced. These results establish a novel mechanism for regulating the amount of TGF-β1, specifically, the intracellular cleavage of proTGF-β1 in the ER.

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Introduction: Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL), an autosomal recessive inherited cerebral small vessel disease (CSVD), involves severe leukoaraiosis, multiple lacunar infarcts, early-onset alopecia, and spondylosis deformans. High-temperature requirement serine peptidase A1 (HTRA1) gene mutations cause CARASIL by decreasing HTRA1 protease activity. Although CARASIL is a recessive inherited disease, heterozygous mutations in the HTRA1 gene were recently identified in 11 families with CSVD. Because CSVD is frequently observed in elderly individuals, it is unclear which mutants truly contribute to CSVD pathogenesis. Here, we found heterozygous mutations in the HTRA1 gene in individuals with CSVD and investigated the differences in biochemical characteristics between these mutant HTRA1s and mutant HTRA1s observed in homozygotes. Methods: We recruited 113 unrelated index patients with clinically diagnosed CSVD. The coding sequences of the HTRA1 gene were analyzed. We evaluated HTRA1 protease activities using casein assays and oligomeric HTRA1 formation using gel filtration chromatography. Results: We found 4 heterozygous missense mutations in the HTRA1 gene (p.G283E, p.P285L, p.R302Q, and p.T319I) in 6 patients from 113 unrelated index patients and in 2 siblings in 2 unrelated families with p.R302Q. These mutant HTRA1s showed markedly decreased protease activities and inhibited wild-type HTRA1 activity, whereas 2 of 3 mutant HTRA1s reported in CARASIL (A252T and V297M) did not inhibit wild- type HTRA1 activity. Wild-type HTRA1 forms trimers; however, G283E and T319I HTRA1, observed in manifesting heterozygotes, did not form trimers. P285L and R302Q HTRA1s formed trimers, but their mutations were located in domains that are important for trimer-associated HTRA1 activation; in contrast, A252T and V297M HTRA1s, which have been observed in CARASIL, also formed trimers but had mutations outside the domains important for trimer- associated HTRA1 activation. Conclusions: The mutant HTRA1s observed in manifesting heterozygotes might result in an impaired HTRA1 activation cascade of HTRA1 or be unable to form stable trimers.