Two Unique Mutations in HTRA1-Related Cerebral Small Vessel Disease in North America and Africa and Literature Review

CADASIL: A NOTCH3-associated cerebral small vessel disease

BACKGROUND

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is the most common hereditary cerebral small vessel disease (CSVD), pathologically characterized by a non-atherosclerotic and non-amyloid diffuse angiopathy primarily involving small to medium-sized penetrating arteries and leptomeningeal arteries. In 1996, mutation in the notch receptor 3 gene (NOTCH3) was identified as the cause of CADASIL. However, since that time other genetic CSVDs have been described, including the HtrA serine peptidase 1 gene-associated CSVD and the cathepsin A gene-associated CSVD, that clinically mimic the original phenotype. Though NOTCH3-associated CSVD is now a well-recognized hereditary disorder and the number of studies investigating this disease is increasing, the role of NOTCH3 in the pathogenesis of CADASIL remains elusive.

AIM OF REVIEW

This review aims to provide insights into the pathogenesis and the diagnosis of hereditary CSVDs, as well as personalized therapy, predictive approach, and targeted prevention. In this review, we summarize the current progress in CADASIL, including the clinical, neuroimaging, pathological, genetic, diagnostic, and therapeutic aspects, as well as differential diagnosis, in which the role of NOTCH3 mutations is highlighted.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In this review, CADASIL is revisited as a NOTCH3-associated CSVD along with other hereditary CSVDs.

Heterozygous Pathogenic and Likely Pathogenic Symptomatic HTRA1 Variant Carriers in Cerebral Small Vessel Disease

High temperature requirement serine peptidase A1 (HTRA1) related cerebral small vessel disease (CSVD) includes both symptomatic heterozygous HTRA1 variant carrier and cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) patients. Presently, most reported symptomatic heterozygous HTRA1 variant carrier cases are sporadic family reports with a lack of specific characteristics. Additionally, the molecular mechanism of heterozygous HTRA1 gene variants is unclear. We conducted this review to collect symptomatic carriers of heterozygous HTRA1 gene variants reported as of 2022, analyzed all pathogenicity according to American College of Medical Genetics and Genomics (ACMG) variant classification, and summarized the cases with pathogenic and likely pathogenic HTRA1 variants gender characteristics, age of onset, geographical distribution, initial symptoms, clinical manifestations, imaging signs, HTRA1 gene variant information and to speculate its underlying pathogenic mechanisms. In this review, we summarized the following characteristics of pathogenic and likely pathogenic symptomatic HTRA1 variant carriers: to date, the majority of reported symptomatic HTRA1 carriers are in European and Asian countries, particularly in China which was found to have the highest number of reported cases. The age of first onset is mostly concentrated in the fourth and fifth decades. The heterozygous HTRA1 gene variants were mostly missense variants. The two variant sites, 166-182 aa and 274-302 aa, were the most concentrated. Clinicians need to pay attention to de novo data and functional data, which may affect the pathogenicity analysis. The decrease in HtrA1 protease activity is currently the most important explanation for the genetic pathogenesis.

HeterozygousHTRA1Mutations Cause Cerebral Small Vessel Diseases

Background and Objectives

Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) is a rare hereditary cerebrovascular disease caused by homozygous or compound heterozygous variations in the high-temperature requirement A serine peptidase 1 (HTRA1) gene. However, several studies in recent years have found that some heterozygousHTRA1mutations also cause cerebral small vessel disease (CSVD). The current study aims to report the novel genotypes, phenotypes, and histopathologic results of 3 pedigrees of CSVD with heterozygousHTRA1mutation.

Methods

Three pedigrees of familiar CSVD, including 11 symptomatic patients and 3 asymptomatic carriers, were enrolled. Whole-exome sequencing was conducted in the probands for identifying rare variants, which were then evaluated for pathogenicity according to the American College of Medical Genetics and Genomics guidelines. Sanger sequencing was performed for validation of mutations in the probands and other family members. The protease activity was assayed for the novel mutations. All the participants received detailed clinical and imaging examinations and the corresponding results were concluded. Hematoma evacuation was performed for an intracerebral hemorrhage patient with the p.Q318H mutation, and the postoperative pathology including hematoma and cerebral small vessels were examined.

Results

Three novel heterozygousHTRA1mutations (p.Q318H, p.V279M, and p.R274W) were detected in the 3 pedigrees. The protease activity was largely lost for all the mutations, confirming that they were loss-of-function mutations. The patients in each pedigree presented with typical clinical and imaging features of CVSD, and some of them displayed several new phenotypes including color blindness, hydrocephalus, and multiple arachnoid cysts. In addition, family 1 is the largest pedigree with heterozygousHTRA1mutation so far and includes homozygous twins, displaying some variation in clinical phenotypes. More importantly, pathologic study of a patient with p.Q318H mutation showed hyalinization, luminal stenosis, loss of smooth muscle cells, splitting of the internal elastic lamina, and intramural hemorrhage/dissection-like structures.

Discussion

These findings broaden the mutational and clinical spectrum of heterozygousHTRA1-related CSVD. Pathologic features were similar with the previous heterozygous and homozygous cases. Moreover, clinical heterogeneity was revealed within the largest single family, and the mechanisms of the phenotypic heterogenetic remain unclear. Overall, heterozygous HTRA1-related CSVD should not be simply taken as a mild type of CARASIL as previously considered.

Identified novel heterozygous HTRA1 pathogenic variants in Chinese patients with HTRA1-associated dominant cerebral small vessel disease

Background: Homozygous and compound heterozygous mutations in HTRA1 cause cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL). Recently, heterozygous pathogenic variants in HTRA1 were described in patients with autosomal dominant cerebral small vessel disease (CSVD). Here, we investigated the genetic variants in a cohort of Chinese patients with CSVD.

Methods: A total of 95 Chinese index patients with typical characteristics of CSVD were collected. Whole exome sequencing was performed in the probands, followed by Sanger sequencing. Pathogenicity prediction software was applied to evaluate the pathogenicity of the identified variants.

Results: We detected five heterozygous HTRA1 pathogenic variants in five index patients. These pathogenic variants included four known variants (c.543delT, c.854C>T, c.889G>A, and c.824C>T) and one novel variant (c.472 + 1G>A). Among them, c.854C>T, c.824C>T, and c.472 + 1G>A have never been reported in China and c.889G>A was once reported in homozygous but never in heterozygous. Three of them were distributed in exon 4, one in exon 2, and another splicing variant in intron 1. Four out of five probands presented typical features of CARASIL but less severe. The common clinical features included lacunar infarction, cognitive decline, alopecia, and spondylosis. All of them showed leukoencephalopathy, and the main involved cerebral area include periventricular and frontal area, centrum semiovale, thalamus, and corpus callosum. Anterior temporal lobes and external capsule involvement were also observed. Three probands had intracranial microbleeds.

Conclusion: Our study expanded the mutation spectrum of HTRA1, especially in Chinese populations, and provided further evidence for “hot regions” in exon 1–4, especially in exon 4, in heterozygous HTRA1 pathogenic variants. Our work further supported that patients with heterozygous HTRA1 pathogenic variants presented with similar but less-severe features than CARASIL but in an autosomal dominantly inherited pattern.