Research

Overview

Approximately 46 unrelated families worldwide have been identified with RVCL-S. Our families are located not only  in the USA but also in Australia, Germany, France, the Netherlands, Spain, Mexico, Japan, China, Turkey, Switzerland and Italy.

This autosomal dominant disorder features three types of mutations all of which are located at or beyond valine-235 of the 314 amino acid protein. The most dominant mutations are frame-shift mutations of TREX1 that produce a truncated protein. A second type of mutation is a Stop mutation in which the translation of TREX1 abruptly ends. Another type of mutation simply deletes amino acids in this region but it retains the amino acids after the deletion. While all of these mutation have not yet been modeled, we believe the common effect is the production of a protein that is mislocalized throughout cells instead of being anchored on the endoplasmic reticulum. We have called this aberrant localization process “TREX on the loose.”

Structure

The wild-type protein translated from the TREX1 gene consists of 314 amino acids (~ 32 kDa) and is encoded by a single exon on chromosome 3p21. TREX1 consists of three domains important for its role as a DNA repair enzyme (i.e., exonuclease domains I, II, and III). TREX1 also has an extended C-terminal “tail” domain of ~ 80 amino acids containing a leucine-rich sequence required for its endoplasmic reticulum (ER) localization.

As described above, RVCL-S results from a mutation in the tail domain that changes the protein reading frame, introduces a stop codon, and thus causes a premature termination leading to partial loss of the tail. As a result, TREX1 loses it ability to remain in the ER and becomes mislocalized throughout the cell. At present, 23 different mutations in TREX1 have been identified (Liszewski, K. & Atkinson, J.P summary as of 11/15/2022). However, of the 46 unrelated families worldwide with RVCL-S, 18 unrelated kindreds have a particular mutation identified on the protein level described as V235G fs*6. This means that in TREX1, the amino acid valine changes to a glycine at amino acid #235. Although normal TREX1 protein is 314 amino acids long, this mutation causes the protein to interrupt its normal sequence, add 6 ‘foreign’ amino acids and then terminate abruptly knocking out the normal tail sequence.

For reasons that are as yet unclear, most people who carry these heterozygous TREX1 tail mutations live a fairly normal life until middle age, when vision and brain damage begin. Progressive deterioration proceeds over a 5 to 15 year period. New studies conducted by our laboratory have identified that TREX1 is expressed by a subset of human brain microglia cells that also are often close to blood vessels. However, TREX1 clusters densely in microglia cells in proximity to brain lesions of RVCL patients. Further studies are needed to determine how clustered TREX1 may impact the symptoms of RVCL.

In addition to RVCL-S, mutations in other areas of TREX1 play major roles in a variety of neurovascular and autoimmune-related disorders such as Aicardi-Goutieres syndrome, chilblain lupus, and Cree encephalitis. TREX1 mutations also have been linked to systemic lupus erythematosus (SLE) and Sjögren’s syndrome.

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Function

There are two known functions for TREX1:

DNA sensor

TREX1 was originally discovered and identified as the “3 prime repair exonuclease 1” that cleans up and digests DNA debris that is generated when tissue is damaged or cells die. It degrades single-stranded DNA ~4-fold more efficiently than double-stranded DNA. TREX1 is also important for handling DNA from viruses that invade and infect cells.

Sugar sensor

A newly discovered function of TREX1 is in overseeing the ‘sugar polishing’ step (i.e., glycosylation) for newly made proteins. Thus, TREX1 interacts with cellular machinery (the oligosaccharyltransferase or “OST” complex) to add N-glycans to proteins as they are produced. These glycans are important for modulating the function and stability of proteins.

When the sugar polishing function is disrupted, TREX1 no longer can bind to its partner proteins in the OST complex. This leads to the buildup of glycans. The disruption of this important interaction also may diminish blood vessel life-span and integrity and lead to disturbances in the immune system. These studies also identified an anthracycline antibiotic (Aclarubicin) that corrected the defect in both mouse disease models and in human patient cells. Studies are underway to better understand and expand on these exciting discoveries. Additionally, human clinical trials are in development.

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Disease models

Our research utilizes two types of models: human lymphoblast cell lines and mouse disease models. For the former, we prepare immortalized EBV-transformed patient B cells. Secondly, we and others have developed TREX1 mouse models that knock out of mouse TREX1 (TREX1-/-) or knock in human RVCL-S TREX1 mutation (V235fs-KI).

Parul Kothari, MD, PhD, who studied this disease at Washington University, wrote her doctoral dissertation on these studies.

TREX1 reagents

Our teams have produced recombinant human TREX1 and recombinant RVCL-S TREX1 as well as antibodies to normal TREX1. These reagents are valuable tools to study the disease in vitro (in the test tube) as well as in vivo (in living systems).

Researchers at the RVCL-S Research Center continue to seek a better understanding of the disease process and to find an effective treatment to replace, bypass, correct or negate the effects of defective TREX1 protein.

For additional information on RVCL-S, see our review for the National Organization for Rare Disorders (NORD).

RVCL review on NORD website

Clinical studies and RVCL-S as a model of cerebral small vessel disease

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