Retinal Vasculopathy with Cerebral Leukoencephalopathy and Systemic manifestations (RVCL-S): An update on basic science and clinical perspectives

Genetics

RVCL-S is caused by heterozygous frameshift or nonsense mutations in the C-terminal of the TREX1 gene (3′−5′ repair exonuclease 1) located on chromosome 3 [1,8]. Up to date, there are less than 15 pathogenic variants of TREX1 known to cause RVCL-S. The frameshift mutation V235Gfs*6 is the most frequently diagnosed pathogenic variant.

TREX1 is the most abundant mammalian 3′−5′ exonuclease with a high affinity for single strand DNA. It is responsible for the removal of nucleoside monophosphates from the 3′-ends of DNA [9,10]. TREX1 consists of a single exon and encodes a protein of 314 amino acids. The protein contains three exonucleases sequence motifs, Exo I, Exo II and Exo III and a hydrophobic c-terminal region containing a transmembrane domain. TREX1 is ubiquitously expressed in most tissues and cell types. The exonuclease domain is required for maintaining innate immune tolerance to cytosolic self-DNA by degrading substrates to prevent initiation of autoimmunity. Whereas the C-terminal domain is necessary for the localization to and interaction with the endoplasmic reticulum (ER) [1,9,11,12]. The pathogenic mutations causing RVCL-S lead to a truncated TREX1 protein. The subcellular localization of TREX1 is shifted, altering its interaction with the endoplasmic reticulum, while the exonuclease function of TREX1 remains unaffected.[11,12]

Biological effects of RVCL-S associated mutations

It remains unclear how this truncated TREX1 protein exactly causes disease. The underlying lesion of RVCL-S is a nonatherosclerotic, amyloid-negative angiopathy involving small arteries and capillaries, primarily in the retina and brain, but also in other organs.[1,7,8] Impaired in vivo endothelial function and elevated markers of chronic endothelial activation (e.g. von Willebrand factor and angiopoietin-2) and impaired cerebrovascular reactivity are found in patients as signs of small vessel pathology [13], [14], [15].

Furthermore, in morphologic studies signs of small vessel disease and endothelial dysfunction have been demonstrated. In patients with RVCL-S, thicker multilaminated basement membranes and fibrous thickening of small vessel walls were seen [8]. TREX1 was found to be expressed in a subset of microglia in the normal human brain, often in close proximity to the microvasculature. Interestingly, in ischemic lesions, the amount of TREX1-positive microglia was increased, indicating a possible role for TREX1 and microglia in vessel homeostasis and response to ischemic injury [16,17]. Moreover, TREX1 was found to be expressed in endothelial cells in the brain of a RVCL-S patient indicating a possible direct effect on endothelium function [17]. Additionally, RVCL-S transgenic knock-in mice show increased mortality, increased infarct volume with experimental stroke and evidence of abnormal vascular function [18].

Auto-inflammation also seems to be a possible disease mechanism. Both erythrocyte-sedimentation rate (ESR) and Fibrinogen levels were elevated in RVCL-S patients.[7] Immune activation in RVCL-S could be due to immunogenic free glycans release demonstrated in lymphoblasts of patients with RVCL-S.[11] This increase in free glycan release is most likely duo to altered oligosaccharyltransferase activity [11]. Furthermore, in a conditional knock-in mice model of RVCL-S (V235fs mutation), a distinct autoantibody profile was found compared with wild type mice [19].

Clinical course and progression

RVCL-S is a progressive disorder, but the rate of progression varies greatly between patients. While symptoms may remain stable for long periods of time, progression may also occur precipitously. Visual impairment is frequently the presenting symptom, while cognitive impairment appears to have a later onset [7]. Neurologic symptoms may be mild until the age of 50, after which symptoms may progress rapidly during the last stage of the disease. Life expectancy in RVCL-S is decreased, with one study demonstrating an average age of death 53 years (standard deviation 9.6, range 32 to 72) [8]. Time from onset to death appears to vary greatly. While there are cases in which death occurs within 5 to 10 years after symptom onset, [20] there are also patients who survived for more than 25 years following the first symptoms. Moreover, this is complicated, as in known mutation carriers, signs and symptoms of the disease are more easily recognized creating a bias that gives the impression that there might be a longer time between onset and death for some patients.

Retinopathy

All RVCL-S patients appear to develop a symptomatic retinopathy, often leading to progressive blindness. While typically presenting around the age of 40, early signs of asymptomatic vascular retinopathy have been demonstrated in patients in their early twenties [3,8]. Therefore, ophthalmologists play an important role in recognizing this underdiagnosed condition.

In the early stages, the vascular retinopathy in RVCL-S is characterized by telangiectasia, micro-aneurysms and cotton wool spots. Later, perifoveal capillary obliterations and neovascularizations may occur, predominantly affecting the posterior pole (Fig. 1) [8]. Fluorescein angiography often shows focal and extended areas of ischemia, capillary obliteration and microaneurysms, telangiectasias, neovascularization and fluorescein leaks. Complications of the retinopathy may occur as the disease progresses, such as secondary glaucoma and macula edema.

Neurologic manifestations and neuroimaging

Systemic features

A wide range of systemic features are part of the clinical spectrum of RVCL-S (Table 2, Fig. 3). Typically, the disease starts from the age of 20 with Raynaud's phenomenon and (pre-symptomatic) vascular retinopathy. From age of 35–40 onwards, patients may develop multiple internal organ disease, justifying regular screening [7].

Microvascular renal disease occurs in approximately 30 to 50% of the patients [7,8]. Commonly, the renal involvement has a mild phenotype, however, it may progress to severe kidney disease, and occasionally fatal stage 4 kidney disease occurs.[8,12,20]

Evidence of liver dysfunction was found in 78% of RVCL-S patients [8]. In the majority of the cases, there were (mildly) elevated levels of alkaline phosphatase and gamma-glutamyltransferase.

Additionally, RVCL-S patients appear to suffer frequently from hypertension [7,8]. Raynaud's phenomenon is present in approximately 40% and is usually mild without causing ischemic injury [7,8]. Subclinical hypothyroidism was present in 37% of RVCL-S patients over age 40 [7]. Mild to moderate anemia occurs frequently, this may be due to microscopic gastrointestinal bleeding that occurs in RVCL-S [7,8]. There are some reports of avascular necrosis of the femoral head, cardiomyopathy and skin lesions also being part of the clinical spectrum [12,20,28].

Ophthalmological markers

While retinal imaging techniques could potentially serve as biomarkers for the retinopathy in RVCL-S, their use is most likely not limited to this. Since the retina is an extension of the brain and shares a common embryological origin with the central nervous system, there is an intense interest in using retinal imaging technology to diagnose and monitor neurological diseases. In vivo optical coherence tomography (OCT) imaging of the retina provides near histologic level resolution of the different retinal layers, including neurosensory tissue.

Recently, the potential of using OCT findings as early biomarker for RVCL-S was shown [30]. A decreased peripapillary retinal nerve fiber layer (pRNFL) thickness and total macular volume (TMV) was demonstrated in TREX1 mutation carriers compared with controls. The pRNFL contains unmyelinated axons of ganglion cells found in the ganglion cell layer, thus showing loss of intraocular axonal integrity. Additionally, thinning of the inner plexiform layer was found, where dendrites from inner nuclear layer neurons are located. Indicating an effect of RVCL-S on retinal neurons [30]. Notably, this decreased pRNFL and TMV were already present in 9 out of 12 eyes with normal funduscopic examination.[30] As OCT is a noninvasive, safe and precise diagnostic tool, OCT measurements may be useful as a (pre)clinical biomarker in RVCL-S.

Neuroradiological markers

In RVCL-S, white matter volumes declined linearly and declined faster in RVCL-S patients compared to controls [21]. Its role as a biomarker for disease progression deserves further investigating.

Cerebrovascular reactivity (CVR) is known as the cerebrovascular reserve capacity and is a marker for cerebrovascular health. It can be measured as change in cerebral blood flow (CBF) to a hypercapnic stimulus. CVR was decreased in both gray and white matter of RVCL-S patients compared with controls [15]. Interestingly, CVR in white matter was already lower in RVCL-S patients aged < 40 years [15].

Endothelial markers

Von Willebrand factor (VWF) is considered a reliable circulating marker of endothelial dysfunction, and its level is increased after endothelial damage and during acute phase responses. Another marker associated with endothelial damage or activation is angiopoietin-2. Elevated levels of VWF and angiopoietin-2 have been found in RVCL-S patients compared with healthy controls. Levels of VWF and angiopoietin-2 were also elevated in mutation carriers aged <40 years, indicating that they may serve as early biomarkers of disease activity [14]. Moreover, a correlation with endothelial markers and markers of kidney and liver disease and inflammation (i.e., systemic symptoms of RVCL-S) was demonstrated [14].

Diagnosis

The diagnosis of RVCL-S should be suspected when middle-aged adults present with vascular retinopathy with focal or global neurologic deficits and/or signs of internal organ disease (Table 1). Especially if there is a positive family history of retinopathy or neurological disease. Still, a negative family history does not exclude RVCL-S because de novo TREX1 mutations have also been described [12]. Furthermore, family members with RVCL-S may be misdiagnosed with other conditions. Demonstration of a frameshift or nonsense pathogenic variant in the C-terminus of TREX1 confirms the diagnosis.

Treatment

As of now, there is no specific disease modifying treatment for RVCL-S and given the rarity of the disease, there are limited data available on the disease management. Many disease modifying treatments have been tried so far, among which are immunosuppressors such as corticosteroids, cyclophosphamide, methotrexate, chloroquine and azathioprine [2,6,12,20,26,31,32]. Moreover, intravenous immunoglobins and plasmapheresis have also been tried [22]. Unfortunately, there have been no clear benefits to starting such treatments as of today. Research is hampered by the rareness of the disorder and our limited understanding of its pathophysiology. One clinical trial studied aclarubicin as it may correct glycan and immune defects caused by dysregulation of oligosaccharyltransferase (For further information about the rational see Section 2.2 Biological effects of RVCL-S associated mutations). This trial was discontinued after only four participants were included (NCT02723448). There is an ongoing trial studying the effectiveness of crizanlizumab (NCT04611880) for treating RVCL-S of which there are no results yet. Further research is needed to shed light on the pathophysiology of RVCL-S so that more targeted treatment can be evaluated. Another promising avenue of research includes gene therapy. In the future, physicians might employ gene editing technologies that will fully correct disease-causing mutations in the TREX1 gene.

We advise to monitor mutation carriers yearly to detect treatable complications and symptoms. Recommended tests include ophthalmological evaluation, brain MRI with gadolinium, neurological examination, assessment of blood pressure and assessment of renal function, liver enzymes, thyroid function and complete blood count and hemoglobin levels (Table 3). More frequent monitoring may be necessary, depending on the extent of the symptoms.

Most symptoms of RVCL-S can be treated with standard care (Table 3). Retinopathy and macular edema should be treated with retinal laser photocoagulation and intravitreal anti-VEGF therapy. Treatment of secondary glaucoma may also be necessary. Control of hypertension is important, especially in the case of renal disease. Anemia can be treated with iron supplementation and in severe cases blood transfusions may be required. We recommend treating symptomatic tumefactive white matter lesions (pseudotumors) with surrounding vasogenic cerebral edema with a course of intravenous methylprednisolone followed by oral glucocorticoids. This seems a reasonable intervention, however, evidence of efficacy is lacking.

Of note, we recommend to not treat RVCL-S patients with acute ischemic stroke with intravenous thrombolytic (IVT) therapy. There is no proof that IVT is effective in RVCL-S and the bleeding risk may be increased, as RVCL-S is an angiopathy. Otherwise we advise to manage acute ischemic events in RVCL-S according to the general principles of stroke medicine. For secondary stroke prevention in patients with RVCL-S who have a MRI confirmed symptomatic ischemic event, available risk reduction strategies should be considered, including antiplatelet therapy, statin treatment, lifestyle modifications, and if present treatment for hypertension and diabetes mellitus. However, there is no specific evidence that these measures are effective in reducing stroke risk in RVCL-S.