Telomere Therapy for Chronic Kidney Disease

Abstract

Chronic kidney disease (CKD) is estimated to affect almost 10% of individuals worldwide and is one of the leading causes of morbidity and mortality. Renal fibrosis, a central pathway in CKD progression (irrespective of etiology), is associated with shortened or dysfunctional telomeres in animal studies. Telomeres are specialized nucleoprotein structures located at the chromosome end that maintain genomic integrity. The mechanisms of associations between telomere length and CKD have not yet been fully elucidated, however, CKD patients with shorter telomere length may have decreased renal function and a higher mortality rate. A plethora of ongoing research has focused on possible therapeutic applications of telomeres with the overall goal to preserve telomere length as a therapy to treat CKD.

Plain language summary

Chronic kidney disease or CKD is one of the leading causes of illness and death worldwide. Scarring of the kidney tissue that occurs in CKD has been associated with shorter telomeres in studies using rats. Telomeres, said to act as the cellular ‘shoelace caps’, maintain the structure of chromosomes, allowing for genetic material inside cells to divide correctly. The length of telomeres (TL) is influenced by diverse factors such as genetics and lifestyle. The underlying processes for the associations between TL and CKD are still not understood, however, patients with CKD and shorter TL have reduced kidney function and an increased death rate. Therefore, research is focused on possible ways to preserve TL and treat CKD.

Tweetable abstract

Preserving telomere length may reduce rates of chronic kidney disease, thereby decreasing morbidity & mortality worldwide.

Keywords: :

• chronic kidney disease
• renal fibrosis
• telomere length
• telomere therapy

The kidneys maintain body homeostasis by regulating fluid, electrolyte and acid-base balance. They are also involved in blood pressure regulation, endocrine functions, blood filtration, metabolism and waste removal [1]. In a healthy adult, about 100–110 ml of glomerular filtrate flows through the kidneys per min, which can decline to <15 ml/min during kidney disease [2]. Chronic kidney disease (CKD) is defined as a glomerular filtration rate (GFR) <60 ml/min per 1.73 m² decrease in GFR by ≥35%, increase in serum creatinine by >50% or markers of kidney damage lasting >3 months [3]. It is a progressive disease that has become a global public health issue due to its increasingly high prevalence [4–6]. A total of 9.1% of the world’s population is estimated to have CKD [7]. CKD is known to cluster in families [8] and eGFR shows a strong heritable component [9].

Despite the associated morbidity and mortality, the current management strategy for CKD is essentially based on the management of existing comorbidities, such as diabetes, hypertension and obesity, to delay its progression [10,11]. The heterogeneous pathogenesis of CKD poses additional challenges for therapeutic interventions [4,12]. The most prevalent risk factors for CKD are family history, diabetes mellitus (DM), hypertension and obesity (Figure 1), for a review, see Kazancioğlu [13]. CKD patients may be asymptomatic in the early stages.

Figure 1. Telomeres, chronic kidney disease and pathophysiology overview.

As renal function decreases, there is an accumulation of urea, toxins and other waste products resulting in uremia. Uremia induces immune deficiency, oxidative stress and inflammation. Oxidative stress, inflammation and immune deficiency are associated with increased telomere attrition. Shorter telomeres predispose to kidney fibrosis and decrease the tissue repair capacity of the kidney in response to injury. This increases susceptibility to onset of CKD and exacerbates existing CKD.

CKD: Chronic kidney disease.

Image created with BioRender.com.

Regardless of the origin, there is a progressive loss of nephrons at the onset of CKD. As nephrons become irreversibly lost, there are fewer functional nephrons, leading to compensatory hyperfiltration and increased glomerular filtration pressure. The ensuing proteinuria and renin–angiotensin–aldosterone system (RAAS) activation results in interstitial inflammation, tubular fibrosis and glomerular sclerosis. Together these further accelerate the rate of nephron loss. This declining renal function leads to accumulation of urea and toxins and elevation of blood urea nitrogen and serum creatinine, hallmarks of uremia. Decreased renal clearance also alters general homeostasis, impacting other organs in the body. A causal player in the progression of CKD may be inherent in the loss of integrity of renal cells leading to tubulointerstitial fibrosis. In that regard, telomeres that protect from DNA damage, cell senescence and/or apoptosis play an important role (Figure 1). This review focuses on the role of telomeres in the pathophysiology of CKD and the prospects of telomere therapy in CKD management.

Telomeres

Telomeres are specialized nucleoprotein structures located at the chromosome end. They maintain genomic integrity and are essential in the replication and proliferation of cells. Telomeric DNA contains a double-stranded region and terminates with a single-stranded region that consists of hundreds of G-rich base-pair strands (Figure 2) [14,15]. The telomere cap is a highly conserved six nucleotide repeat of 5′-TTAGGG-3′ at the end of the chromosome that binds to domains of the shelterin complex to form a protective telomeric loop that maintains telomere integrity and stability [16]. The binding of the six-member shelterin protein complex to telomeric DNA protects the chromosomes from degradation and fusion with one another. It also prevents the stimulation of the DNA double-strand-break repair machinery in telomeres [16,17].

Figure 2. Telomere shortening.

Due to the end replication problem, telomeres progressively shorten with each cell division.

Image created with BioRender.com.

In linear chromosomes, DNA polymerase cannot completely replicate the DNA end and telomeres serve as buffers against the end replication problem [18]. Therefore, telomere attrition occurs with each DNA replication because the end of the lagging strand of the telomeric DNA is not replicated (Figure 2) [18,19]. Telomeres progressively shorten with each cell division until they reach a critical short length that activates the DNA damage response (DDR) [20,21]. The DDR induces cell cycle arrest by inhibiting cyclin-dependent kinases (CDKs) via the p53/p21 or p16/Rb pathways resulting in replicative senescence or apoptosis [22,23].

The telomerase enzyme can maintain telomere length (TL) by adding single-stranded G-rich sequences (5′-TTAGGG-3′) to extend the 3′ overhang region [15,24,25]. Human telomerase is an enzymatic ribonucleoprotein complex made up of a catalytic subunit human TERT, a template for reverse transcription human TERC and regulatory protein subunit dyskerin [15,24]. Baseline TL and rate of telomere shortening vary among individuals and across different organs in the same individual [26]. TL is influenced by genetic and lifestyle factors including lack of physical activity, stress, smoking and alcohol abuse [27,28]. Oxidative damage induced by inflammation and oxidative stress can accelerate telomere shortening [29–32]. Thus, the rate of telomere shortening is dependent on both cell replication and DNA damage.

Telomeres & CKD

Evidence from animal studies

As shown in Table 1, there are a few studies reported in animal models on CKD and telomeres, the general consensus of which is that telomeres play an important role in maintaining healthy kidney structure and function. In a mouse model of telomere dysfunction, a recent study from the Blasko group demonstrated that deletion of Trf1, a shelterin component, induced kidney fibrosis, a precursor to end-stage kidney disease [33]. Telomerase-deficient mice (G3 TERT^-/-) with short telomeres developed kidney fibrosis and severe kidney dysfunction after the administration of low-dose folic acid [33]. A similar dose of folic acid did not induce fibrosis in wild-type mice [33], indicating that mild kidney injury could increase the predisposition to develop kidney failure in the presence of shortened telomeres. Deprotected telomeres either due to telomere shortening or shelterin dysfunction undergo changes in their epigenetic pattern [34]. The lncRNAs, telomeric repeat-containing RNA (TERRA) and telomere dysfunction-induced lncRNA (tdilncRNA), are transcribed at deprotected telomeres [35,36]. Growing evidence shows that lncRNAs may be implicated in the pathogenesis of renal fibrosis [37] and TERRA has been associated with pulmonary fibrosis [38]. TERRA and tdilncRNA may mediate telomere dysfunction and induce renal fibrosis.

Table 1. Studies of associations between telomeres and chronic kidney disease.

Animal studies
Sample type	Main findings	Association	Source	Measurement method	Ref.
Uremic rats	Shorter cardiac TL correlates with expressed ROS in uremic rats. Treatment with antioxidant (vitamin C) increases TL and decreases ROS expression	Positive	Heart	Flow FISH^†	[45]
TERT and TERC knockout mice exposed to ischemia/reperfusion injury	TERT and TERC knockout significantly delayed renal recovery after ischemia/reperfusion-induced renal injury	Positive	Not specified	Not specified	[40]
Cats with CKD	Shorter kidney TL independently associated with CKD; no association between liver or skin TL and CKD. Liver telomerase activity highest in CKD group compared with other groups; no difference in kidney telomerase activity across groups	Positive	Kidney, liver, skin	qFISH^† TRAP assay^‡	[42]
Cyclosporine-induced nephrotoxic rats	Decreased kidney TL correlates with decline in renal function. Cyclosporine-induced oxidative stress could be responsible for renal telomere attrition and ensuing kidney senescence. Renal telomerase activity not affected by cyclosporine-A	Positive	Kidney	Southern blot (TRF analysis)^† TRAP assay^‡	[44]
TERT and Trf1 knockout mice	Trf1 deletion leads to development of kidney fibrosis. Telomerase-deficient mice treated with low-dose folic acid develop kidney fibrosis and severe kidney dysfunction	Positive	Kidney	qFISH^†	[33]
Telomerase knockout mice	Kidney injury results in shorter kidney TL in rats; recovery rate from injury is associated with TL	Positive	Kidney	qFISH^†	[39]
Rats with induced renal failure and myocardial infarction	Severe renal failure, but not mild renal failure, results in cardiac telomere attrition compared with control	Positive	Heart	qPCR^†	[41]
Cell lines
Cell type	Main findings	Association	Tissue source	Measurement method	Ref.
Human glomerular mesangial cells	High glucose environment promotes telomere attrition in kidney cells	Positive	Human glomerular mesangial cells	Southern blot (TRF analysis)^†	[64]
Human studies
Study population (sample size)	Main findings	Association	Tissue source	Measurement method	Ref.
Coronary heart disease patients (n = 954)	Decreased renal function associated with shorter baseline LTL and greater telomere attrition after 5-year follow-up; no longer significant after adjusting for age	Age-dependent	Leukocytes	qPCR^†	[69]
End-stage renal disease patients (n = 281)	Telomere attrition independently associated with end-stage renal disease. Decreased T-cell TL may explain uremia-associated immune deficiency in patients with loss of renal function	Positive	T-cells	Flow FISH^†	[46]
HD patients (n = 136)	Shorter PBMC TL associated with HD independent of age. HD patients with vitamin D supplementation had longer TL compared with untreated patients	Positive	PBMCs	Southern blot (TRF analysis)^†	[55]
Prevalent HD patients (n = 175)	LTL predicts mortality in CKD patients independent of age and gender	Positive	Leukocytes	qPCR^†	[54]
CKD patients (n = 222)	CKD associated with decreased PBMC TL. Telomerase activity not correlated with TL	Positive	PBMCs	qPCR^† TRAP assay^‡	[50]
Kidney transplant donors (n = 257)	LTL and intrarenal TL correlated; shorter TL not associated with renal function (eGFR) but with intrarenal arteriosclerosis	Positive	Leukocytes, kidney	qPCR^†	[66]
Kidney transplant recipients (n = 134)	5 years post-transplantation, intrarenal TL associated with renal arteriosclerosis, dependent on donor characteristics	Positive	Kidney	qPCR^†	[67]
Patients with cardiovascular risk factors (n = 766)	Shorter LTL associated with decreased renal function (eGFR); stronger correlation in non-CKD patients	Positive	Leukocytes	qPCR^†	[72]
CKD patients (n = 4926)	Shorter LTL in CKD patients associated with increased risk of mortality independent of kidney function	Age-dependent	Leukocytes	qPCR^†	[51]
Type 1 diabetes mellitus patients (n = 176)	Shorter LTL predicts incident microalbuminuria and diabetic nephropathy progression in Type 1 diabetes mellitus patients	Positive	Leukocytes	Southern blot (TRF analysis)^†	[60]
Type 2 diabetes mellitus patients (n = 691)	Shorter LTL independently predicts albuminuria progression in Type 2 diabetes mellitus patients with preserved renal function	Positive	Leukocytes	qPCR^†	[48]
Incident HD patients (n = 59)	Patients with increased leukocyte count had longer LTL after first year of dialysis. Patients with lower eGFR and higher albumin levels had higher telomere attrition	Positive	Leukocytes	qPCR^†	[47]
CKD and HD patients (n = 150)	Increased PBMC telomerase activity in CKD patients compared with control. Telomerase activity is highest in stage 5 CKD patients		PBMCs	TRAP assay^‡	[76]
CKD patients (n = 153)	Decreased PBMC telomerase activity and human TERT expression in CKD group compared with dialysis and control groups. Control group had highest telomerase activity		PBMCs	TRAP assay^‡ RT-qPCR^§	[77]
CKD patients (including dialysis and kidney transplantation patients) (n = 159)	CKD associated with leukocyte telomere attrition. Telomere attrition more pronounced in renal replacement therapy group compared with dialysis group after 1 year	Positive	Leukocytes	qPCR^†	[56]
Non-CKD patients with and without diabetes mellitus (n = 403)	LTL not associated with eGFR and urinary microalbumin-to-creatinine ratio in non-CKD patients	None	Leukocytes	qPCR^†	[74]
CKD patients (n = 10568)	Decreased LTL correlates with decreasing eGFR	Positive	Leukocytes	qPCR^†	[63]
Kidney transplant patients (n = 140)	T-cell TL shorter in end-stage renal disease patients compared with control; remains unchanged 1-year post kidney transplantation. Uremia-associated immunological aging not reversed by kidney transplantation	Positive	T-cells	flow FISH^†	[58]
Kidney transplant patients (n = 124)	Mycophenolic acid-treated patients have increased PBMC telomere attrition compared with mTOR inhibitor-treated patients	Positive	PBMCs	qPCR^†	[57]
Patients without CKD and CVD (n = 253)	TL independently correlated with albuminuria. Association between urea or creatinine and TL is age-dependent	Positive, age-dependent		qPCR^†	[73]
HD patients (n = 30)	HD patients have shorter PBMC TL associated with inflammation biomarkers	Positive	PBMCs	flow FISH^†	[53]
CKD patients with and without diabetes mellitus (n = 889)	Shorter LTL associated with CKD progression in active smokers and/or diabetes mellitus patients	Positive	Leukocytes	qPCR^†	[65]
CKD patients with and without diabetes mellitus (n = 4955)	Shorter LTL in CKD patients predicts cardiovascular disease incidence. Association between albuminuria or eGFR and TL is age-dependent	Age-dependent	Leukocytes	qPCR^†	[68]
CKD patients (n = 4955)	Patients with CKD for <6 months and >5 years had comparable LTL, longer than patients with CKD between 6 months and 5 years	Positive	Leukocytes	qPCR^†	[49]
End-stage renal disease patients on chronic HD (n = 66)	No significant difference in PBMC TL among end-stage renal disease patients and control. Patients on longer duration of HD had shorter TL	Positive	PBMCs	Southern blot (TRF analysis)^† TRAP assay^‡	[52]
Male patients with Type 2 diabetes mellitus (n = 84)	LTL associated with progression of kidney disease in Type 2 diabetes mellitus patients	Positive	Leukocytes	TRF analysis^†	[59]
HD patients (n = 81)	Telomerase activity decreased in PBMCs of HD patients; inversely correlated with duration of HD treatment		PBMCs	TRAP assay^‡	[78]
Chronic heart failure patients (n = 610)	LTL associated with declining renal function in chronic heart failure patients	Positive	Leukocytes	qPCR^†	[61]
Chronic heart failure patients (n = 866)	Shorter LTL associated with decreased renal function and may affect kidney’s ability to cope with metabolic changes in heart failure patients	Positive	Leukocytes	qPCR^†	[62]
Healthy participants (n = 609)	No association between LTL changes and renal function in healthy patients	None	Leukocytes	TRF analysis^†	[75]
Healthy participants (n = 139)	LTL reduces with age in healthy participants; correlates with eGFR	Positive	Leukocytes	Southern blot (TRF analysis)^†	[71]

† TL.

‡ Telomerase activity.

§ TERT/TERC expression.

CKD: Chronic kidney disease; HD: Hemodialysis; HPA: Hybridization protection assay; LTL: Leukocyte telomere length; PBMC: Peripheral blood mononuclear cell; qFISH: Quantitative fluorescence in situ hybridization; qPCR: Quantitative polymerase chain reaction; ROS: Reactive oxygen species; RT-qPCR: Real-time quantitative polymerase chain reaction; TL: Telomere length; TRAP: Telomeric repeat amplification protocol; TRF: Terminal restriction fragment.

To determine whether telomere shortening or dysfunction could impair the regenerative capacity of the kidney, TERT and TERC knockout mice have been used as models. TERT and TERC knockout significantly delayed renal recovery after ischemia/reperfusion-induced renal injury and the animals with relatively longer TL showed better recovery [39,40]. Concurrent occurrences of shortened TL and conditions that increase the functional load of the kidney such as hypertension and diabetes may accelerate the development of CKD. In addition, pre-existing CKD may lead to telomere shortening in other organs, which may cause further complications in CKD. Wong et al. [41] reported that kidney failure after myocardial infarction (MI) did not lead to excessive cardiac telomere shortening compared with that observed in kidney failure-only rats [41]. An association between CKD and shorter renal TL has also been reported in cats [42,43].

The association between telomerase and CKD remains unclear in animal studies. Cyclosporine-induced oxidative stress in non-CKD rats led to renal telomere shortening without altering renal telomerase activity [44]. This indicates the possibility of oxidative stress modulating TL independent of the telomerase pathway. In cats, kidney telomerase activity was similar across CKD and control groups, while liver telomerase activity was highest in the CKD group compared with the control group [42]. The increase in liver telomerase activity may be compensatory, as uremic toxins are known to directly inhibit enzyme activation in the liver. On the other hand, severe kidney failure and uremia resulted in cardiac telomere shortening in rats [41,45]. Although oxidative stress has been implicated in animal models, the pathways involved are yet to be fully clarified. The shorter aortic TL of uremic rats has been correlated with increased oxidative stress and treatment with an antioxidant (vitamin C) resulted in longer TL in the treated group [45]. Conversely, similar antioxidant treatment did not decrease oxidative stress in nonuremic aging rats [45]. These findings suggest that uremia-induced oxidative stress may initiate telomere shortening in rats. Cyclosporine-induced oxidative stress has also been associated with nephrotoxicity and renal telomere shortening in non-CKD rats [44]. Other factors that increase oxidative stress may accelerate kidney aging and telomere shortening.

Human studies

CKD patients: positive associations between TL & CKD

Shorter leukocyte TL (LTL) has been associated with decreased renal function in CKD patients [46–48]. A study by Raschenberger et al. [49] found a U-shaped association between LTL and CKD such that patients who have had CKD for between 6 months and 5 years exhibit shorter LTL than those patients who have CKD for <6 months and >5 years [49]. This result suggests that TL change may not be unidirectional and may be related to the CKD stage. On the other hand, CKD may exacerbate telomere shortening in patients, accounting for the decreased TL between 6 months and 5 years of CKD. Longer telomeres in patients with CKD for >5 years may be due to survival bias. Shorter peripheral blood mononuclear cells (PBMCs) TL and LTL have also been reported to be independent predictors of morbidity and mortality in CKD patients [50,51]. Therefore, patients with shorter TL may have a higher mortality rate that may result in patients with long disease duration having comparably longer telomeres.

Incident kidney disease resulting in TL shortening and a further decline in renal function is another hypothesis. Betjes et al. [46] observed that shortened T-cell TL in kidney failure may be responsible for the uremia-associated immune deficiency seen in CKD patients [46]. In 222 patients at different CKD stages, the uremic state was associated with shorter PBMC TL and immune senescence [50]. In this study, there was no difference in DNA damage, which is a marker of telomere-independent immune senescence, between uremic and nonuremic CKD patients [50]. It was concluded that uremia-induced immune senescence might depend on telomere replicative senescence and not on telomere-independent immune senescence. Uremia may accelerate telomere shortening through inflammation and immunodeficiency.

Furthermore, other factors could contribute to this process. Renal replacement therapies, hemodialysis (HD) and kidney transplantation, for stage 4 and 5 CKD patients are linked with telomere attrition. A longer duration of HD correlates with shorter PBMC TL, possibly due to increased oxidative stress because of recurring exposure of blood within the extracorporeal circuit [52]. Studies have reported that decreased LTL and PBMC TL are associated with inflammation biomarkers observed in patients on HD, which may contribute to telomere attrition [53,54]. Increased oxidative stress due to both the uremic state characteristic of kidney disease and persistent inflammation resulting from dialysis therapy may be largely responsible for the observed telomere attrition.

Patients with increased baseline leukocyte count have been reported to have longer LTL after the first year of dialysis therapy while those with longer baseline LTL had higher telomere attrition [47]. This may be reflective of leukocyte proliferation in patients with high leukocyte count at the onset, highlighting the role of dialysis in oxidative stress in CKD. In a case-control study among HD patients, antioxidant vitamin D supplementation resulted in longer PBMC TL compared with untreated HD patients [55]. This further underscores the possible role of oxidative stress in inducing telomere shortening due to HD therapy. For stage 4 and 5 CKD patients on HD, the use of antioxidants to attenuate the effect of oxidative stress and prevent further telomere shortening may be explored.

Reduction of the prooxidative and proinflammatory state in CKD alone may not result in immediate attenuation of telomere attrition. Kidney transplantation stabilizes the uremic milieu and increases long-term survival compared with dialysis therapy; however, kidney transplantation has been linked with increased telomere attrition compared with dialysis [56]. The choice of immunosuppressive therapy may correlate with the degree of telomere shortening. Increased telomere attrition is observed in blood cells of kidney transplant patients on mycophenolic acid compared with those on less potent immunosuppressants such as azathioprine or mTOR inhibitors [56,57]. Furthermore, immunological aging and a decrease in T-cell TL associated with uremia are not reversed by kidney transplantation [58]. In kidney transplant patients, while increased telomere attrition in blood cells of more immunosuppressed patients may be due to immune deficiency, factors such as comorbidities and donor characteristics may also have implications.

Patients with comorbidities: positive associations

Shorter LTL in Type 1/Type 2 DM (T1DM/T2DM) and cardiovascular disease (CVD) patients [59–62] has been associated with decreased renal function. Several studies (see Table 1) also show that telomere shortening could be an independent predictor of deteriorating renal function in T1DM and T2DM patients [48,59,60,63]. This suggests that telomere shortening may be involved in the pathogenesis of kidney disease in DM patients. A plausible mechanism is through oxidative stress characteristic of the hyperglycaemic milieu. Exposure of human kidney cells to a stressful environment of high glucose, resulting in increased oxidative stress, led to shorter TL [64]. Similarly, other conditions that induce oxidative stress may accelerate telomere shortening and kidney disease progression. A strong association was observed between LTL and CKD progression in CKD patients who were also active smokers and/or DM patients [65]. In addition, shorter TL in CKD patients may predispose them to the development of comorbidities. Shorter LTL and renal TL have been associated with renal arteriosclerosis [66,67]. The resulting endothelial dysfunction and arteriosclerosis may play a role in the pathogenesis of CVD.

CKD patients: lack of associations

Some studies have reported that the association between renal function and TL may be age-dependent in CKD patients [51,68,69]. As discussed, TL may vary with CKD stage and the relationship between telomere shortening and renal function might be CKD-stage dependent. Variation of CKD stage among study participants could be responsible for the lack of association observed. The complex pathogenesis of renal disease, which could affect the rate of initial decline in renal function in the early stages of CKD, may also explain the discrepancies.

General population

A recent Mendelian randomization study using data from the CKDGen consortium and UK Biobank showed a causal link between genetically predicted telomere attrition and increased CKD risk [70]. Shorter LTL has been associated with decreased renal function in healthy participants [71]. For example, Eguchi et al. [72] found that the correlation between LTL and eGFR is stronger in non-CKD patients, which may further explain the age-dependent association reported in some CKD patients. Although, an age-dependent association [73] and a lack of associations [74,75] between LTL and renal function in non-CKD patients have been reported. These observations may occur because age is associated with both telomere attrition and renal function decline. In addition, the correlation between TL and renal function may differ depending on the renal function parameter assessed. In a cohort of healthy participants, the association between TL and some measures of renal function, such as glomerular filtration, urea and creatinine, was age-dependent while TL was independently correlated with albuminuria [73]. TL may only be indicative of renal function using specific parameters in healthy people.

CKD patients: telomerase activity

Telomerase activity may provide insights into TL changes across CKD stages. Kidir et al. [76] observed increased telomerase activity in CKD patients compared with controls with the highest telomerase activity in stage 5 CKD patients. In contrast, another study reported low telomerase activity in CKD patients with the highest telomerase activity in controls [77]. Since the latter study analyzed telomerase activity in all CKD groups together, varying TL across CKD stages may have affected the overall results. Patients with longer LTL at baseline have been shown to experience higher telomere attrition [47,75], suggesting that telomerase activity may be more pronounced in short telomeres. Telomerase activity seems to decrease in the PBMCs of HD patients compared with controls and inversely correlates with the duration of HD treatment [78]. More recently, a study reported no difference in PBMC telomerase activity between HD patients and healthy controls [52]. Here, the long duration of HD treatment was associated with positive telomerase activity and relatively longer PBMC TL among HD patients [52]. Increased oxidative stress during dialysis may stimulate telomerase activity. An increase in telomerase activity over the long duration of HD may further explain the longer LTL in patients with CKD for >5 years compared with patients with CKD for between 6 months and 5 years. A prospective study found no difference in PBMC telomerase activity between CKD and non-CKD patients and no correlation between telomerase activity and TL in PBMCs [50]. Hence, telomerase activity may not be indicative of the development of CKD.

Current therapeutic applications of telomeres

Telomere biology has been implicated in the pathophysiology of a wide range of diseases with a plethora of ongoing research focused on possible therapeutic applications of telomeres. The goal is to increase or maintain TL in diseases associated with aging, whereas the opposite is true in cancer, where the focus is on reducing the proliferative capacity of tumor cells. Telomerase, TERT, TERC and telomere-associated proteins are current therapeutic targets as a result. Adeno-associated viruses for cardiac-specific TERT expression increased TL and improved prognosis without increasing cancer susceptibility in a preclinical mouse model of MI [80]. Polysaccharides of Cistanche deserticola increased telomerase activity in mice hearts and brains [81]. Female mice on dietary supplementation with telomerase activator TA-65 extracted from Astragalus membranaceus had enhanced health span and lengthening of short telomeres with no increase in the risk of developing cancer [82]. These effects may be exerted through a telomerase-dependent pathway since TA-65 increased average TL and decreased the percentage of critically short telomeres in mouse embryonic fibroblasts (MEFs) with a single copy of the telomerase RNA TERC gene (G3 TERC^+/-) but not in telomerase-deficient G3 TERC^-/- MEFs [82].

Adults on a health maintenance program consisting of TA-65 (10–50 mg daily) and a dietary supplement pack had a lengthening of critically short telomeres after 1 year [83]. Low dose TA-65 (250 U) also increased LTL among healthy cytomegalovirus-positive participants over a 1-year period [84]. High-dose TA-65 (1000 U) did not result in a statistically significant increase in TL [84], suggesting a dose-dependent effect of telomerase activators. Ginkgo biloba extracts increased telomerase activity in cultured human endothelial progenitor cells [85]. Telomerase activity of human HeLa cells increased after treatment with root extracts of Ashwagandha (Withania somnifera) [86].

Metformin inhibited telomerase activity in endometrial and human breast cancer cell lines [87–89]. Despite their telomerase inhibiting activity in cancer cells, evidence shows that antidiabetic agents may promote telomere maintenance in noncancerous cells. Sitagliptin significantly increased LTL in newly diagnosed T2DM patients after 2 months of therapy [90]. Decreased leukocyte telomere attrition rate was reported among T2DM patients using antidiabetic agents compared with those not using medication [91]. However, specific antidiabetic agents may not be protective against telomere attrition. Independent of glycemic control, patients whose antidiabetic therapy incorporated the use of acarbose had a higher telomere attrition rate compared with patients using acarbose-free antidiabetic therapy [91]. A 6-year follow-up study of T2DM patients receiving oral antidiabetic therapy and/or insulin therapy showed that, despite good glycemic control in all study participants, 70% of patients showed LTL attrition while 30% of patients had an increase in LTL [92]. Insulin was independently associated with the telomere attrition observed in these patients [92]. Acarbose and insulin may promote telomere attrition, while metformin and some other antidiabetic agents may decrease telomere shortening.

Hypertensive BPH/2J mice show renal telomere attrition only after the development of hypertension [93]. Alleviation of telomere attrition in hypertension may prove beneficial to delay hypertension-induced renal damage. Treatment with losartan resulted in significantly longer renal TL in young spontaneously hypertensive rats [94]. These effects were sustained for a prolonged period even after discontinuation of therapy. Zhang et al. [95] reported increased LTL and accompanying lower systolic blood pressure and pulse pressure in patients on calcium-channel blocker (CCB) or angiotensin-receptor blocker (ARB) therapy but not on diuretic therapy during a 2-year follow-up. A study on postmenopausal women with arterial hypertension found that, independent of blood pressure control, moxonidine therapy increased telomerase activity in blood monocytes while bisoprolol decreased telomerase activity [96]. Diuretics, β-blockers and ARBs but not CCBs nor angiotensin-converting enzyme inhibitors are independent protective factors against decreasing TERT expression in peripheral blood leukocytes [97]. These studies suggest that antihypertensives are associated with telomere maintenance; however, their action on telomere biology may be class specific.

In cancer therapy, oligonucleotides and small molecule inhibitors such as BIBR1532 that prevent telomere extension by direct inhibition of telomerase activity have been identified [98,99]. Telomere-specific antisense oligonucleotides that target lncRNAs transcribed at telomeres hold potential for use in diseases such as CKD by mitigating the effect of fibrosis and other phenotypic changes resulting from deprotected telomeres [35]. In addition, senolytics that selectively clear senescent cells attenuated renal fibrosis in obese mice and there are ongoing clinical trials on their use in CKD [100,101]. TERT oncolytic adenovirus and vaccines targeting TERT tumor-associated antigens and G-quadruplex stabilizing compounds that alter telomerase function are also being studied for their anticancer effects [98]. Telomerase is being combined with existing therapies to form new nanoplatforms that can increase the specificity of tumor cell identification and combat chemotherapy multidrug resistance [102–105]. Natural telomerase inhibitors such as telomestatin in combination with chemotherapeutic agents potentiated anticancer effects in K562 leukemia cell lines [15]. Despite the prospects that telomeres hold, the only telomere therapeutics in advanced stages of clinical trials are for the treatment of cancer. Imetelstat (GRN163L), an oligonucleotide telomerase inhibitor that competitively inhibits telomerase catalytic activity by binding to TERC, has completed phase II clinical trials for breast cancer, non-small cell lung cancer and multiple myeloma [106]. Human TERT-based vaccines (GV1001, GRNVAC1 and combinations Vx-001) that induce the killing of telomerase-positive tumor cells through CD4⁺ and CD8⁺ T-cell cytotoxic responses with no significant toxicity to noncancerous cells are also being tested in different stages of clinical trials [106].

Limitations

Most of the reviewed studies on telomeres and CKD in humans measured TL or telomerase activity in the blood (LTL and PBMCs). A validation study from the Genotype-Tissue Expression Consortium has shown that LTL can be a proxy for renal TL [79]. This allows for the use of LTL for noninvasive measurement and prognosis management of CKD in the future. Furthermore, many of the studies discussed had a small number of subjects, therefore, not precluding the possibility of false associations.

Conclusion

With almost 10% of the world’s population impacted by CKD, there is a need to find new effective therapies. Several studies indicate a correlation between shorter length of telomere and onset of CKD. Although many of these studies were conducted using leukocytes rather than kidney tissue and had small samples, there is compelling evidence for this association. This association has brought to the foreground the important therapeutic role of the enzyme telomerase, which can maintain TL. Studies have demonstrated that TERT knockout significantly delays renal recovery. The causality of the association between CKD and shorter telomeres also remains unclear, but fibrosis seems to play a central role here. Another intriguing piece of data suggests that CKD patients with shorter TL may have a higher mortality rate, again supporting the evidence that maintaining TL is a treatment avenue. Further work to clarify these associations and their therapeutic potential is warranted.

Future perspective

With increasing investigation of the associations between telomeres and CKD, novel management and treatment strategies for CKD are on the horizon. Telomerase, shelterin protein complex and telomere maintenance genes are potential therapeutic targets (Figure 3). Telomere-specific antisense oligonucleotides that deplete TERRA and tdilncRNAs may be a strategy to alleviate renal fibrosis. TL assessment could also serve as a diagnostic biomarker for the early detection of CKD.

Figure 3. Telomere therapies for chronic kidney disease.

Treatment possibilities include gene therapy using genes in the telomerase subunits TERT and TERC, proteins from the shelterin complex (TRF1, TRF2, TIN2, POT1, RAP1, TPP1) and other telomere maintenance genes, including PARN, CTC1, TCBA1, NHP2, RTEL1, NOP10, DKC1 and NAF1. Use of telomerase activators such as Astragalus membranaceus, Centella asiatica, oleanolic acid and maslinic acid can also be explored.

Image created with BioRender.com.

Executive summary

• Chronic kidney disease (CKD) is a complex multifactorial condition with increasing global prevalence. Regardless of the etiology, renal fibrosis is an integral pathway for CKD progression.

Telomeres & CKD: evidence from animal studies

• Renal fibrosis is associated with shortened or dysfunctional telomeres and may provide insights into the role of telomeres in CKD pathogenesis.

Telomeres & CKD: human studies

• Telomeres have been linked with the development and progression of CKD and shorter telomeres may predispose to the development of CKD complications.

Current therapeutic applications of telomeres

• Telomerase activator supplements have increased telomere length without increasing the risk of developing cancer.

Future directions

• Telomeres and telomere maintenance genes are potential therapeutic intervention targets for the early detection and management of CKD.

Author contributions

OA Akinnibosun: Conception and design; literature search; manuscript draft; critical revision; final approval of published version; agreement to be accountable for all aspects of the work. MC Maier: literature search; manuscript draft; final approval of published version; agreement to be accountable for all aspects of the work. J Eales: literature search; critical revision; final approval of published version; agreement to be accountable for all aspects of the work. M Tomaszewski: literature search; critical revision; final approval of published version; agreement to be accountable for all aspects of the work. FJ Charchar: conception and design; critical revision; final approval of published version; agreement to be accountable for all aspects of the work.

Financial & competing interests disclosure

The British Heart Foundation project grant (PG/06/097/21331) and National Health Medical Research Foundation of Australia project grant (GNT1123472) both fund the open access of this paper. OA Akinnibosun is funded by Destination Australia Scholarship. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Additional information

Funding

The British Heart Foundation project grant (PG/06/097/21331) and National Health Medical Research Foundation of Australia project grant (GNT1123472) both fund the open access of this paper. OA Akinnibosun is funded by Destination Australia Scholarship. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.