Background
Homocysteine (Hcy) is a sulfhydryl amino acid situated at a branch point of methionine metabolism that is metabolized via two interrelated processes of remethylation and transsulfuration [
1]. Numerous studies have found that elevated serum Hcy is a nontraditional independent risk factor for cardiovascular disease (CVD) [
2,
3]. Elevated serum Hcy is a common finding in patients with chronic kidney disease (CKD) [
4], which is attributed to reduced renal clearance of circulating Hcy and impaired Hcy metabolism [
5]. Furthermore, elevated serum Hcy was confirmed to be related to the prevalence and progression of CKD [
6,
7].
IgA nephropathy (IgAN) is the most common primary glomerulonephritis [
8]. Approximately 30–40% of IgAN patients eventually develop end-stage renal disease (ESRD) within 20–30 years of renal biopsy [
9]. A small retrospective study from Duan et al. revealed that elevated Hcy was associated with poor renal outcome in IgAN patients [
10]. Another study by Mendelian randomization (MR) analysis observed positive effects of Hcy on serum creatinine, blood pressure (BP), and pathogenic T lesions in IgAN patients [
11]. However, only the two small-sample studies mentioned above have investigated the relationship between Hcy and IgAN patients. Clinical research on the relationship between Hcy and pathology has not yet been highlighted.
In this study, we investigated the relationship between Hcy and clinicopathologic characteristics in IgAN patients in our center, especially at an early stage with a normal estimated glomerular filtration rate (eGFR) (≥ 90 ml/min/1.73 m2).
Discussion
In this study, we found that the serum Hcy of IgAN patients was significantly higher than that of healthy controls in our center. IgAN patients with elevated Hcy displayed more severe T lesions, even in subgroups of patients with normal renal function. Multivariate logistic regression analysis showed that pathologic T was an independent risk factor for elevated Hcy in IgAN patients and persisted in a subgroup analysis according to eGFR ≥90 ml/min/1.73 m2.
In this study, we found that Hcy was positively correlated with age in the whole cohort of 337 IgAN patients. A serious shortage of vitamin B12 and vitamin B6 was common in elderly populations, and a lack of cofactors reduced the metabolism of Hcy. In addition, the decline in renal function with aging also caused elevation of Hcy [
14]. Significantly higher serum levels of Hcy were observed in male patients with IgAN than that in female patients [13.5 (10.5, 17.5) vs. 10.7 (8.1, 13.6) μmol/L,
P < 0.001], which was similar to a previous study [
15,
16]. Moreover, males were independently associated with elevated Hcy in our study. One possible reason was the difference between the sexes in lifestyle variables that are known to have an impact on Hcy [
17]. In addition, estrogen could decrease plasma Hcy levels [
18]. As previously reported [
19], patients with elevated Hcy had higher ALB levels. The mechanism was not well elucidated. Scholars have explained that Hcy covalently binds to ALB, resulting in the lack of filtration and absorption processes of Hcy in the kidney [
20]. However, more studies are needed to probe the link between ALB and Hcy and its underlying mechanism.
From the results in this study, it can be seen that serum Hcy was closely related to kidney function. A previous study demonstrated that serum Hcy was very likely elevated in patients with CKD and gradually increased with the progression of CKD stage [
4]. Hcy was markedly elevated at the ESRD stage [
21], and was 3-5 times that of healthy participants [
22]. In our study, the serum Hcy of IgAN patients was similarly elevated compared to that of sex- and age- matched healthy controls, even in a subgroup of patients with normal renal function, and increased with the decline in eGFR. However, there was a question as to whether the elevated Hcy was a cause for CKD or rather the result of CKD.
It has been demonstrated that the kidney is a major site for the removal of plasma Hcy and plays an important role in the regulation of plasma Hcy levels [
23]. In our study, multivariate logistic regression analysis found that pathologic T (OR = 2.87, 95% CI = 1.20-6.89,
P = 0.018) was an independent risk factor for elevated Hcy in IgAN patients, even after adjustment for eGFR and in the subgroup with normal renal function (OR = 14.20, 95% CI = 1.68-120.03,
P = 0.015). Zhang et al. observed the association between Hcy and pathologic T lesions in IgAN [
11]. Given impairment in remethylation, patients with CKD were likely more dependent on the pathway of Hcy transsulfuration, which was catalyzed by cystathione-β-synthase (CBS) and γ-cystathionase (CTL). Meanwhile, James D et al. expressed that CBS was enriched in the proximal convoluted tubule cells. CTLs exhibit higher enrichment patterns in proximal straight tubule cells [
24]. Li et al. found that renal proximal tubules were major sites for Hcy metabolism in the kidney [
25]. When tubular atrophy and interstitial fibrosis occurr, the transsulfuration pathway is reduced, leading to the elevation of Hcy. It seemed that the increase in Hcy was the result of renal injury, especially T lesions.
The Oxford classification score is a well-regarded prognostic indicator for IgAN. Tubular interstitial lesions are the most valuable histological parameter and a final pathway for most progressive kidney diseases. T lesions are common in IgAN patients even with normal renal function; however, factors associated with T lesions in IgAN have not been elucidated. Possible mechanisms to explain the T lesions are that with aggravating renal impairment, renal units will be damaged accompanied by fibrosis, resulting in loss of normal excretory function and reduced clearance of Hcy. On the other hand, IgAN is an inflammatory disease, and Hcy is related to the inflammatory response and stimulates the production of cytokines and proinflammatory molecules, which may be related to renal fibrosis. We found that nearly half of IgAN patients had elevated Hcy levels and displayed more severe clinicopathologic characteristics, even in patients with normal renal function with eGFR ≥90 ml/min/1.73 m
2. A longitudinal prospective study showed that elevated plasma Hcy levels may be a predictor of accelerated decline in renal function and future incidence of CKD [
6]. An animal study implicated elevated Hcy related to glomerular injury [
26]. In both essential hypertension and general adult populations, high Hcy was associated with albuminuria independent of renal function [
27,
28]. A prospective study showed that Hcy was an independent determinant of the development of albuminuria even after adjustment for eGFR [
29]. In fact, Hcy is associated with vascular damage, endothelial dysfunction and cell proliferation, oxidative stress, lipid and lipoprotein metabolism and inflammation [
30].
In addition, serum Hcy was positively correlated with hsCRP and ESR, but negatively correlated with SOD in the whole cohort of IgAN patients. Gori et al. revealed that interleukin-6 and interleukin-1 were independent predictors of Hcy concentrations in elderly individuals [
31]. Decreased SOD activity was observed in the kidneys of high Hcy rats [
32]. SOD inhibits antioxidants and catalyzes superoxide conversion to H
2O
2. Hcy directly inhibits the activity of antioxidants, thereby disrupting SOD. On the other hand, elevated Hcy could induce oxidative stress, resulting in excessive superoxide production and SOD consumption [
33]. The clinical results supported the findings of animal and cell model studies that Hcy promoted inflammation, redox imbalance and oxidative stress [
34], which contributed to renal vascular damage. As a consequence, high Hcy levels may potentially induce renal injury and not only be the result of impaired renal function.
Of note, we investigated the relationship between Hcy and clinicopathologic characteristics in the whole cohort of 337 IgAN patients and then in the subgroup with eGFR ≥90 ml/min/1.73 m2 to modify the effect of renal impairment. Our study showed consistent results that higher Hcy was associated with higher Scr, BUN, UA, ALB and T lesions but lower eGFR and HDL. However, there were opposite results in the correlation between Hcy and age, 24-h urine protein, and ESR at the early stage of 156 IgAN patients, which might be partially explained by the small sample size.
To date, whether Hcy is just a marker or plays a causal role in IgAN remains to be elucidated. The main strength of this study was that we assessed the association between serum Hcy and clinicopathologic characteristics in IgAN patients with normal renal function. We observed that elevated Hcy displayed more severe clinicopathologic characteristics, and pathologic T was an independent risk factor for elevated Hcy even in the early stage of IgAN. Our study prompted clinicians to pay more attention to Hcy levels in IgAN patients, especially at early stages. Previously, a randomized controlled trial in China suggested that Hcy-lowering therapy can significantly delay the progression of renal impairment among patients with mild-to-moderate CKD [
35]. Further studies are needed in the future to discover the controversial relationship between kidney function and Hcy and confirm whether Hcy concentration intervention is beneficial to IgAN patients.
The present study had some limitations. First, because of the cross-sectional design of this study, we cannot obtain any causal inferences from the data. Second, as a single-center study, our study could not exclude the limits of races and regions, and its external validity may be limited. Third, the serum Hcy cutoff points differed among the previous studies [
4,
11,
36]. However, Chinese expert consensus on hyperhomocysteinemia published in 2020 defined hyperhomocysteinemia as Hcy levels greater than 10 μmol/L. A study of Hcy and Chinese patients with IgA nephropathy also defined hyperhomocysteinemia as Hcy levels greater than 10 μmol/L [
11]. The population in our study was from China, so 10 μmol/L was selected as the Hcy cutoff in this study. Fourth, Hcy may be affected by folate, vitamin B12 and vitamin B6 status, which were not assessed in this study. In addition, there is a lack of follow-up data to assess the impact of baseline serum Hcy on renal and CVD outcomes in patients with IgAN.
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