Effects of n-Acetyl-l-Cysteine on the Progression of Kidney Dysfunction in Acadian Variant Fanconi Syndrome: A Case Series

Abstract

Introduction

Acadian variant Fanconi syndrome (AVFS) is an autosomal recessive disease caused by a mutation in the NDUFAF6 gene which results in mitochondrial dysfunction and oxidative damage to the kidneys. It presents as a slowly progressive chronic kidney disease and proximal tubular dysfunction that, in contrast to typical Fanconi syndrome, leads to end-stage renal disease. It is diagnosed clinically and is present only in Acadian people. The objective of this study was to evaluate the effect of n-acetyl-l-cysteine (NAC), an antioxidant, on the progression of loss of renal function in patients with AVFS.

Case Presentation

Chart reviews were conducted on 4 active cases of AVFS not on kidney replacement therapy. Data on age, estimated glomerular filtration rate (eGFR), creatinine, medications, and blood pressure were collected. Progression of eGFR was evaluated in each patient before and after NAC prescription, and the projected time to kidney replacement therapy (eGFR = 10 mL/min/1.73 m²) with and without the use of NAC was calculated for each case. NAC had a positive effect on slowing the rate of eGFR loss in each patient. The projected need for kidney replacement therapy was delayed by an average of 14 years in the treated patients who otherwise would have required it by an estimated mean age of 28 years.

Conclusion

This study suggests that NAC has a beneficial effect on slowing the progression of kidney disease in patients with AVFS. This may be due to the modulation of oxidative stress by NAC. Further studies are needed to evaluate the potential benefits of NAC in other chronic kidney conditions.

Introduction

Fanconi syndrome (FS) is a rare dysfunction of the proximal tubule of the kidney, either inherited or acquired, that leads to abnormal urinary excretion of glucose, amino acids, phosphate, bicarbonate, and uric acid, which in turn can cause acidosis, rickets, growth retardation, and electrolyte imbalances [1]. The Acadian variant of Fanconi syndrome (AVFS) is an autosomal recessive form of FS found in the Acadian people on the East Coast of Canada that exhibit a founder effect [2, 3].

Similarly to FS, AVFS has historically been managed by electrolytes and bicarbonate replacement [1, 3]. This clinical management, however, does not prevent the worsening of kidney disease in AVFS patients [2, 3], for which no specific treatment exists. To elaborate, in contrast to FS, in AVFS, the proximal tubule-associated kidney disease progresses to end-stage renal disease, eventually leading to the need for kidney replacement therapy (KRT) at an early age [2]. KRT includes various therapeutic modalities such as hemodialysis, peritoneal dialysis, and kidney transplantation [4]. These treatment options are associated with a steep reduction in quality of life for those patients [5, 6]. At our institution, individuals diagnosed with AVFS have required KRT at an average of 30 years of age.

Hartmannová et al. [2] isolated the mutation responsible for AVFS in the NDUFAF6 gene, encoding the NADH:ubiquinone oxidoreductase complex assembly factor 6, a structural protein of the complex I of the electron transport chain of the mitochondrion. Defects in complex I assembly result in impaired mitochondrial ATP production and are associated with an increased production of reactive oxygen species (ROS) [2]. Because of the high energy requirement of the reabsorption process, the tubular epithelial cells have been shown to be particularly vulnerable to mitochondrial injuries leading to metabolic or oxidative stress, which are associated with decline of kidney function [7, 8].

Despite evidence linking elevated levels of ROS to chronic kidney disease (CKD) development and progression, attempts to prevent or slow CKD progression using varied low-molecular-weight antioxidants to remove ROS excess have not been successful due to the diversity of pathophysiological processes involved [9]. However, given the known NDUFAF6-mediated complex I deficiency in AVFS patients, using antioxidant agents specifically targeting the mitochondrial ROS production and defense mechanisms seems promising. Glutathione (GSH) is a major actor in the oxidation-reduction (redox) homeostasis, predominantly located in mitochondria, that can decrease ROS-mediated damages by protecting proteins from oxidation by reversible posttranslational modification (glutathionylation) [10]. One of the main limits to GSH biosynthesis is the availability of its nonessential amino acid precursor, cysteine. One orally bioavailable cysteine source is n-acetyl-l-cysteine (NAC), a safe and inexpensive antioxidant that has received FDA approval for treatment of hepatotoxic doses of acetaminophen [11]. NAC is also approved for various conditions with abnormal mucus secretion, including pneumonia and bronchitis [11], and has shown great promises in clinical trials for various psychiatric and neurological disorders [12]. Besides GSH replenishment, NAC has been shown to promote scavenging of free radicals under varied, albeit specific conditions, and new antioxidative and cytoprotective roles are still being discovered [13].

The aim of this study was to assess the potential of NAC to prevent or slow the progression of CKD to end-stage renal disease in AFVS. We first describe 4 known cases of AVFS with moderate CKD that have been prescribed NAC for the last 6 to 8 years and compare their kidney function before and after its introduction. We then estimate the projected age at which KRT would be initiated with and without NAC.

Case Presentations

This is a case series utilizing medical record reviews of four active cases of AVFS, not transplanted nor on dialysis. Medical chart reviews were conducted to collect data regarding age, weight, height, estimated glomerular filtration rate (eGFR), creatinine, medications, and blood pressure. eGFR was calculated using the CKiD U25 equation up to the age of 25, and the CKD-EPI equation for ages above that.

A scatter plot of eGFR values over time was drawn for each patient. Linear regression models were fitted to the data series to estimate the slopes of eGFR change. The difference in slopes before and during treatment with NAC was computed for each patient. Because of the small sample size (n = 4), formal parametric inference was avoided. Instead, an exact sign-flip permutation test and a nonparametric bootstrap procedure (10,000 resamples) were used to estimate the 95% confidence interval (CI) of the mean slope difference [14]. Regression beta coefficients were then used to estimate the amount of time needed for each series to reach a prespecified eGFR threshold where KRT is required. For the purposes of this study, KRT was postulated to be necessary at an eGFR of 10 mL/min/1.73 m² [15, 16]. Analyses were conducted using SAS 9.4 (SAS Institute, Inc., Cary, NC, USA).

Case 1

The first case is that of a female patient of Acadian descent who was clinically diagnosed with FS at 8 years of age. Her symptoms included hypophosphatemic rickets, renal tubular acidosis, normoglycemic glucosuria, amino aciduria, and CKD. Genetic confirmation of the diagnosis was subsequently obtained. The patient was prescribed NAC at the age of 21, at a dosage of 500 mg PO bid. The steady decline of eGFR slowed upon initiation of treatment with NAC (shown in Fig. 1a).

Fig. 1.

a–d Graphical representation of progression of kidney function in each patient up to a projected eGFR of 10, yellow: eGFR values and projection pre-NAC, green: eGFR values and projection post-NAC.

Case 2

The second case is that of a female patient of Acadian descent that was clinically diagnosed with FS in her early teens. Symptoms included severe bilateral genu valgum, hypophosphatemic rickets, normoglycemic glucosuria, amino aciduria, proximal tubular acidosis, and CKD. Genetic confirmation of the diagnosis was subsequently obtained. The patient was prescribed NAC 500 mg PO bid at the age of 18. The rapid decline of eGFR slowed upon start of treatment with NAC (shown in Fig. 1b).

Case 3

The third case is that of a male patient of Acadian descent, with a strong family history of AVFS, who was clinically diagnosed with FS at 10 years of age. He also underwent kidney biopsy at age 14, which showed chronic tubulointerstitial nephritis and normocellular glomeruli (shown in Fig. 2). Genetic confirmation of the diagnosis was subsequently obtained. Symptoms included proximal tubular acidosis, rickets, amino aciduria, phosphaturia, glucosuria, and CKD. The patient was prescribed NAC 500 mg PO bid at the age of 26 (shown in Fig. 1c). This patient was also prescribed SGLT-2 inhibitors for a short period of time a few years after the initiation of NAC, but due to the lack of apparent benefit these were discontinued.

Fig. 2.

Kidney biopsy image of case 3.

Case 4

The last case is that of a male patient of Acadian descent who was clinically diagnosed with FS at around 14 years of age. Symptoms included normoglycemic glucosuria, hypercalciuria, intermittent phosphaturia, renal tubular acidosis, mild uremia, and CKD. Genetic confirmation of the diagnosis was subsequently obtained. Prescribed medications included hydrochlorothiazide, which was discontinued within a year after starting NAC 500 mg PO bid at the age of 18 (shown in Fig. 1d).

Patients were prescribed NAC for a mean duration of 7 ± 1 years at the time of the study. Figure 1a–d provides a graphical representation of kidney function progression pre- and post-NAC. Table 1 summarizes individual slope estimates and the projected age at initiation of KRT. The mean slope difference in eGFR (pre-post-NAC) was 2.46 ± 0.82 mL/min/1.73 m² per year, with a bootstrap 95% CI of [1.17, 3.76]. Although an exact paired sign-flip permutation test did not reach statistical significance (p = 0.13), the consistent direction of change across patients and a CI excluding zero suggests a trend toward improved eGFR slopes following NAC. As such, it took longer for eGFR to reach the KRT threshold of 10 mL/min/1.73 m² in all patients when taking NAC. Pre-NAC, patients lost eGFR at an average rate of −3.74 ± 0.92 mL/min/1.73 m² per year. This would have led to KRT at an average projected age of 28 ± 3. With the treatment of NAC, eGFR declined at an average rate of −1.27 ± 0.31 mL/min/1.73 m² per year, leading to an average age of 42 ± 3 years before initiation of KRT. Moreover, this resulted in an estimated gain of 14 years without the need for dialysis or renal transplantation in patients who received NAC.

Table 1.

Individual patient eGFR slopes, projected age of KRT, and differences with and without NAC treatment

Patient	Sex	Age at treatment onset, years	eGFR at treatment onset, mL/min/1.73 m2	Rate of eGFR loss, mL/min/1.73 m2/year			Projected age of KRT, years
				pre-NAC	post-NAC	difference	pre-NAC	post-NAC	difference
Patient 1	Female	21	39	−2.49	−0.93	+1.57	32	47	15
Patient 2	Female	18	55	−4.51	−1.47	+3.04	26	41	15
Patient 3	Male	26	29	−3.63	−1.58	+2.05	31	40	9
Patient 4	Male	18	28	−4.35	−1.10	+3.26	23	40	17

eGFR, estimated glomerular filtration rate; KRT, kidney replacement therapy; NAC, N-acetyl-L-cysteine.

While patient 2 started at the highest baseline eGFR (55 mL/min/1.73 m²), this patient was projected to require KRT at the age of 41 years, which is 1 year later than patient 4, who had the lowest eGFR at treatment onset (28 mL/min/1.73 m²) and who is projected to require KRT at the age of 40. Patients 2 and 4 also happen to be the youngest patients in the cohort. Patient 3, who was the oldest at time of treatment initiation, is projected to require KRT at the age of 40, similar to patients 2 and 4. These differences may be due to variations in disease severity, muscle mass, dietary effect, and metabolism.

Data on blood pressure were collected before and after receiving NAC. No clinically significant difference was found between the pre- and post-NAC systolic blood pressure, of 125 ± 11 mm Hg and 124 ± 9 mm Hg, respectively. There was no noted effect of NAC on the proteinuria or the metabolic consequences of AVFS.

Discussion

The aim of this study was to evaluate the effects of NAC on the progression of kidney function in patients with AVFS. Our findings suggest that, from a clinical perspective, NAC has a preventative effect with regard to requiring KRT. Estimated 14 years of KRT were saved with treatment of NAC, with projected KRT being required at a mean age of 42 rather than at a projected age of 28 without NAC. This estimated gain also compares favorably with past cases in our institution who had not received NAC and required KRT at an average age of 30. Furthermore, while these past cases typically had an acceleration in the loss of kidney function over time, our cohort experienced a slowing of the progression after receiving NAC. These results support the use of this antioxidant in patients with AVFS as a viable treatment providing years of potentially greater quality of life [5, 6].

In our study, the mean difference in rate of decline in eGFR pre-post-NAC was 2.46 mL/min/1.73 m²/year. This slowing in eGFR decline is comparable to effective treatment effects seen in other kidney disease populations. For instance, absolute differences in eGFR decline of 1.1 mL/min/1.73 m²/year were observed with the use of angiotensin-converting enzyme inhibitors in type 1 diabetic nephropathy [17], and a reduction of 1.27 mL/min/1.73 m²/year with the use of tolvaptan in autosomal polycystic kidney disease [18]. None of the patients were prescribed ACE inhibitors at any point in time during this study.

Renal indications for NAC have historically included prevention of contrast-induced acute nephropathy, though results on its efficacy to prevent preventing KRT are lacking [19]. Chronic NAC use has traditionally been limited, being mainly prescribed for its respiratory mucolytic properties [20]. While articles exploring chronic NAC use in CKD are scarce [21], a systematic review and meta-analysis by Ye et al. [22] demonstrated that eGFR data were improved in patients with CKD receiving NAC. These findings should be contextualized in the setting of NAC possibly decreasing serum creatinine without having an impact on kidney function, as was described in a systematic review and meta-analysis by Huang et al. [23]. In the setting of our study, however, NAC was being prescribed orally rather than intravenously, which would not lead to the high concentrations that may cause the more significant interference documented by the study [22]. Furthermore, one can also look at patient 3, who was 26 years old at treatment onset and was projected to require dialysis around 30 years of age without NAC intervention. Continuous follow-up shows that, despite being in their mid-thirties now, the patient has yet to present signs of requiring KRT. Moreover, a recent study by Chiu et al. [21] showed that patients with CKD who took NAC 600 mg PO twice a day were less likely to require hemodialysis than the control group. They postulated that this is through modulation of oxidative stress, which in turn modulated serum creatinine and eGFR levels [21]. Finally, one can also look at the graphs of eGFR progression in our patients, in which the velocity of loss of eGFR itself declines after starting NAC. If the benefit were mainly artifact, then the slope at which eGFR is lost should remain the same. As such, while there is still developing literature regarding the use of NAC in the setting of CKD, these results appear to be in line with our findings and further support the apparent effectiveness of NAC in our patient population.

This study has some limitations. Given the very small prevalence of AVFS in the population, our sample treated with NAC was limited. While the generalizability of our results to the AVFS population is relevant, it should be treated with caution. Moreover, the simplistic method used in this study to estimate time to KRT should also be interpreted cautiously. With this being a rare disease, a controlled trial with a larger population is not likely to be feasible, and thus the use of stronger statistical models to estimate time to KRT is not possible. Additionally, while many confounding factors are controlled due to the pre-post-design, some other sources of bias may be unaccounted for. These include compliance to treatment, difference in frequency of checking creatinine between patients, acute episodes of kidney injury, and differences in timing of NAC prescription. With the above being said, the pre-post-study design allowed for all patients to receive the proposed treatment and allowed for analysis to be performed despite the rarity of the disease. Moreover, the predictions made are conservative ones given the setting of eGFR = 10 as estimation of time of KRT, and our projected age of pre-NAC KRT requirement is similar to that of prior cases (28 vs. 30, respectively).

In conclusion, chronic NAC treatment seems to slow down impairment of kidney function in patients with AVFS. These results, in conjunction with its relatively safe side effect profile and its accessibility, make NAC a promising option for the management of AVFS. Further research is needed to better understand the molecular mechanisms of action of NAC and its pharmacological properties in the context of AVFS, as well as to assess the potential of long-term use of NAC in slowing the progression of other causes of CKD.

Statement of Ethics

This study has been approved by the Research Ethics Committee of the Vitalité Health Network (#101701_2023). Written informed consent was obtained from all patients involved in this study. The CARE Checklist for this case report is available as online supplementary material (for all online suppl. material, see https://doi.org/10.1159/000549747).

Conflict of Interest Statement

The authors declare no conflicts of interest.

Funding Sources

This study was not supported by any sponsor or funder.

Author Contributions

M.A.-Q, C.J., and Y.T. participated in the study design. M.A.-Q. participated in data collection and wrote the manuscript. M.A.-Q. and J.G. participated in data analysis. J.G. created graphical figures. All authors contributed to, edited, and approved the manuscript. Y.T. supervised the project.

Funding Statement

This study was not supported by any sponsor or funder.

Data Availability Statement

All data relevant to the findings of this study are contained within the manuscript.