Abstract
Niemann-Pick Disease Type C (NP-C) is a fatal lysosomal storage disorder with progressive neurodegeneration. In addition to the characteristic cholesterol and lipid overload phenotype, we previously found that altered metal homeostasis is also a pathological feature. Increased brain iron in the Npc1−/− mouse model of NP-C may potentially contribute to neurodegeneration, similar to neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases. Deferiprone (DFP) is a brain penetrating iron chelator that has demonstrated effectiveness in preventing neurological deterioration in Parkinson’s disease clinical trials. Therefore, we hypothesized that DFP treatment, targeting brain iron overload, may have therapeutic benefits for NP-C. Npc1−/− mice were assigned to four experimental groups: (1) pre-symptomatic (P15) + 75 mg/kg DFP; (2) pre-symptomatic (P15) + 150 mg/kg DFP; (3) symptomatic (P49) + 75 mg/kg DFP; (4) symptomatic (P49) + 150 mg/kg DFP. Our study found that in Npc1−/− mice, DFP treatment did not offer any improvement over the expected disease trajectory and median lifespan. Moreover, earlier treatment and higher dose of DFP resulted in adverse effects on body weight and onset of ataxia. The outcome of our study indicated that, despite increased brain iron, Npc1−/− mice were vulnerable to pharmacological iron depletion, especially in early life. Therefore, based on the current model, iron chelation therapy is not a suitable treatment option for NP-C.
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Introduction
Niemann-Pick Disease Type C (NP-C) is a fatal lysosomal storage disorder affecting 1:90,000 live births (Wassif et al. 2016). NP-C is further classified as Type C1 (NP-C1; OMIM 257220) or Type C2 (NP-C2; OMIM 607625), based on pathogenic mutations in the NPC1 or NPC2 genes, respectively (Carstea et al. 1997; Naureckiene et al. 2000). NP-C1 is the predominant subtype affecting about 95% of the patient population (Patterson et al. 2013). Emerging structural studies implicate NPC1 and NPC2 proteins, localized to late endosomes and lysosomes, in the transport and intracellular mobilization of cholesterol and sterols (Elghobashi-Meinhardt 2019; Hodoscek and Elghobashi-Meinhardt 2018; Gong et al. 2016; Li et al. 2017; Pfeffer 2019; Xu et al. 2007). The loss-of-function NPC1 and/or NPC2 mutant proteins block cholesterol egress from lysosomes, resulting in an excessive build-up of lysosomal cholesterol. Consequently, toxic cholesterol accumulation results in cellular and organ damage. NP-C patients present with heterogeneous clinical symptoms that include hepatosplenomegaly, cerebellar ataxia, dysarthria, dysphagia, schizophrenia-like psychosis and dementia due to progressive neurodegeneration affecting brain regions that include the cerebellum, basal ganglia, brain stem, corpus callosum, thalamus and hippocampus (Rego et al. 2019). The age of disease onset spans a wide spectrum, from in utero to late life, and typically, inversely correlates with disease severity (Vanier 2010). Childhood onset of NP-C is most common, and based on “crowd-sourced” data, Bianconi et al. (2019) recently reported that NP-C patients have a median age of death at 13. There are limited disease-specific treatment options for NP-C, but none yet approved by the FDA (Hammond et al. 2019).
Previously, we reported metal dyshomeostasis in NP-C1 patient and mouse model plasma and various tissues including brain, liver and spleen (Hung et al. 2014). Dysregulated iron homeostasis is one of the key changes. Iron is significantly elevated in the brain (cerebellum and cerebrum) of BALB/cJ Npc1nih (Npc1−/−) mice; and a trend to an increase in cerebellar iron in limited sample-size of post-mortem human NP-C1 cerebellar tissue. Brain iron elevation and disrupted iron metabolism increase adverse involvement of redox-active iron in Fenton or Haber-Weiss reactions, and thus promote oxidative stress by producing toxic levels of reactive oxygen species and free radicals. Oxidative stress is a pathological feature of NP-C (Reddy et al. 2006; Porter et al. 2010; Klein et al. 2011; Vazquez et al. 2011; Ribas et al. 2012; Vazquez et al. 2012), and abnormal iron metabolism may be a contributing factor.
Pathological brain iron accumulation has been identified as a potential therapeutic target for a number of neurodegenerative diseases that include Alzheimer’s (AD) and Parkinson’s (PD) diseases (Masaldan et al. 2019). Deferiprone (DFP), a brain penetrating iron chelator used to treat patients with iron overload disorders such as thalassemia, sickle cell anemia and hemochromatosis (Hider and Hoffbrand 2018; Fredenburg et al. 1996), is currently under investigation as a treatment option for AD and PD. DFP phase II clinical trials for PD (NCT00943748; NCT01539837) showed promising improvements in neurological outcomes and decreased nigral dentate and caudate nucleus iron levels (Martin-Bastida et al. 2017; Devos et al. 2014). Phase IIb clinical trials of DFP for PD are currently in progress (NCT02655315 and NCT02728843). Additionally, a phase II clinical trial of DFP for AD is underway (NCT03234686). Based on promising potential of DFP as a treatment option for AD and PD, we hypothesized that targeting brain iron accumulation in NP-C by iron chelation with DFP may improve neurological outcome and possibly lifespan in NP-C. To test this hypothesis, we investigated DFP treatment using the Npc1nih mouse model of NP-C.
Methods
Animals
This study used the BALB/cJ Npc1nih (Npc1−/−) mouse model, a commonly used animal model of NP-C disease, which harbors Npc1 null allele due to a spontaneous insertion mutation (Loftus et al. 1997). The Npc1−/− mice closely model the early childhood onset of the human NP-C disease, showing progressive weight loss, motor and cognitive impairments, and have a median lifespan of about 11 weeks (Maue et al. 2012; Voikar et al. 2002). Heterozygous Npc1+/− mice were used to generate Npc1−/− mice (nfemale = 12; nmale = 12) used in this study. The Npc1−/− genotype was identified by real-time polymerase chain reaction with probes designed for Npc1 (Transnetyx, Cordova, TN, USA). Mice were maintained at the Florey Core Animal Services Facility on Barastoc standard rodent feed (Ridley Corporation, Victoria, Australia). Food and water were available ad libitum. After weaning at postnatal day (P) 21, male and female mice were housed separately, with a maximum of six mice of mixed genotype per cage, and maintained on a 12-h light-dark cycle.
Deferiprone treatment
DFP (3-hydroxy-1,2-dimethyl-4(1H)-pyridone; Sigma-Aldrich Cat #379409, Lot #STBG1231V) was dissolved using milliQ H2O (Merck). Dissolved DFP was filter sterilized and warmed to 37 °C prior to use.
The Npc1−/− mice were assigned to four different treatment groups. They were assigned to start DFP treatment at two different ages: (1) pre-symptomatic (P15); and (2) symptomatic (P49). Within each of these groups, Npc1−/− mice were further assigned to be treated with (1) 75 mg/kg or (2) 150 mg/kg DFP. DFP treatment was administered by intraperitoneal (i.p.) injection three time a week (Monday, Wednesday and Friday). The choice of DFP dosage was based on previously reported DFP dosage for animal models of other neurological disorders, taking into consideration the young age of Npc1−/− mice (Ayton et al. 2013; Zhao et al. 2015; Devos et al. 2014).
Npc1−/− mice were weighed prior to receiving DFP by i.p. injection and for disease monitoring. The maximum injected volume of DFP was 1% of total body weight. Mice were sacrificed when they have lost ≥ 15% of their maximal body weight.
Experimental procedures were approved by the Florey Animal Ethics Committee and were conducted in accordance with the Australian Code for the Care and Use of Animals for Scientific Purposes, Eight Edition (2013).
Simon’s two-stage design
Simon’s two-stage design was used to assess the potential of DFP as a therapeutic drug for NP-C. Primary endpoint was extension of Npc1−/− mice lifespan by at least 50%, from expected median lifespan of 70 days to 105 days. An online calculator was used to generate Simon’s two-stage design for this study: http://cancer.unc.edu/biostatistics/program/ivanova/SimonsTwoStageDesign.aspx. We tested the null hypothesis that the probability of DFP elicits a poor drug response is 5% against a one-sided alterative. In the first stage, enrolling n1 = 3 Npc1−/− mice per experimental group. Within each experimental group, if none of the DFP-treated Npc1−/− mice survived to 105 days, the study will terminate. Otherwise, stage 2 will recruit additional n2 = 3 Npc1−/− mice per experimental group to achieve a total of n = 6 Npc1−/− mice. The null hypothesis will be rejected if two or more mice survive for 105 days. This design yields a type I error rate of 2.66% and power of 82.8% when the response probability of DFP as an effective treatment is 50%.
Statistical analysis
Statistical analyses were performed using R version 3.4.3 (R Development Core Team 2017) and the following packages: cowplot (Wilke 2017), dplyr (Wickham et al. 2017), ggplot2 (Wickham 2009), ggpubr (Kassambara 2017), moments (Komsta and Novomestky 2015), rcompanion (Mangiafico 2018). To ensure satisfaction of distributional assumptions for statistical testing, the normality of data was checked visually by histogram and by Shapiro-Wilk test. Due to the small sample size, data that showed deviation from normal distribution were further tested for skewness. Data with a skewness value between − 2 and 2 were considered to be within the acceptable range for normal distribution. Analysis of variance (ANOVA) and multiple comparisons of means [Tukey Honest Significant Difference test (TukeyHSD)] were used to assess differences in lifespan, onset of weight loss and onset of ataxia between the experimental groups. A one-sample Wilcoxon signed-rank test was used to compare the median lifespan of DFP-treated Npc1−/− mice against the expected median lifespan of 70 days (based on ≥15% loss of maximal body weight). The analyses also examined the interaction between sex, DFP start time, and DFP dose.
Results
To evaluate the potential of DFP as a treatment for NP-C, we treated commonly used Npc1nih (Npc1−/−) mouse model for NP-C with DFP in this study. To minimize the number of mice required for this evaluation, we adopted Simon’s two-stage design for single-arm phase II clinical trials (Simon 1989), which allows for early termination of the study in the event that DFP does not elicit positive improvements with respect to median lifespan as the primary endpoint. The expected median lifespan was 70 days (Hung et al. 2016), which was determined based on the criterion to sacrifice the mice when they have lost ≥ 15% of their maximal body weight.
Progressive weight loss is a part of the disease trajectory in the Npc1−/− mouse model. The onset of weight loss depended on the sex of Npc1−/− mice and their age at DFP treatment initiation (p = 0.0004; Fig. 1). There was no difference in the onset of weight loss between male and female Npc1−/− mice when DFP treatment began at P15. However, when DFP treatment begins at P49, the onset of weight loss was 7.3 days later in female Npc1−/− mice compared with male Npc1−/− mice. There was no difference in the onset of weight loss between the two doses tested.
Due to neurodegeneration of the cerebellum, Npc1−/− mice develop ataxia as the disease progresses. In this study, we visually monitored the onset of ataxia in Npc1−/− mice as a readout of their response to DFP treatment. Our analysis found an interaction between DFP dose and the sex of Npc1−/− mice (p = 0.02; Fig. 2a), and an interaction between DFP dose and age of DFP treatment initiation (p = 0.05; Fig. 2b). The onset of ataxia was observed eight days earlier in female Npc1−/− mice treated with 150 mg/kg DFP compared with those treated with 75 mg/kg DFP (Fig. 2a). There was no dose-dependent response observed for male Npc1−/− mice. For Npc1−/− mice that began DFP treatment at P15, the onset of ataxia began 7.9 days earlier in mice treated with 150 mg/kg DFP compared with those treated with 75 mg/kg (Fig. 2b). For Npc1−/− mice that began DFP treatment at P49, there was no dose-dependent onset of ataxia.
The expected median lifespan for Npc1−/− mice was 70 days. There was no difference in median lifespan of DFP-treated Npc1−/− mice with respect to their sex (Fig. 3a), age of DFP treatment initiation (Fig. 3b), or DFP dose (Fig. 3c). There was no change in the median lifespan of DFP-treated Npc1−/− mice compared with the expected median lifespan (p = 0.77). As none of the Npc1−/− mice survived longer than the expected median lifespan of 70 days, the study was terminated at the end of stage 1 based on our pre-determined criteria for Simon’s two-stage design.
Discussion
Previously, our metallomic analysis revealed significantly elevated brain iron in Npc1−/− mice and a trend to increased cerebellar iron in post-mortem human NP-C brain (Hung et al. 2014). We hypothesized that, similar to abnormal iron accumulation in AD and PD, this increase in NP-C brain iron may contribute to disease pathogenesis, and in particular, the underlying oxidative stress by promoting Fenton and/or Haber-Weiss reactions. Iron chelation therapy with DFP has demonstrated neuroprotective effects and improved motor and cognitive outcomes in PD (Devos et al. 2014; Martin-Bastida et al. 2017) and are in further phase II clinical trials for PD (NCT02655315 and NCT02728843) and AD (NCT03234686). Therefore, we hypothesized that iron chelation therapy with DFP, targeting the abnormal brain iron accumulation in NP-C, may elicit similar neuroprotective therapeutic benefits. However, contrary to our expectations, DFP was ineffective in slowing the disease progression or prolong the lifespan of Npc1−/− mice. Moreover, our results showed that higher dose of DFP and initiating treatment at an earlier age (P15) resulted in increased toxicity.
The ineffectiveness of DFP in treating Npc1−/− mice in this study may be attributed a combination of significantly decreased hepatic iron levels in Npc1−/− mice (Hung et al. 2014) and administration of DFP by i.p. injection. DFP is orally bioavailable and many studies administer DFP in drinking water (Carboni et al. 2017; Casu et al. 2016; Zhao et al. 2015; Ayton et al. 2013). However, voluntary consumption may be variable between animals and produce confounding results. To ensure precise and accurate oral dosing, the preferred method is oral gavage. It is possible to give oral gavage to pre-weaning and young mice (Moser et al. 2005), but repeated dosing required by this study risks damage to the thin esophageal tissues of young animals used. Consequently, we chose to administer DFP by i.p. injections based on similarity in pharmacokinetics between this delivery route and that of oral administration (Turner et al. 2011). Absorption of drugs delivered by i.p. occurs primarily through portal circulation and can thus undergo hepatic metabolism before systemic distribution (Lukas et al. 1971). The detrimental effect of DFP treatment found in this study may be the result of DFP further depleting the already deficient hepatic iron content in Npc1−/− mice.
Another explanation for the negative outcome of iron chelation therapy with DFP in Npc1−/− mice is the potential detrimental effects exerted by lowering brain iron during sensitive neurodevelopmental periods. In healthy human brain, high levels of iron accumulates in the basal ganglia with age (Ramos et al. 2014). Age-related iron accumulation may contribute to neurotoxicity and consequently, apoptosis and ferroptosis that result in neurodegeneration (Masaldan et al. 2019). For late life neurodegenerative disorders such as AD and PD, where age-related iron accumulation and compromised brain iron metabolism have been implicated in disease pathogenesis, removal of excess iron by iron chelation therapy resulted in neurological improvements (Devos et al. 2014; Martin-Bastida et al. 2017). In this study, the age of iron chelation treatment with DFP for Npc1−/− mice began at P15 and P49, which are equivalent to human age of about 3 months and 13.4 years old, respectively (Dutta and Sengupta 2016). Iron is critical to brain development and in particular, the rapid axon myelination during these developmental periods (Snaidero and Simons 2014; Wessling-Resnick 2017). Optimal brain development requires balanced iron levels; iron deficiency or overload would result in neurological deficits. The brain iron elevation in Npc1−/− mice may represent greater iron influx due to Npc1 deficiency and/or greater need for iron that remains to be determined. This scenario could be similar to the accumulation of iron in Friedreich’s ataxia caused by mutation of frataxin, where defects in the transfer of iron to iron-sulfur clusters (ISCs) leads to both a metabolic deficiency of ISCs as well as an upstream toxic build-up of iron (Ast et al. 2019). The increase in brain iron in Npc1−/− mice may contribute to their neurological phenotype, but the concentration may not have reached the same neurotoxic level as in old age. It is possible that the DFP doses used in this study may have caused excessive iron depletion, rather than restore the iron homeostasis and facilitate neuroprotection.
In conclusion, our pilot study showed that in a relevant mouse model of NP-C, targeting brain iron accumulation in early life is ineffective and potentially harmful. These findings may bear implications for other rare genetic childhood-onset disorders manifesting as Neurodegeneration with Brain Iron Accumulation (NBIAs), in which iron chelation, although potentially effective (Klopstock et al. 2019), must be considered with care to avoid over depletion of iron critical to brain development.
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Acknowledgements
The authors wish to thank Dr. Ibrahima Diouf for statistical advice. This work was supported by research grants from the Australian NPC Disease Foundation, Inc. to Y.H.H. and A.I.B.; the Australia-Israel Medical Research Foundation (AUSiMED) to A.L. and A.I.B.; Victoria Israel Science Innovation and Technology Scheme Grants for Feasibility and Proof of Concept Projects to A.L. and A.I.B.; Cooperative Research Centre for Mental Health to A.I.B. The Florey Institute of Neuroscience and Mental Health acknowledges the strong support of the Victorian Government and in particular funding from the Operational Infrastructure Support Grant.
Funding
This work was supported by research grants from the Australian NPC Disease Foundation, Inc.; the Australia-Israel Medical Research Foundation (AUSiMED); Victoria Israel Science Innovation and Technology Scheme Grants for Feasibility and Proof of Concept Projects; Cooperative Research Centre for Mental Health; and the Victorian Government Operational Infrastructure Support Grant
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A.I.B. and A.L. conceived the idea for this study. Y.H.H. and A.L. designed and planned the experiments, and performed data analysis. A.S. and S.Y. performed the experiments and contributed to interpretation of results. Y.H.H. wrote the manuscript with inputs from all authors.
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A.I.B. is a shareholder of Prana Biotechnology Ltd, Cogstate Ltd, Brighton Biotech LLC, Grunbiotics Pty Ltd, Eucalyptus Pty Ltd, and Mesoblast Ltd. He is a paid consultant for, and has a profit share interest in Collaborative Medicinal Development LLC.
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Animal experimental procedures were approved by the Florey Animal Ethics Committee (AEC# 17–042).
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Hung, Y.H., Lotan, A., Yeshurun, S. et al. Iron chelation by deferiprone does not rescue the Niemann-Pick Disease Type C1 mouse model. Biometals 33, 87–95 (2020). https://doi.org/10.1007/s10534-020-00233-5
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DOI: https://doi.org/10.1007/s10534-020-00233-5