Oligodendrocyte development and myelinogenesis are not impaired by high concentrations of phenylalanine or its metabolites

DNAJC12 in Monoamine Metabolism, Neurodevelopment, and Neurodegeneration

Recent studies show that pathogenic variants in DNAJC12, a co-chaperone for monoamine synthesis, may cause mild hyperphenylalaninemia with infantile dystonia, young-onset parkinsonism, developmental delay and cognitive deficits. DNAJC12 has been included in newborn screening, most revealingly in Spain, and those results highlight the importance of genetic diagnosis and early intervention in combating human disease. However, practitioners may be unaware of these advances and it is probable that many patients, especially adults, have yet to receive molecular testing for DNAJC12. Hence, this review summarizes genotype-phenotype relationships and treatment paradigms for patients with pathogenic variants in DNAJC12. It provides an overview of the structure of DNAJC12 protein, known genetic variants, domains, and binding partners, and elaborates on its role in monoamine synthesis, disease etiology, and pathogenesis. © 2023 International Parkinson and Movement Disorder Society.

Neurological and imaging phenotypes of adults with untreated phenylketonuria: new cases and literature review

OBJECTIVES

Phenylketonuria (PKU) is the most prevalent congenital disease of amino acid metabolism. Neurological manifestations usually complicate PKU in untreated adult patients. This study describes neurological and imaging phenotypes of adult patients with untreated PKU.

METHODS

We investigated a cohort of 320 unrelated adult patients with suspected genetic leukoencephalopathies using whole-exome sequencing (WES). We analyzed the phenotypic features of adult PKU patients in our cohort and summarized cases reported in the literature.

RESULTS

We identified 10 patients in our cohort and 12 patients in the literature, who presented with neurological manifestations and were diagnosed with PKU in adulthood. Approximately 60% of these patients had onset of clinical features in adulthood. The most common neurological symptoms of patients presenting in adulthood were cognitive disturbance and spastic paralysis, followed by vision loss, cerebellar ataxia, weakness of limbs, and seizure. This differed from that of patients presenting with PKU features in childhood, who consistently had mental retardation with various neurological complications emerging during a broad age range. Imaging findings were similar between patients presenting with clinical features in childhood compared with adulthood, comprising symmetric periventricular white matter hyperintense on T2-weighted imaging and diffusion-weighted imaging predominantly in the parietal and occipital lobes. Also, normal brain imaging and diffuse leukoencephalopathies were observed in both patient groups.

CONCLUSION

PKU with clinical features presenting in adulthood is an atypical subtype and should be considered during diagnosis of adults with neurological symptoms and leukoencephalopathy. DWI seems to be most helpful to distinguish patients with PKU. Additionally, we demonstrate that PKU constitutes a part (3.1%) of adult genetic leukoencephalopathies.

Myelin basic protein recovery during PKU mice lifespan and the potential role of microRNAs on its regulation

Untreated phenylketonuria (PKU) patients and PKU animal models show hypomyelination in the central nervous system and white matter damages, which are accompanied by myelin basic protein (MBP) impairment. Despite many assumptions, the primary explanation of the mentioned cerebral outcomes remains elusive. In this study, MBP protein and mRNA expression on brains of wild type (WT) and phenylketonuric (ENU2) mice were analyzed throughout mice lifespan (14-60-180-270-360-540 post-natal days, PND). The results confirmed the low MBP expression at first PND times, while revealed an unprecedented progressive MBP protein expression recovery in aged ENU2 mice. Unexpectedly, unaltered MBP mRNA expression between WT and ENU2 was always observed. Additionally, for the same time intervals, a significant decrease of the phenylalanine concentration in the peripheral blood and brain of ENU2 mice was detected, to date, for the first time. In this scenario, a translational hindrance of MBP during initial and late cerebral development in ENU2 mice was hypothesized, leading to the execution of a microRNA microarray analysis on 60 PND brains, which was followed by a proteomic assay on 60 and 360 PND brains in order to validate in silico miRNA-target predictions. Taken together, miR-218 - 1-3p, miR - 1231-3p and miR-217-5p were considered as the most impactful microRNAs, since a downregulation of their potential targets (MAG, CNTNAP2 and ANLN, respectively) can indirectly lead to a low MBP protein expression. These miRNAs, in addition, follow an opposite expression trend compared to MBP during adulthood, and their target proteins revealed a complete normalization in aged ENU2 mice. In conclusion, these results provide a new perspective on the PKU pathophysiology understanding and on a possible treatment, emphasizing the potential modulating role of differentially expressed microRNAs in MBP expression on PKU brains during PKU mouse lifespan.

High phenylalanine concentrations induce demyelination and microglial activation in mouse cerebellar organotypic slices

Phenylketonuria (PKU) is an inborn error of metabolism. Mutations in the enzyme phenylalanine hydroxylase (PAH)-encoding gene lead to a decreased metabolism of the amino acid phenylalanine (Phe). The deficiency in PAH increases Phe levels in blood and brain. Accumulation of Phe can lead to delayed development, psychiatric problems and cognitive impairment. White matter (WM) damage is a neuropathological hallmark of PKU and can be seen even in early detected and treated PKU patients. The mechanisms linking high Phe concentrations to WM abnormalities remain unclear. We tested the effects of high Phe concentrations on myelin in three in vitro models of increasing complexity: two simple cell culture models and one model that preserves local brain tissue architecture, a cerebellar organotypic slice culture prepared from postnatal day (P) 8 CD-1 mice. Various Phe concentrations (0.1–10 mM) and durations of exposure were tested. We found no toxic effect of high Phe in the cell culture models. On the contrary, the treatment promoted the maturation of oligodendrocytes, particularly at the highest, non-physiological Phe concentrations. Exposure of cerebellar organotypic slices to 2.4 mM Phe for 21 days in vitro (DIV), but not 7 or 10 DIV, resulted in a significant decrease in myelin basic protein (MBP), calbindin-stained neurites, and neurites co-stained with MBP. Following exposure to a toxic concentration of Phe, a switch to the control medium for 7 days did not lead to remyelination, while very active remyelination was seen in slices following demyelination with lysolecithin. An enhanced number of microglia, displaying an activated type morphology, was seen after exposure of the slices to 2.4 mM Phe for 10 or 21 DIV. The results suggest that prolonged exposure to high Phe concentrations can induce microglial activation preceding significant disruption of myelin.

Genetic etiology and clinical challenges of phenylketonuria

This review discusses the epidemiology, pathophysiology, genetic etiology, and management of phenylketonuria (PKU). PKU, an autosomal recessive disease, is an inborn error of phenylalanine (Phe) metabolism caused by pathogenic variants in the phenylalanine hydroxylase (PAH) gene. The prevalence of PKU varies widely among ethnicities and geographic regions, affecting approximately 1 in 24,000 individuals worldwide. Deficiency in the PAH enzyme or, in rare cases, the cofactor tetrahydrobiopterin results in high blood Phe concentrations, causing brain dysfunction. Untreated PKU, also known as PAH deficiency, results in severe and irreversible intellectual disability, epilepsy, behavioral disorders, and clinical features such as acquired microcephaly, seizures, psychological signs, and generalized hypopigmentation of skin (including hair and eyes). Severe phenotypes are classic PKU, and less severe forms of PAH deficiency are moderate PKU, mild PKU, mild hyperphenylalaninaemia (HPA), or benign HPA. Early diagnosis and intervention must start shortly after birth to prevent major cognitive and neurological effects. Dietary treatment, including natural protein restriction and Phe-free supplements, must be used to maintain blood Phe concentrations of 120-360 μmol/L throughout the life span. Additional treatments include the casein glycomacropeptide (GMP), which contains very limited aromatic amino acids and may improve immunological function, and large neutral amino acid (LNAA) supplementation to prevent plasma Phe transport into the brain. The synthetic BH4 analog, sapropterin hydrochloride (i.e., Kuvan®, BioMarin), is another potential treatment that activates residual PAH, thus decreasing Phe concentrations in the blood of PKU patients. Moreover, daily subcutaneous injection of pegylated Phe ammonia-lyase (i.e., pegvaliase; PALYNZIQ®, BioMarin) has promised gene therapy in recent clinical trials, and mRNA approaches are also being studied.

Neurotoxicity of phenylalanine on human iPSC-derived cerebral organoids

Phenylketonuria (PKU) is a common genetic metabolic disorder that causes phenylalanine accumulation in the blood. The most serious symptoms are related to the brain, as intellectual disability, seizure, and microcephaly are commonly found in poorly treated PKU patients and the babies of maternal PKU. However, the mechanism of hyperphenylalaninemia on human neurodevelopment is still unclear. Here we utilized human induced pluripotent stem cell (iPSC)-derived cerebral organoids to investigate the neurotoxicity of hyperphenylalaninemia. Cerebral organoids at days 40 or 100 were treated with different concentrations of phenylalanine for 5 days. After phenylalanine treatments, the cerebral organoids displayed alterations in organoid size, induction of apoptosis, and depletion of neural progenitor cells. However, phenylalanine did not have an impact on neurons and glia, including astrocytes, immature oligodendrocytes, and mature oligodendrocytes. Remarkably, a reduction in the thickness of the cortical rosettes and a decrease in myelination at the intermediate zone were inspected with the elevated phenylalanine concentrations. RNA-seq of phenylalanine-treated organoids revealed that gene sets related to apoptosis, p53 signaling pathway, and TNF signaling pathway via NF-kB were enriched in upregulated genes, while those related to cell cycle and amino acid metabolism were enriched in downregulated genes. In addition, there were several microcephaly disease genes, such as ASPM, LMNB1, and CENPE, ranked at the top of the downregulated genes. These findings indicate that phenylalanine exposure may contribute to microcephaly, abnormal cortical expansion, and myelination lesions in the developing human brain.

Engineering Organoids for in vitro Modeling of Phenylketonuria

Phenylketonuria is a recessive genetic disorder of amino-acid metabolism, where impaired phenylalanine hydroxylase function leads to the accumulation of neurotoxic phenylalanine levels in the brain. Severe cognitive and neuronal impairment are observed in untreated/late-diagnosed patients, and even early treated ones are not safe from life-long sequelae. Despite the wealth of knowledge acquired from available disease models, the chronic effect of Phenylketonuria in the brain is still poorly understood and the consequences to the aging brain remain an open question. Thus, there is the need for better predictive models, able to recapitulate specific mechanisms of this disease. Human induced pluripotent stem cells (hiPSCs), with their ability to differentiate and self-organize in multiple tissues, might provide a new exciting in vitro platform to model specific PKU-derived neuronal impairment. In this review, we gather what is known about the impact of phenylalanine in the brain of patients and highlight where hiPSC-derived organoids could contribute to the understanding of this disease.

Phenylketonuria

Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.

Oxidative stress in phenylketonuria-evidence from human studies and animal models, and possible implications for redox signaling

Phenylketonuria (PKU) is one of the commonest inborn error of amino acid metabolism. Before mass neonatal screening was possible, and the success of introducing diet therapy right after birth, the typical clinical finds in patients ranged from intellectual disability, epilepsy, motor deficits to behavioral disturbances and other neurological and psychiatric symptoms. Since early diagnosis and treatment became widespread, usually only those patients who do not strictly follow the diet present psychiatric, less severe symptoms such as anxiety, depression, sleep pattern disturbance, and concentration and memory problems. Despite the success of low protein intake in preventing otherwise severe outcomes, PKU's underlying neuropathophysiology remains to be better elucidated. Oxidative stress has gained acceptance as a disturbance implicated in the pathogenesis of PKU. The conception of oxidative stress has evolved to comprehend how it could interfere and ultimately modulate metabolic pathways regulating cell function. We summarize the evidence of oxidative damage, as well as compromised antioxidant defenses, from patients, animal models of PKU, and in vitro experiments, discussing the possible clinical significance of these findings. There are many studies on oxidative stress and PKU, but only a few went further than showing macromolecular damage and disturbance of antioxidant defenses. In this review, we argue that these few studies may point that oxidative stress may also disturb redox signaling in PKU, an aspect few authors have explored so far. The reported effect of phenylalanine on the expression or activity of enzymes participating in metabolic pathways known to be responsive to redox signaling might be mediated through oxidative stress.

Diffusion kurtosis imaging detects subclinical white matter abnormalities in Phenylketonuria

OBJECTIVE

Phenylketonuria (PKU) is an autosomal recessive disorder whereby deficiencies in phenylalanine metabolism cause progressive neurological dysfunction. Managing PKU is challenging, with disease monitoring focussed on short-term phenylalanine control rather than measures of neuronal damage. Conventional imaging lacks sensitivity, however diffusion kurtosis imaging (DKI), a new MRI method may reveal subclinical white matter structural changes in PKU.

METHODS

This cohort study involved adults with PKU recruited during routine clinical care. MRI, neurocognitive assessment and historical phenylalanine (Phe) levels were collected. A hypothesis-generating case study comparing diet-compliant and non-compliant siblings confirmed that DKI metrics are sensitive to dietary adherence and prompted a candidate metric (K_rad/K_FA ratio). We then tested this metric in a Replication cohort (PKU = 20; controls = 43).

RESULTS

Both siblings scored outside the range of controls for all DKI-based metrics, with severe changes in the periventricular white matter and a gradient of severity toward the cortex. K_rad/K_FA provided clear separation by diagnosis in the Replication cohort (p < 0.001 in periventricular, deep and pericortical compartments). The ratio also correlated negatively with attention (r = -0.51 & -0.50, p < 0.05) and positively with 3-year mean Phe (r = 0.45 & 0.58, p < 0.01).

CONCLUSION

DKI reveals regionally-specific, progressive abnormalities of brain diffusion characteristics in PKU, even in the absence of conspicuous clinical signs or abnormalities on conventional MRI. A DKI-based marker derived from these scores (K_rad/K_FA ratio) was sensitive to cognitive impairment and PKU control over the medium term and may provide a meaningful subclinical biomarker of end-organ damage.

White matter disturbances in phenylketonuria: Possible underlying mechanisms

White matter pathologies, as well as intellectual disability, microcephaly, and other central nervous system injuries, are clinical traits commonly ascribed to classic phenylketonuria (PKU). PKU is an inherited metabolic disease elicited by the deficiency of phenylalanine hydroxylase. Accumulation of l-phenylalanine (Phe) and its metabolites is found in tissues and body fluids in phenylketonuric patients. In order to mitigate the clinical findings, rigorous dietary Phe restriction constitutes the core of therapeutic management in PKU. Myelination is the process whereby the oligodendrocytes wrap myelin sheaths around the axons, supporting the conduction of action potentials. White matter injuries are implicated in the brain damage related to PKU, especially in untreated or poorly treated patients. The present review summarizes evidence toward putative mechanisms driving the white matter pathology in PKU patients.

Tamoxifen promotes differentiation of oligodendrocyte progenitors in vitro

The most promising therapeutic approach to finding the cure for devastating demyelinating conditions is the identification of clinically safe pharmacological agents that can promote differentiation of endogenous oligodendrocyte precursor cells (OPCs). Here we show that the breast cancer medication tamoxifen (TMX), with well-documented clinical safety and confirmed beneficial effects in various models of demyelinating conditions, stimulates differentiation of rat glial progenitors to mature oligodendrocytes in vitro. Clinically applicable doses of TMX significantly increased both the number of CNPase-positive oligodendrocytes and protein levels of myelin basic protein, measured with Western blots. Furthermore, we also found that OPC differentiation was stimulated, not only by the pro-drug TMX-citrate (TMXC), but also by two main TMX metabolites, 4-hydroxy-TMX and endoxifen. Differentiating effects of TMXC and its metabolites were completely abolished in the presence of estrogen receptor (ER) antagonist, ICI182780. In contrast to TMXC and 4-hydroxy-TMX, endoxifen also induced astrogliogenesis, but independent of the ER activation. In sum, we showed that the TMX prodrug and its two main metabolites (4-hydroxy-TMX and endoxifen) promote ER-dependent oligodendrogenesis in vitro, not reported before. Given that differentiating effects of TMX were achieved with clinically safe doses, TMX is likely one of the most promising FDA-approved drugs for the possible treatment of demyelinating diseases.

Unusual Case of Phenylketonuria With Atypical Brain Magnetic Resonance Imaging Findings

Phenylketonuria is a treatable inborn error of amino acid metabolism caused by deficiency of the enzyme phenylalanine hydroxylase, responsible for converting phenylalanine to tyrosine. We report a 10-month-old boy with psychomotor regression and infantile spasms. He was diagnosed with classic phenylketonuria and West syndrome. Treatment was initiated with phenylalanine-restricted diet and vigabatrin. After 5 months of treatment, he persists with developmental delay, severe hypotonia, swallowing disorder, and drug-resistant epilepsy. Brain magnetic resonance imaging showed the typical abnormalities in supratentorial white matter and exceptional infratentorial and basal ganglia compromise. Severity of white matter abnormalities and neurologic symptoms correlates with blood levels of phenylalanine. Infratentorial changes occur in severe cases. Other mechanisms could take part in cases like this with atypical neuroimaging abnormalities of the basal ganglia.

Automatic robust neurite detection and morphological analysis of neuronal cell cultures in high-content screening

Cell-based high content screening (HCS) is becoming an important and increasingly favored approach in therapeutic drug discovery and functional genomics. In HCS, changes in cellular morphology and biomarker distributions provide an information-rich profile of cellular responses to experimental treatments such as small molecules or gene knockdown probes. One obstacle that currently exists with such cell-based assays is the availability of image processing algorithms that are capable of reliably and automatically analyzing large HCS image sets. HCS images of primary neuronal cell cultures are particularly challenging to analyze due to complex cellular morphology. Here we present a robust method for quantifying and statistically analyzing the morphology of neuronal cells in HCS images. The major advantages of our method over existing software lie in its capability to correct non-uniform illumination using the contrast-limited adaptive histogram equalization method; segment neuromeres using Gabor-wavelet texture analysis; and detect faint neurites by a novel phase-based neurite extraction algorithm that is invariant to changes in illumination and contrast and can accurately localize neurites. Our method was successfully applied to analyze a large HCS image set generated in a morphology screen for polyglutamine-mediated neuronal toxicity using primary neuronal cell cultures derived from embryos of a Drosophila Huntington's Disease (HD) model.