Congenital hydranencephalic-hydrocephalic syndrome associated with mitochondrial dysfunction

An Analysis of the Neurological and Molecular Alterations Underlying the Pathogenesis of Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by amyloid beta (Aβ) plaques, neurofibrillary tangles, and neuronal loss. Unfortunately, despite decades of studies being performed on these histological alterations, there is no effective treatment or cure for AD. Identifying the molecular characteristics of the disease is imperative to understanding the pathogenesis of AD. Furthermore, uncovering the key causative alterations of AD can be valuable in developing models for AD treatment. Several alterations have been implicated in driving this disease, including blood–brain barrier dysfunction, hypoxia, mitochondrial dysfunction, oxidative stress, glucose hypometabolism, and altered heme homeostasis. Although these alterations have all been associated with the progression of AD, the root cause of AD has not been identified. Intriguingly, recent studies have pinpointed dysfunctional heme metabolism as a culprit of the development of AD. Heme has been shown to be central in neuronal function, mitochondrial respiration, and oxidative stress. Therefore, dysregulation of heme homeostasis may play a pivotal role in the manifestation of AD and its various alterations. This review will discuss the most common neurological and molecular alterations associated with AD and point out the critical role heme plays in the development of this disease.

Expanding the clinical spectrum of Fowler syndrome: Three siblings with survival into adulthood and systematic review of the literature

Proliferative vasculopathy and hydranencephaly-hydrocephaly syndrome (PVHH, OMIM 225790), also known as Fowler syndrome, is a rare autosomal recessive disorder of brain angiogenesis. PVHH has long been considered to be prenatally lethal. We evaluated the phenotypes of the first three siblings with survival into adulthood, performed a systematic review of the Fowler syndrome literature and delineated genotype-phenotype correlations using a scoring system to rate the severity of the disease. Thirty articles were included, describing 69 individual patients. To date, including our clinical reports, 72 patients have been described with Fowler syndrome. Only 6/72 (8%) survived beyond birth. Although our three patients carry the same mutations (c.327T>A-p.Asn109Lys and c.887C>T-p.Ser296Leu) in FLVCR2, only two of them presented with the same cerebral features, ventriculomegaly and cerebral calcifications, as affected fetuses. The third sibling has a surprisingly milder clinical and radiological phenotype, suggesting intrafamilial variability. Although no clear phenotype-genotype correlation exists, some variants appear to be associated with a less severe phenotype compatible with life. As such, it is important to consider Fowler syndrome in patients with gross ventriculomegaly, cortical malformations and/or cerebral calcifications on brain imaging.

Unraveling the Role of Heme in Neurodegeneration

Heme (iron-protoporphyrin IX) is an essential co-factor involved in several biological processes, including neuronal survival and differentiation. Nevertheless, an excess of free-heme promotes oxidative stress and lipid peroxidation, thus leading to cell death. The toxic properties of heme in the brain have been extensively studied during intracerebral or subarachnoid hemorrhages. Recently, a growing number of neurodegenerative disorders have been associated to alterations of heme metabolism. Hence, the etiology of such diseases remains undefined. The aim of this review is to highlight the neuropathological role of heme and to discuss the major heme-regulated pathways that might be crucial for the survival of neuronal cells. The understanding of the molecular mechanisms linking heme to neurodegeneration will be important for therapeutic purposes.

Mitochondrial Targeting in Neurodegeneration: A Heme Perspective

Mitochondrial dysfunction has achieved an increasing interest in the field of neurodegeneration as a pathological hallmark for different disorders. The impact of mitochondria is related to a variety of mechanisms and several of them can co-exist in the same disease. The central role of mitochondria in neurodegenerative disorders has stimulated studies intended to implement therapeutic protocols based on the targeting of the distinct mitochondrial processes. The review summarizes the most relevant mechanisms by which mitochondria contribute to neurodegeneration, encompassing therapeutic approaches. Moreover, a new perspective is proposed based on the heme impact on neurodegeneration. The heme metabolism plays a central role in mitochondrial functions, and several evidences indicate that alterations of the heme metabolism are associated with neurodegenerative disorders. By reporting the body of knowledge on this topic, the review intends to stimulate future studies on the role of heme metabolism in neurodegeneration, envisioning innovative strategies in the struggle against neurodegenerative diseases.

Heme and FLVCR-related transporter families SLC48 and SLC49

Heme is critical for a variety of cellular processes, but excess intracellular heme may result in oxidative stress and membrane injury. Feline leukemia virus subgroup C receptor (FLVCR1), a member of the SLC49 family of four paralogous genes, is a cell surface heme exporter, essential for erythropoiesis and systemic iron homeostasis. Disruption of FLVCR1 function blocks development of erythroid progenitors, likely due to heme toxicity. Mutations of SLC49A1 encoding FLVCR1 are noted in patients with a rare neurodegenerative disorder: posterior column ataxia with retinitis pigmentosa. FLVCR2 is highly homologous to FLVCR1 and may function as a cellular heme importer. Mutations of SLC49A2 encoding FLVCR2 are observed in Fowler syndrome, a rare proliferative vascular disorder of the brain. The functions of the remaining members of the SLC49 family, MFSD7 and DIRC2 (encoded by the SLC49A3 and SLC49A4 genes), are unknown, although the latter is implicated in hereditary renal carcinomas. SLC48A1 (heme responsive gene-1, HRG-1), the sole member of the SLC48 family, is associated with the endosome and appears to transport heme from the endosome into the cytosol.

The Fowler syndrome-associated protein FLVCR2 is an importer of heme

Mutations in FLVCR2, a cell surface protein related by homology and membrane topology to the heme exporter/retroviral receptor FLVCR1, have recently been associated with Fowler syndrome, a vascular disorder of the brain. We previously identified FLVCR2 to function as a receptor for FY981 feline leukemia virus (FeLV). However, the cellular function of FLVCR2 remains unresolved. Here, we report the cellular function of FLVCR2 as an importer of heme, based on the following observations. First, FLVCR2 binds to hemin-conjugated agarose, and binding is competed by free hemin. Second, mammalian cells and Xenopus laevis oocytes expressing FLVCR2 display enhanced heme uptake. Third, heme import is reduced after the expression of FLVCR2-specific small interfering RNA (siRNA) or after the binding of the FY981 FeLV envelope protein to the FLVCR2 receptor. Finally, cells overexpressing FLVCR2 are more sensitive to heme toxicity, a finding most likely attributable to enhanced heme uptake. Tissue expression analysis indicates that FLVCR2 is expressed in a broad range of human tissues, including liver, placenta, brain, and kidney. The identification of a cellular function for FLVCR2 will have important implications in elucidating the pathogenic mechanisms of Fowler syndrome and of phenotypically associated disorders.

A near infrared spectroscopy study investigating oxygen utilisation in hydrocephalic rats

Determination of hydrocephalus and its severity is important for optimal management of the condition. We have used near infrared spectroscopy (NIRS) to assess changes in concentrations of oxygenated (O2Hb), deoxygenated (HHb), total haemoglobin (tHb) and cytochrome c oxidase (Caa3) in normal and hydrocephalic Texas (HTx) rats in response to a 5 min head down tilt and a sodium pentobarbitone (NaPB) challenge. The former was used to test vascular responses and the latter to test metabolic responses. The haemoglobin oxygenation index (HbD) was derived which provides information regarding oxygen utilisation ([HbD]=[O2Hb]-[HHb]). With the tilt challenge, a significant (P=0.001) difference was observed in [HbD] between normal (n=24) and hydrocephalic (n=14) rats (-3.50 (-6.00 to 0.00) microM cm(-1 )and 7.50 (0.75 to 14.25) microM cm(-1), respectively). In another experiment we tested the response of ten rats to NaPB administration and observed a significant difference (P=0.008) in [Caa3] between normal (n=5) and hydrocephalic (n=5) rats (-6.60 (-7.55 to -5.50) microM cm(-1 )and -2.20 (-5.60 to -1.05) microM cm(-1), respectively). Coronal sections of these ten rat brains were analysed and significant (P<0.05) relationships were found between some of the NIRS parameters and cortical thickness or lateral ventricle area measurements. Our studies demonstrate that a significant difference in cerebral oxygenation and haemodynamics can be observed between normal and hydrocephalic HTx rats using NIRS.

Epidemiology of pediatric mitochondrial respiratory chain disorders in northwest Spain

Our knowledge of mitochondrial respiratory chain diseases has increased dramatically in recent years, but relatively little information is available about their prevalence and incidence, either in pediatric or adult patients. This study reports incidence and prevalence estimates, and summarizes the clinical, biochemical, histologic, and genetic characteristics of 51 patients age 0-16 years. The overall annual incidence of all mitochondrial respiratory chain diseases was estimated to be 1.43 cases per 10(5) in the population as a whole, and 2.85 cases per 10(5) in the under-6 population. The overall prevalence of all mitochondrial respiratory chain diseases was estimated as 7.5 cases per 10(5) in the under-19 population, and 8.7 cases per 10(5) in the under-16 population. These incidence and prevalence estimates are higher than in most previous studies of pediatric populations. Estimated prevalences of specific mitochondrial respiratory chain diseases were 2.05 cases per 10(5) for Leigh syndrome, 0.68 per 10(5) for mitochondrial deoxyribonucleic acid (mtDNA) deletions and deletions-duplications, 1.59 per 10(5) for mtDNA depletions, and 0.45 per 10(5) for mtDNA point mutations. Leigh syndrome was the most frequent clinical syndrome. The estimates of the prevalences of mtDNA deletions, deletions-duplications, and point mutations set forth here are lower than in similar previous studies, whereas the estimate of the prevalence of mtDNA depletions is rather higher. Sixteen of these patients manifested phenotypic syndromes that have not been previously reported in association with mitochondrial respiratory chain diseases.

Congenital hydranencephalic-hydrocephalic syndrome with proliferative vasculopathy: a possible relation with mitochondrial dysfunction

We report the case of a fetus aborted at gestation week 20 because of hydranencephalic-hydrocephalic syndrome. The fetus was the third pregnancy of a nonconsanguineous couple whose first child exhibited congenital hydranencephalic-hydrocephalic syndrome associated with muscle histology findings consistent with mitochondrial cytopathy and deficiency of complexes III and IV of the respiratory chain and whose second pregnancy had terminated in an elective abortion on detection of progressive hydrocephalus at gestation week 19. The third pregnancy had a normal course according to obstetric and ultrasonography examinations performed at gestation weeks 5, 10, and 15, and negative results were obtained in standard serologic and polymerase chain reaction (PCR) tests for prenatal infections of the mother. However, the ultrasonography examination at gestation week 18 revealed hydrocephalus, in response to which the parents requested an abortion, which was performed at gestation week 20; the fetus was male and with no evident external malformations. Histopathologic studies of the brain and medulla oblongata revealed proliferative vasculopathy (glomeruloid vessels, intracytoplasmic inclusions, and microcalcifications) and intracytoplasmic inclusions in the voluntary muscle. Microbiologic and PCR tests of hepatic and spleen tissue were negative for prenatal infections. In view of the precedent of a sister with mitochondrial dysfunction, these findings raise the pos sibility that at least some cases of familial syndrome of congenital hydranencephalic-hydrocephalic syndrome with proliferative vasculopathy can be attributed to alterations in the mitochondrial respiratory chain.