Rab GTPases implicated in inherited and acquired disorders

The endocytotic machinery imports, transports and exports receptors and associated molecules between the plasma membrane and various cytoplasmic chambers resulting in selective recycling, degradation, or secretion of molecules and signaling complexes. Trafficking of receptors, growth factors, nutrients, cytokines, integrins as well as pathogens dictates the kinetics and magnitude of signal transduction cascades. Understandably, alterations in the 'fate' of such cargo complexes have profound physiologic and pathophysiologic implications. Rab GTPases regulate endocytosis by decorating intracellular vesicles and targeting these vesicles along with their cargoes to appropriate subcellular compartments. In the last decade, the number of genetic diseases driven by germline mutations in Rab GTPases or their interacting proteins, has increased and there is growing evidence of aberrant Rab GTPase function in acquired pathophysiologies such as immune deficiency, infection, obesity, diabetes and cancer.     

Vitamin B12 absorption: Mammalian physiology and acquired and inherited disorders

Background

Vitamin B12 (cobalamin) is a cobalt-containing compound synthesized by bacteria and an essential nutrient in mammals, which take it up from diet. The absorption and distribution of dietary vitamin B12 to the organism is a complex process involving several gene products including carrier proteins, plasma membrane receptors and transporters. Disturbed cellular entry, transit or egress of vitamin B12 may lead to low vitamin B12 status or deficiency and eventually hematological and neurological disorders.

Objective

The aim of this review is to summarize the causes leading to vitamin B12 deficiency including decreased intake, impaired absorption and increased requirements. Under physiological conditions, vitamin B12 bound to the gastric intrinsic factor is internalized in the ileum by a highly specific receptor complex composed by Cubilin (Cubn) and Amnionless (Amn). Following exit of vitamin B12 from the ileum, general cellular uptake from the circulation requires the transcobalamin receptor CD320 whereas kidney reabsorption of cobalamin depends on Megalin (Lrp2).Whereas malabsorption of vitamin B12 is most commonly seen in the elderly, selective pediatric, nondietary-induced B12 deficiency is generally due to inherited disorders including the Imerslund-Gräsbeck syndrome and the much rarer intrinsic factor deficiency. Biochemical, clinical and genetic research on these disorders considerably improved our knowledge of vitamin B12 absorption.This review describes basic and recent findings on the intestinal handling of vitamin B12 and its importance in health and disease.     

RNA surveillance. Unforeseen consequences for gene expression, inherited genetic disorders and cancer

Messenger RNAs are monitored for errors that arise during gene expression by a mechanism called RNA surveillance, with the result that most mRNAs that cannot be translated along their full length are rapidly degraded. This ensures that truncated proteins are seldom made, reducing the accumulation of rogue proteins that might be deleterious. The pathway leading to accelerated mRNA decay is referred to as nonsense-mediated mRNA decay (NMD). The proteins that catalyze steps in NMD in yeast serve two roles, one to monitor errors in gene expression and the other to control the abundance of endogenous wild-type mRNAs as part of the normal repertoire of gene expression. The NMD pathway has a direct impact on hundreds of genetic disorders in the human population, where about a quarter of all known mutations are predicted to trigger NMD.     

Magnesium homeostasis: Mechanisms and inherited disorders

Mechanisms of magnesium homeostasis intensively studied over the last 10–15 years by means of pathophysiological and molecular genetic approaches have been considered. Impairments of magnesium homeostasis causes the development of magnesium-deficient states, which have been found in many common diseases (diabetes mellitus, cardiovascular diseases, chronic fatigue syndrome, alcoholism, psychiatric and neurologic diseases, etc.), stress condition, effects of some environmental factors as well as therapy with some drugs. Special attention is paid to familial hypomagnesemias caused by genetic defects of magnesium transport systems. The review considers clinical and biochemical characteristics of twelve familial disorders including mechanisms of their development. Deeper understanding of mechanisms of regulation of magnesium homeostasis will results in the development of new approaches in diagnostics, prophylaxis and treatment of magnesium-deficient conditions.     

Adeno-associated virus-mediated antiapoptotic gene delivery: in vivo gene therapy for neurological disorders

Apoptosis is an important mechanism of physiological and pathological cell death and is known to occur in various neurological disorders. Apoptosis is associated with activation of genetic programs in which apoptosis-effector genes promote cell death, thereby opposing repressor genes that enhance cell survival. In this review, we describe various apoptotic pathways, with a special reference to the caspase cascade and discuss the role of individual antiapoptotic factors in various target diseases. Apoptosis could be suppressed by in vivo gene delivery of antiapoptotic factors directly into the central nervous system. The adeno-associated virus (AAV) vector is a good candidate for such gene therapy because it can infect postmitotic neurons. We also describe our in vivo system for overexpression of apoptotic protease activating factor-1 (Apaf-1) caspase recruitment domain as an Apaf1-dominant negative inhibitor (Apaf-1-DN) to regulate the mitochondrial caspase cascade. Apaf-1-DN delivery using an AAV vector system inhibited mitochondrial apoptotic signaling pathway and prevented dopaminergic cell death in a mouse model of Parkinson's disease. Our results suggest that AAV-Apaf-1-DN is potentially useful as an antimitochondrial apoptotic gene therapy for neurodegenerative disorders such as Parkinson's disease.     

Genetic reversion of inherited skin disorders

Human epidermis is a squamous stratified epithelium whose integrity relies on balanced processes of cell attachment, proliferation, and differentiation. In monogenic skin dermatoses, such as mecano-bullous diseases, or DNA repair deficiencies such as the xeroderma pigmentosum (XP), alterations of skin integrity may have devastating consequences as illustrated by the extremely high epidermal cancer proneness of XP patients. The lack of efficient pharmacological treatments, the easy accessibility of skin, and the possibility of long term culture and genetic manipulations ex vivo of epidermal keratinocytes, have encouraged approaches toward gene transfer and skin therapy prospects. We review here some of the human genetic disorders that exhibit major traits in skin, as well as requirements and difficulties inherent to approaches aimed at stable phenotypic correction.     

Rare acquired bleeding disorders

Accurate diagnosis of the cause of bleeding is a prerequisite for determination of the optimal therapeutic response. Clinicians are generally aware of the more prevalent hemorrhagic syndromes but some rare acquired conditions are also of importance. In many of these, inhibitors of coagulation factors or of platelet adhesion/aggregation cause bleeding. These inhibitors are generally, but not always, immunoglobulins. In this review, the less common inhibitors of coagulation and hemostasis, as well as some important but rare nutritional, iatrogenic and disease associated hemorrhagic disorders, are described.     

Zinc deficiency and its inherited disorders -a review

Zinc is an essential trace element required by all living organisms because of its critical roles both as a structural component of proteins and as a cofactor in enzyme catalysis. The importance of zinc in human metabolism is illustrated by the effects of zinc deficiency, which include a diminished immune response, reduced healing and neurological disorders. Furthermore, nutritional zinc deficiency can be fatal in newborn or growing animals. While zinc deficiency is commonly caused by dietary factors, several inherited defects of zinc deficiency have been identified. Acrodermatitis enteropathica is the most commonly described inherited condition found in humans. In several of the few cases that have been reported, this disorder is associated with mutations in the hZIP4 gene, a member of the SLC39 family, whose members encode membranebound putative zinc transporters. Mutations in other members of this family or in different genes may account for other cases of acrodermatitis in which defects in hZIP4 have not been detected. Another inherited form of zinc deficiency occurs in the lethal milk mouse, where a mutation in ZnT4 gene, a member of the SLC30 family of transmembrane proteins results in impaired secretion of zinc into milk from the mammary gland. A similar disorder to the lethal milk mouse occurs in humans. In the few cases studied, no changes in ZnT4 orthologue, hZnT4, were detected. This, and the presence of several minor phenotypic differences between the zinc deficiency in humans and mice, suggests that the human condition is caused by defects in genes that are yet to be identified. Taking into account the fact that there are no definitive tests for zinc deficiency and that this disorder can go undiagnosed, plus the recent identification of multiple members of the SCL30 and SLC39, it is likely that mutations in other genes may underlie additional inherited disorders of zinc deficiency.     

l-carnitine supplementation as a potential antioxidant therapy for inherited neurometabolic disorders

In recent years increasing evidence has emerged suggesting that oxidative stress is involved in the pathophysiology of a number of inherited metabolic disorders. However the clinical use of classical antioxidants in these diseases has been poorly evaluated and so far no benefit has been demonstrated. l-Carnitine is an endogenous substance that acts as a carrier for fatty acids across the inner mitochondrial membrane necessary for subsequent beta-oxidation and ATP production. Besides its important role in the metabolism of lipids, l-carnitine is also a potent antioxidant (free radical scavenger) and thus may protect tissues from oxidative damage. This review addresses recent findings obtained from patients with some inherited neurometabolic diseases showing that l-carnitine may be involved in the reduction of oxidative damage observed in these disorders. For some of these diseases, reduced concentrations of l-carnitine may occur due to the combination of this compound to the accumulating toxic metabolites, especially organic acids, or as a result of protein restricted diets. Thus, l-carnitine supplementation may be useful not only to prevent tissue deficiency of this element, but also to avoid oxidative damage secondary to increased production of reactive species in these diseases. Considering the ability of l-carnitine to easily cross the blood–brain barrier, l-carnitine supplementation may also be beneficial in preventing neurological damage derived from oxidative injury. However further studies are required to better explore this potential.     

The role of amino acid transporters in inherited and acquired diseases

Amino acids are essential building blocks of all mammalian cells. In addition to their role in protein synthesis, amino acids play an important role as energy fuels, precursors for a variety of metabolites and as signalling molecules. Disorders associated with the malfunction of amino acid transporters reflect the variety of roles that they fulfil in human physiology. Mutations of brain amino acid transporters affect neuronal excitability. Mutations of renal and intestinal amino acid transporters affect whole-body homoeostasis, resulting in malabsorption and renal problems. Amino acid transporters that are integral parts of metabolic pathways reduce the function of these pathways. Finally, amino acid uptake is essential for cell growth, thereby explaining their role in tumour progression. The present review summarizes the involvement of amino acid transporters in these roles as illustrated by diseases resulting from transporter malfunction.     

Dominantly inherited,non-coding microsatellite expansion disorders

Dominantly inherited diseases are generally caused by mutations resulting in gain of function protein alterations. However, a CTG expansion located in the 3' untranslated portion of a kinase gene was found to cause myotonic dystrophy type 1, a multisystemic dominantly inherited disorder. The recent discovery that an untranslated CCTG expansion causes the same constellation of clinical features in myotonic dystrophy type 2 (DM2), along with other recent discoveries on DM1 pathogenesis, have led to the understanding that both DM1 and DM2 mutations are pathogenic at the RNA level. These findings indicate the existence of a new category of disease wherein repeat expansions in RNA alter cellular function. Pathogenic repeat expansions in RNA may also be involved in spinocerebellar ataxia types 8, 10 and 12, and Huntington's disease-like type 2.     

Ezetimibe: A biomarker for efficacy of liver directed UGT1A1 gene therapy for inherited hyperbilirubinemia

As recently demonstrated in patients with factor IX deficiency, adeno-associated virus (AAV)-mediated liver-directed therapy is a viable option for inherited metabolic liver disorders. Our aim is to treat Crigler-Najjar syndrome type I (CN I), an inherited severe unconjugated hyperbilirubinemia, as a rare recessive inherited disorder. Because the number of patients eligible for this approach is small, the efficacy can only be demonstrated by a beneficial effect on the pathophysiology in individual patients. Serum bilirubin levels in potential candidates have been monitored since birth, providing an indication of their pathophysiology. Adjuvant phototherapy to prevent brain damage reduces serum unconjugated bilirubin (UCB) levels in CN I patients to the level seen in the milder form of the disease, CN type II. This therapy increases the excretion of UCB, thereby complicating the use of UCB and conjugated bilirubin levels in serum as biomarkers for the gene therapy we try to develop. Therefore, a suitable biomarker that is not affected by phototherapy is currently needed. To this end, we have investigated whether estradiol, ethinylestradiol or ezetimibe could be used as markers for uridine 5'-di-phospho-glucuronosyltransferase isoform 1A1 (UGT1A1) activity restored by AAV gene therapy in Gunn rats, a relevant animal model for CN I. Of these compounds, ezetimibe appeared most suitable because its glucuronidation rate in untreated control Gunn rats is low. Subsequently, ezetimibe glucuronidation was studied in both untreated and AAV-treated Gunn rats and the results suggest that it may serve as a useful serum marker for restored hepatic UGT1A1 activity.     

Unstable mutations, new challenges for genetic counseling of inherited disorders

Unstable mutations or amplification of DNA tandem repeats sequences constitute a new kind of genetic alteration discovered in the 90's that cause hereditary diseases. This mutation has been found inside or near important genes involved in the normal neurological function in human beings. In some cases, the presence of the amplification causes altered expression of the genes, their inactivation or the synthesis of a protein with new functions. Some common characteristics of these diseases are that they affect the central nervous system and are degenerative in nature. Most of them show genetic anticipation meaning that the severity of the manifestations increases in each generation and appear at an earlier age. In most cases, the severity of the symptoms is positively correlated with the size of the amplification. Twenty illnesses caused by this kind of mutations have been identified so far. Briefly, this work reviews the current knowledge about this topic.     

Cell therapy for inherited diseases of the hematopoietic system

Cell therapy was born in 1968 with the first allogeneic transplantation of hematopoietic stem cells for two immune deficiency disorders: the Wiskott-Aldrich syndrome and the Severe Combined ImmunoDeficiency (SCID). From this pioneering experience, thousands of patients affected with inherited or acquired diseases of the hematopoietic system have benefited from this therapeutic approach. Unfortunately, immunologic obstacles, represented by the compatibility in the major histocompatibility HLA system, still dictate today important limitations for a larger therapeutic utilization of hematopoietic stem cells (HSC). In this review, we have summarized the difficulties and the scientific advances leading us to improve the clinical results; the therapeutic research's track for primary immunodeficiencies is also discussed.     

Retinal ganglion cell neurodegeneration in mitochondrial inherited disorders

Since the early days of mitochondrial medicine, it has been clear that optic atrophy is a very common and sometimes the singular pathological feature in mitochondrial disorders. The first point mutation of mitochondrial DNA (mtDNA) associated with the maternally inherited blinding disorder, Leber's hereditary optic neuropathy (LHON), was recognized in 1988. In 2000, the other blinding disorder, dominant optic atrophy (DOA) Kjer type, was found associated with mutations in the nuclear gene OPA1 that encodes a mitochondrial protein. Besides these two non-syndromic optic neuropathies, optic atrophy is a prominent feature in many other neurodegenerative diseases that are now recognized as due to primary mitochondrial dysfunction.We will consider mtDNA based syndromes such as LHON/dystonia/Mitochondrial Encephalomyopahty Lactic Acidosis Stroke-like (MELAS)/Leigh overlapping syndrome, or nuclear based diseases such as Friedreich ataxia (mutations in FXN gene), deafness-dystonia-optic atrophy (Mohr-Tranebjerg) syndrome (mutations in TIMM8A), complicated hereditary spastic paraplegia (mutations in SPG7), DOA “plus” syndromes (mutations in OPA1), Charcot-Marie-Tooth type 2A (CMT2A) with optic atrophy or hereditary motor and sensory neuropathy type VI (HMSN VI) (mutations in MFN2), and Costeff syndrome and DOA with cataract (mutations in OPA3). Thus, genetic errors in both nuclear and mitochondrial genomes often lead to retinal ganglion cell death, a specific target for mitochondrial mediated neurodegeneration. Many mechanisms have been studied and proposed as the bases for the pathogenesis of mitochondrial optic neuropathies including bioenergetic failure, oxidative stress, glutamate toxicity, abnormal mitochondrial dynamics and axonal transport, and susceptibility to apoptosis.     

Gene therapy for lipid disorders

Lipid disorders are associated with atherosclerotic vascular disease, and therapy is associated with a substantial reduction in cardiovascular events. Current approaches to the treatment of lipid disorders are ineffective in a substantial number of patients. New therapies for refractory hypercholesterolemia, severe hypertriglyceridemia, and low levels of high-density lipoprotein cholesterol are needed: somatic gene therapy is one viable approach. The molecular etiology and pathophysiology of most of the candidate diseases are well understood. Animal models exist for the diseases and in many cases preclinical proof-of-principle studies have already been performed. There has been progress in the development of vectors that provide long-term gene expression. New clinical gene therapy trials for lipid disorders are likely to be initiated within the next few years.