Current Molecular Medicine - Volume 14, Issue 8, 2014

Neurogenetics presents research that contributes to better understanding of the genetic basis of normal and abnormal function of the nervous system. According to the American Academy of Neurology, Section of Neurogenetics, “Genetic susceptibility involves nearly every branch of neurology, including a wide range of conditions ranging from multiple sclerosis, epilepsy, dementia, sleep, neuromuscular disease and movement disorders, to childhood disorders, such as autism, intellectual disability and language based learning disabilities. Genetic testing for "causal" genes as well as susceptibility genes is becoming more prevalent and there is a need for section engagement along with multiple policy domains.” The clinical practice of neurogenetics is complex, challenging, and rewarding. However, with the evolution of molecular biology techniques, the impact of the genetic etiology in neurology is becoming crucial for multiple aspects: prenatal diagnosis, genetic counselling, therapeutic strategies and so on. The last 25 years have been the golden age of genetics, with the disease genes responsible for many genetic neurological disorders are now identified. The molecular age of neurogenetics began in 1987, with the description of the genetic basis of Duchenne muscular dystrophy. Since then, we have known hundreds different genes causing all kinds of neurological diseases, with childhood or adulthood onset, with central and/or periheral nervous system involvement. It is a fact that, with the advent of next generation sequencing, most, if not all, causative genes will be identified in the next few years. The Neurological Institute of the University of Pisa, in collaboration with the Neurogenetics Group of the Italian Society of Neurology, had organised a scientific meeting in May, 2013 in Pisa, Italy. Leading scientists from Italy gathered to discuss recent progress in the field of Neurogenetics. It has been a great experience to serve as the Guest Editor for this special thematic issue of Current Molecular Medicine. I hope the publication of the proceedings of this meeting would give some momentum in advancing the science of Neurogenetics. I would like to express my special gratitude to the Editorial Board of the Current Molecular Medicine, and to Professor David W. Li in particular, for offering us this unique opportunity. I also deeply thank all the contributing authors of this issue for their timely and superb efforts in composing this monograph. Finally, my very special thanks to my unequalled teacher Prof. Salvatore ‘Billi’ DiMauro, for his contagious passion for mitochondrial diseases and neurogenetics, and for always treating his professional team as family.

Limb girdle muscular dystrophies (LGMD) are a heterogeneous group of inherited progressive muscle disorders affecting predominantly the shoulder and pelvic girdle muscles. They present both with autosomal dominant and autosomal recessive patterns of inheritance. Recent development, including results from Next Generation Sequencing technology, expanded the number of recognised forms. Therefore a revised genetic classification that takes into account the novel entities is needed, allowing clinicians and researchers to refer to a common nomenclature for diagnostic and research purposes.

Inherited white matter (WM) disorders include a heterogenous group of disorders affecting brain white matter and associated with myelin, axonal and glial cells or vascular pathology. Often a wide range of overlapping neurological manifestations possibly associated with variable systemic involvement are found in these disorders making clinical diagnosis challenging. Advances in molecular genetics enabled the identification of the responsible genes of an increasing number of previously undefined forms. This review focuses on genetic leukoencephalopathies with exclusive adulthood presentation, most of which have an autosomal dominant inheritance. The most common forms are related to vascular pathology, such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL), COL4A1-related leukoencephalopathy, retinal vasculopathy with cerebral leukodystrophy (RVCL), and polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL). Also cerebroretinal microangiopathy with cysts and calcifications (CRMCC), which presents a prevalent infantile onset, will be detailed because of the vascular based myelin damage and the recent genetic characterization. Other adult onset (AO) leukoencephalopathies, such as the recently genetically defined hereditary diffuse leukoencephalopathy with axonal spheroids (HDLS), adult-onset autosomal dominant leukodystrophy (ADLD) due to LMNB1 duplication, adult polyglucosan body disease (APBD), and fragile X-associated tremor/ataxia syndrome (FXTAS) will be detailed shortly. Short notes on the clinical and MRI features of late onset variants of the classical infantile-onset leukodystrophies mostly related to metabolic disorders will also be given. Finally, palliative, curative and experimental treatment options are here summarized.

Metabolic ataxias are rare. They usually start in the childhood and often have autosomal recessive inheritance. They may also present in adulthood. The diagnosis is important since some patients may be successfully managed with diet and treatments.

Glycogenosis II (GSDII) is an autosomal recessive lysosomal storage disorder resulting from acid alpha-glucosidase (GAA) deficiency, subsequent lysosomal accumulation of glycogen in muscles, impairment of autophagic processes and progressive cardiac, motor and respiratory failure. The infantile form usually appears in the first month of life, progresses rapidly and presents with severe cardiac involvement and complete deficiency of alpha-glucosidase activity (< 1% of normal controls). The late-onset form is characterized by great variability of the phenotypical spectrum. Main findings are muscle weakness and severe respiratory insufficiency while cardiac involvement may be completely absent. Residual GAA enzyme activity may correlate with severity of phenotype but many adult patients sharing the same mutations present with a wide variability in residual enzyme activity, age of onset and rate of disease progression, thus supporting a role for other factors, i.e., post-translational modifications and modifier genes, in modulating disease presentation. Enzyme replacement therapy (ERT) with alglucosidase alfa stabilizes the disease or improves muscle and/or respiratory function. However, efficacy of ERT may be influenced by several factors including age when ERT begins, extent of muscle damage, degree of defective autophagy, diversity in muscle fiber composition, difficulties in delivery of the therapeutic agent and antibody production. Further studies should be warranted to investigate factors determining the differences in clinical expression and therapeutic response in order to achieve better clinical and therapeutic management of these patients.

Stroke is a complex disease resulting from the interplay of genetics and environment. In some instances (mainly in young adults) stroke is the direct result of a monogenic disease. Among the monogenic causes of stroke, the diseases which are most frequently encountered in the adult general neurological practice are CADASIL, Fabry and mitochondrial diseases. Brain MRI and clinical features may frequently lead to a correct molecular diagnosis. Here we review the single-gene causes of ischemic stroke, with special regard to the associated features which may help in the diagnostic approach.

Over two decades have elapsed since the first mtDNA point mutation was associated with Leber’s hereditary optic neuropathy (LHON) in 1988. We have subsequently witnessed a substantial understanding of the molecular basis of hereditary optic neuropathies, as well as of their clinical features and pathogenic mechanisms. It became clear that the large majority of genetic optic neuropathies have a primary or an indirect involvement of mitochondrial functions, justifying the definition of “mitochondrial optic neuropathies”. Despite this progress many unsolved features remain to be understood, such as incomplete penetrance and variable clinical expressivity in LHON and dominant optic atrophy (DOA), gender prevalence in LHON, and complex gene/environment interactions in both LHON and DOA. The most recent advancement in our understanding of the molecular basis of mitochondrial optic neuropathies is the topic of this review. In particular, we analyze the role that mitochondrial biogenesis may play in the compensatory mechanisms that underlie incomplete penetrance and clinical expressivity, a scenario relevant for the possible design of future therapeutic approaches.

The genetics of neurodegenerative diseases has an important role to clarify the pathogenetic mechanism, the diagnosis and finally the therapeutic and ethical implications. Moreover, the genetic approach to the study of the main clinical forms of dementia (Alzheimer’s disease-AD and Frontotemporal Dementia-FTD) suggests clinical guidelines for helping families to navigate through these complexities. AD and FTD are multifactorial, genetically complex diseases involving many candidate genes. Mutations in three genes (i.e. Amyloid Precursor Protein, APP; presenilin 1, PSEN1; presenilin 2, PSEN2) have been linked to 50% of all familial forms of AD (FAD). Genome wide association studies (GWAS) have involved an increasing number of genes with a possible role in the disease pathogenesis. Up to now, the genetics of familial forms of FTD is related to 7 genes: the microtubule-associated protein tau (MAPT) progranulin (GRN), the valosin-containing protein (VCP), chromatin-modifying 2B (CHMP2B), the TARDNA binding protein 43 encoding gene (TARBDP), fused in sarcoma (FUS) and the last hexanucleotide expansion repeats in the open reading frame of chromosome 9 (C9orf72). Pre-test counseling and the identification of genetic defects are important in both patients and asymptomatic at risk family members.

Mitochondrial Disorders (MD) include a heterogeneous group of inherited disorders due to molecular defects mainly affecting the mitochondrial oxidative phosphorylation system. Because the respiratory chain is under control of two different genomes (nuclear DNA-nDNA and mitochondrial DNA-mtDNA), mitochondrial genetics is quite complex and may justify the extreme clinical heterogeneity of these diseases. Clinically, MD usually involve multiple tissues, mainly affecting organs with high energy request as central nervous system and skeletal muscle. They may present at any age, with different onsets, clinical presentation and progression from an isolated involvement of vision or hearing to a multisystemic degenerative disorders with stroke-like episodes, peripheral neuropathy, ophthalmoparesis, seizures, cardiopathy, hepatopathy, endocrinopathies, etc. Over the last 50 years, it became evident that MD represent an important part of the general medicine. The complexity of clinical and genetic spectrum of those disorders is still increasing. The aim of this review is to walk through mitochondrial genetics, highlighting novel clinical entities.

Charcot-Marie-Tooth disease (CMT) and related neuropathies are a genetically highly heterogeneous group of neurodegenerative disorders. CMT affects both the sensory and motor nerves, distal Hereditary Motor Neuropathies (dHMN) are phenotypically similar disorders involving only motor nerves, while Hereditary Sensory and Autonomic Neuropathies (HSAN) are rare distinct disorders affecting sensory and sometimes autonomic nerves. Almost 70 genes have been identified as responsible for these disorders. It is astonishing to learn how diverse are the cellular sublocalisation and the functional roles of the encoded proteins of CMT-associated genes which all lead to similar disorders of the peripheral nervous system. Myelin formation and maintenance, mitochondrial dynamics, cytoskeleton organization, axonal transport, and vesicular trafficking are the most frequently involved pathways. However, dysfunction of several activities from the nucleus to the neuromuscular junction forms the basis for these hereditary neuropathies, making it challenging predicting the functions of newly identified mutated genes. In this review we will discuss the function and related phenotypes of all the genes thus far associated with CMT, dHMN, and HSAN.

The hereditary spastic paraplegias (HSP) are characterized by spastic gait with weakness in the legs and additional neurological or extra-neurological signs in "complicated" forms. The past two decades have witnessed major advances in our understanding of their molecular bases with the identification of a plethora of loci and the cloning of several SPG genes. Combined genetic and clinical information has permitted a modern, molecularly-driven classification and an improved diagnosis, with several new data on the possible disease mechanisms. Further heterogeneity will rapidly emerge with the diffusion of next-generation sequencing platforms and, under the shadow of common themes in the pathogenesis, new therapeutic options will likely emerge for a great number of patients.

Neuronal Ceroid Lipofuscinoses (NCL) are genetically heterogeneous heritable neurodegenerative disorders with worldwide distribution. They are considered as childhood diseases; however rare adult onset forms are known. NCL have a progressive course, affecting visual, motor and cognitive functions, and are associated with myoclonic epilepsy; behavioural problems can be observed at the onset. The outcome is invariably fatal, mostly during the second or third decade. The denomination is based on pathological criteria, i.e. the presence of intralysosomal storage of autofluorescent lipopigment of glycoprotein origin with characteristic ultrastructural features. The NCL are autosomal recessive diseases (but a rare autosomal dominant form of adult onset). Thirteen NCL associated genes have been identified so far, which allow a definite diagnosis to be reached and provide genetic counselling to the families. Still unidentified NCL genes are foreseen. Allelic heterogeneity is observed in some mutated genes; likewise phenotypic heterogeneity is seen in several NCL. The gene products are either soluble proteins (such as lysosomal enzymes) or membrane proteins related to lysosomes, endoplasmic reticulum, synaptic vesicles. Little is known about pathogenetic mechanisms, leading to storage formation and cell death. Current research is focusing on intracellular trafficking, neurotransmission and storage removal. No cure is available for any form. Innovative treatments led to some results in mouse models related to lysosome hydrolase defects. Evidences that autophagy, oxidative stress, excitotoxicity play roles in NCL cell pathology raise the possibility that selected steps of these processes might become target of treatments, and therefore modify the disease course.

Facioscapulohumeral muscular dystrophy (FSHD) has been classified as an autosomal dominant myopathy, linked to rearrangements in an array of 3.3 kb tandemly repeated DNA elements (D4Z4) located at the 4q subtelomere (4q35). For the last 20 years, the diagnosis of FSHD has been confirmed in clinical practice by the detection of one D4Z4 allele with a reduced number (≤8) of repeats at 4q35. Although wide inter- and intra-familial clinical variability was found in subjects carrying D4Z4 alleles of reduced size, this DNA testing has been considered highly sensitive and specific. However, several exceptions to this general rule have been reported. Specifically, FSHD families with asymptomatic relatives carrying D4Z4 reduced alleles, FSHD genealogies with subjects affected with other neuromuscular disorders and FSHD affected patients carrying D4Z4 alleles of normal size have been described. In order to explain these findings, it has been proposed that the reduction of D4Z4 repeats at 4q35 could be pathogenic only in certain chromosomal backgrounds, defined as “permissive” specific haplotypes. However, our most recent studies show that the current DNA signature of FSHD is a common polymorphism and that in FSHD families the risk of developing FSHD for carriers of D4Z4 reduced alleles (DRA) depends on additional factors besides the 4q35 locus. These findings highlight the necessity to re-evaluate the significance and the predictive value of DRA, not only for research but also in clinical practice. Further clinical and genetic analysis of FSHD families will be extremely important for studies aiming at dissecting the complexity of FSHD.

Mitochondrial disorders are a group of heterogeneous diseases associated with abnormalities of the oxidative phosphorylation (OXPHOS), the most important source of energy for the cell. The number of mitochondrial syndromes and of identified causative genes is constantly increasing. Taken as a whole they are among the most frequent genetic diseases in humans at any age. The respiratory chain is the only metabolic pathway under double genome control and molecular genetics of these disorders is complicated by the existence of strict interactions between mitochondrial DNA and nuclear DNA. In childhood and infancy, clinical presentation differs from mitochondrial disorders with adult onset. The phenotypes are much more severe, often involving brain, frequently presenting as multisystemic disorders and seldom as isolated myopathy. Mutations in nDNA are more frequent than in adulthood. The major phenotypes presenting in infancy are here correlated with genetic defects and biochemical data with the aim to facilitate diagnosis work-up.

Parkinson Disease (PD) is a common neurodegenerative disorder of intricate etiology, caused by progressive loss of aminergic neurons and accumulation of Lewy bodies. The predominant role of genetics in the etiology of the disease has emerged since the identification of the first pathogenetic mutation in SNCA (alpha-synuclein) gene, back in 1997. Mendelian parkinsonisms, a minority among all PD forms, have been deeply investigated, with 19 loci identified. More recently, genome wide association studies have provided convincing evidence that variants in some of these genes, as well as in other genes, may confer an increased risk for late onset, sporadic PD. Moreover, the finding that heterozygous mutations in the GBA gene (mutated in Gaucher disease) are among the strongest genetic susceptibility factors for PD, has widened the scenario of PD genetic background to enclose a number of genes previously associated to distinct disorders, such as genes causative of spinocerebellar ataxias, mitochondrial disorders and fragile X syndrome. At present, the genetic basis of PD defines a continuum from purely mendelian forms (such as those caused by autosomal recessive genes) to multifactorial inheritance, resulting from the variable interplay of many distinct genetic variants and environmental factors.

In the last twenty years the rapid advances in neurogenetic have revolutionized not only the molecular, pathological, inheritance but also the clinical concept of ALS. Here we review the current genetic breakthrough in familial and sporadic ALS, considering how this knowledge has allowed widening of the scenario on the possible pathogenic disease mechanisms and better understanding of the relationship between the genetic, pathological and clinical subtypes.