Prader-Willi and Angelman syndromes: update on genetic mechanisms and diagnostic complexities

Potential therapeutic approaches for Angelman syndrome

INTRODUCTION

Angelman syndrome (AS) is a neurodevelopmental disorder caused by deficiency of maternally inherited UBE3A, an ubiquitin E3 ligase. Despite recent progress in understanding the mechanism underlying UBE3A imprinting, there is no effective treatment. Further investigation of the roles played by UBE3A in the central nervous system (CNS) is needed for developing effective therapies.

AREA COVERED

This review covers the literature related to genetic classifications of AS, recent discoveries regarding the regulation of UBE3A imprinting, alterations in cell signaling in various brain regions and potential therapeutic approaches. Since a large proportion of AS patients exhibit comorbid autism spectrum disorder (ASD), potential common molecular bases are discussed.

EXPERT OPINION

Advances in understanding UBE3A imprinting provide a unique opportunity to induce paternal UBE3A expression, thus targeting the syndrome at its 'root.' However, such efforts have yielded less-than-expected rescue effects in AS mouse models, raising the concern that activation of paternal UBE3A after a critical period cannot correct all the CNS defects that developed in a UBE3A-deficient environment. On the other hand, targeting abnormal downstream cell signaling pathways has provided promising rescue effects in preclinical research. Thus, combined reinstatement of paternal UBE3A expression with targeting abnormal signaling pathways should provide better therapeutic effects.

Epigenetic epidemiology in psychiatry: A translational neuroscience perspective

Accumulating evidence from the field of neuroscience indicates a crucial role for epigenetic regulation of gene expression in development and aging of nervous system and suggests that aberrations in the epigenetic machinery are involved in the etiology of psychiatric disorders. Epidemiologic evidence on epigenetics in psychiatry, however, is currently very sparsely available, but is consistent with a mediating role for epigenetic mechanisms in bringing together inherited and acquired risk factors into a neurodevelopmental etiological model of psychiatric disorders. Here, we review evidence from the epidemiological and neuroscience literature, and aim to converge the evidence into an etiological model of psychiatric disorders that encompasses environmental, genetic and epigenetic contributions. Given the dynamic nature of the epigenetic machinery and the potential reversibility of epigenetic modifications, future well-designed interdisciplinary and translational studies will be of key importance in order to identify new targets for prevention and therapeutic strategies.

Diagnosis and management of lymphatic vascular disease

The lymphatic vasculature is comprised of a network of vessels that is essential both to fluid homeostasis and to the mediation of regional immune responses. In health, the lymphatic vasculature possesses the requisite transport capacity to accommodate the fluid load placed upon it. The most readily recognizable attribute of lymphatic vascular incompetence is the presence of the characteristic swelling of tissues, called lymphedema, which arises as a consequence of insufficient lymph transport. The diagnosis of lymphatic vascular disease relies heavily upon the physical examination. If the diagnosis remains in question, the presence of lymphatic vascular insufficiency can be ascertained through imaging, including indirect radionuclide lymphoscintigraphy. Beyond lymphoscintigraphy, clinically-relevant imaging modalities include magnetic resonance imaging and computerized axial tomography. The state-of-the-art therapeutic approach to lymphatic edema relies upon physiotherapeutic techniques. Complex decongestive physiotherapy is an empirically-derived, effective, multicomponent technique designed to reduce limb volume and maintain the health of the skin and supporting structures. The application of pharmacological therapies has been notably absent from the management strategies for lymphatic vascular insufficiency states. In general, drug-based approaches have been controversial at best. Surgical approaches to improve lymphatic flow through vascular reanastomosis have been, in large part, unsuccessful, but controlled liposuction affords lasting benefit in selected patients. In the future, specifically engineered molecular therapeutics may be designed to facilitate the controlled regrowth of damaged, dysfunctional, or obliterated lymphatic vasculature in order to circumvent or mitigate the vascular insufficiency that leads to edema and tissue destruction.

The clinical spectrum of lymphatic disease

Lymphatic disease is quite prevalent, and often not well clinically characterized. Beyond lymphedema, there is a broad array of human disease that directly or indirectly alters lymphatic structure and function. The symptomatic and objective presentation of these patients can be quite diverse. In this review, we have attempted to provide a systematic overview of the subjective and objective spectrum of lymphatic disease, with consideration of all of the categories of disease that primarily or secondarily impair the functional integrity of the lymphatic system. Lymphedema is discussed, along with chromosomal disorders, lymphangioma, infectious diseases, lymphangioleiomyomatosis, lipedema, heritable genetic disorders, complex vascular malformations, protein-losing enteropathy, and intestinal lymphangiectasia.

Neural phenotypes of common and rare genetic variants

Neuroimaging methods offer a powerful way to bridge the gaps between genes, neurobiology and behavior. Such investigations may be further empowered by complementary strategies involving chromosomal abnormalities associated with particular neurobehavioral phenotypes, which can help to localize causative genes and better understand the genetics of complex traits in the general population. Here we review the evidence from studies using these convergent approaches to investigate genetic influences on brain structure: (1) studies of common genetic variations associated with particular neuroanatomic phenotypes, and (2) studies of possible 'genetic subtypes' of neuropsychiatric disorders with very high penetrance, with a focus on neuroimaging studies using novel computational brain mapping algorithms. Finally, we discuss the contribution of behavioral neurogenetics research to our understanding of the genetic basis of neuropsychiatric disorders in the broader population.

Prader-Willi syndrome: an update and review for the primary pediatrician

Prader-Willi syndrome, the first known human genomic imprinting disorder, is one of the most common micro-deletion syndromes. Prader-Willi syndrome is caused by the absence of certain paternally inherited genes on the long arm of chromosome 15, resulting in a complete absence of the active copy of the genetic information in this region. It is most commonly known for its food-related characteristics of hyperphagia, food-seeking behavior, and consequent obesity. Primary care physicians play an important role in the care of children with Prader-Willi syndrome, from recognizing the presenting signs and symptoms at its various stages to understanding their unique medical, developmental, behavioral, and dietary issues. They can also serve as a valuable source of support and advocacy for the family. This article reviews the current state of knowledge about Prader-Willi syndrome and discusses up-to-date understanding of the management of this condition.

Human chromosome 15q11-q14 regions of rearrangements contain clusters of LCR15 duplicons

Six breakpoint regions for rearrangements of human chromosome 15q11-q14 have been described. These rearrangements involve deletions found in approximately 70% of Prader-Willi or Angelman's syndrome patients (PWS, AS), duplications detected in some cases of autism, triplications and inverted duplications. HERC2-containing (HEct domain and RCc1 domain protein 2) segmental duplications or duplicons are present at two of these breakpoints (BP2 and BP3) mainly associated with deletions. We show here that clusters containing several copies of the human chromosome 15 low-copy repeat (LCR15) duplicon are located at each of the six described 15q11-q14 BPs. In addition, our results suggest the existence of breakpoints for large 15q11-q13 deletions in a proximal duplicon-containing clone. The study reveals that HERC2-containing duplicons (estimated on 50-400 kb) and LCR15 duplicons ( approximately 15 kb on 15q11-q14) share the golgin-like protein (GLP) genomic sequence. Through the analysis of a human BAC library and public databases we have identified 36 LCR15 related sequences in the human genome, most (27) mapping to chromosome 15q and being transcribed. LCR15 analysis in non-human primates and age-sequence divergences support a recent origin of this family of segmental duplications through human speciation.

Additional complexity on human chromosome 15q: identification of a set of newly recognized duplicons (LCR15) on 15q11-q13, 15q24, and 15q26

Several cytogenetic alterations affect the distal part of the long arm of human chromosome 15, including recurrent rearrangements between 12p13 and 15q25, which cause congenital fibrosarcoma (CFS). We present here the construction of a BAC/PAC contig map that spans 2 Mb from the neurotrophin-3 receptor (NTRK3) gene region on 15q25.3 to the proximal end of the Bloom's syndrome region on 15q26.1, and the identification of a set of new chromosome 15 duplicons. The contig reveals the existence of several regions of sequence similarity with other chromosomes (6q, 7p, and 12p) and with other 15q cytogenetic bands (15q11-q13 and 15q24). One region of similarity maps on 15q11-q13, close to the Prader-Willi/Angelman syndromes (PWS/AS) imprinting center. The 12p similar sequence maps on 12p13, at a distance to the ets variant 6 (ETV6) gene that is equivalent on 15q26.1 to the distance to the NTRK3 gene. These two genes are the targets of the CFS recurrent translocations, suggesting that misalignments between these two chromosomes regions could facilitate recombination. The most striking similarity identified is based on a low copy repeat sequence, mainly present on human chromosome 15 (LCR15), which could be considered a newly recognized duplicon. At least 10 copies of this duplicon are present on chromosome 15, mainly on 15q24 and 15q26. One copy is located close to a HERC2 sequence on the distal end of the PWS/AS region, three around the lysyl oxidase-like (LOXL1) gene on 15q24, and three on 15q26, one of which close to the IQ motif containing GTPase-activating protein 1 (IQGAP1) gene on 15q26.1. These LCR15 span between 13 and 22 kb and contain high identities with the golgin-like protein (GLP) and the SH3 domain-containing protein (SH3P18) gene sequences and have the characteristics of duplicons. Because duplicons flank chromosome regions that are rearranged in human genomic disorders, the LCR15 described here could represent new elements of rearrangements affecting different regions of human chromosome 15q.