Cloning and analysis of the murine Fanconi anemia group C cDNA

Fanconi anemia (FA) is one of a group of disorders characterizedat the cellular level by a combination of hypersensitivity toDNA-damaging agents, chromosomal instability, and defectiveDNA repair. Clinical features of FA include pancytopenia, oftenaccompanied by specific congenital malformations, and a predispositionto leukemia. Since the hematological manifestations are thecritical defect in terms of prognosis, FA is a candidate diseasefor gene replacement therapy, and the development of a mousemodel system is essential for the initial stages of this work.Previously, we have cloned the gene defective in FA group Cby complementation of the intrinsic sensitivity of FA cellsto DNA cross-linking agents. We have now cloned the murine homologueof the human FACC cDNA. The mouse cDNA (Facc) shares 79% aminoacid sequence similarity with the human gene product. The expressionof the mouse cDNA in human FA(C) cells restores the cellulardrug sensitivity to normal levels. Thus, the function of theprotein has been conserved despite the significant sequencedivergence. PCR analysis of mouse tissue RNA reveals that thegene is expressed in all adult tissues, while in situ RNA hybridizationexperiments show tissue specific expression at late stages offetal development. Cross-hybridizing sequences exist in DNAfrom other mammals, chicken and Drosophila. These results supportthe hypothesis that the FACC gene product has a role in a basicaspect of cellular protection against DNA damaging agents andthat this function has been conserved during evolution.     

Inactivation of the mouse Magel2 gene results in growth abnormalities similar to Prader-Willi syndrome

Prader-Willi syndrome (PWS) is an imprinted genetic obesity disorder characterized by abnormalities of growth and metabolism. Multiple mouse models with deficiency of one or more PWS candidate genes have partially correlated individual genes with aspects of the PWS phenotype, although the genetic origin of defects in growth and metabolism has not been elucidated. Gene-targeted mutation of the PWS candidate gene Magel2 in mice causes altered circadian rhythm output and reduced motor activity. We now report that Magel2-null mice exhibit neonatal growth retardation, excessive weight gain after weaning, and increased adiposity with altered metabolism in adulthood, recapitulating fundamental aspects of the PWS phenotype. Magel2-null mice provide an important opportunity to examine the physiological basis for PWS neonatal failure to thrive and post-weaning weight gain and for the relationships among circadian rhythm, feeding behavior, and metabolism.     

Identification of a novel paternally expressed gene in the Prader - Willi syndrome region

We have isolated a novel gene from the Prader - Willi syndrome(PWS) smallest region of deletion overlap in proximal humanchromosome 15q. IPW (Imprinted gene in the Prader-Willi syndromeregion) was isolated using the direct selection method and yeastartificial chromosomes localized to the deletion region. IPWis spliced and polyadenylated but its longest open reading framecodes for only 45 amino acids, suggesting that it functionsas an RNA, similar to H19 and XIST. The RNA is widely expressedin adult and fetal tissues and is found in the cytoplasmic fractionof human cells, which is also the case for the H19 non-translatedRNA, but differs from the XIST RNA which is found predominantlyin the nucleus. Using a sequence polymorphism, exclusive expressionfrom the paternal allele in lymphoblasts and fibroblasts wasdemonstrated; monoallelic expression was found in fetal tissues.IPW is located about 150 kb distal to SNRPN, the only otherknown gene in the deletion interval, and about 50 kb proximalto the breakpoint of a translocation which defines the distalend of the PWS region and the proximal end of the Angelman syndrome(AS) region. As is the case with SNRPN, PWS patients with 15q11– q13 deletions do not express IPW, where as expressionis normal in Angelman syndrome patients. Lack of expressionof IPW may contribute to the PWS phenotype directly. Alternatively,the mRNA product of IPW may play a role in the imprinting process,acting either on genes located proximally in the PWS regionor distally in the AS region.     

The role of genomic imprinting in human developmental disorders: lessons from Prader-Willi syndrome

Normal human development involves a delicate interplay of gene expression in specific tissues at narrow windows of time. Temporally and spatially regulated gene expression is controlled both by gene-specific factors and chromatin-specific factors. Genomic imprinting is the expression of specific genes primarily from only one allele at particular times during development, and is one mechanism implicated in the intricate control of gene expression. Two human genetic disorders, Prader-Willi syndrome (PWS, MIM 176270) and Angelman syndrome (AS, MIM 105830), result from rearrangements of chromosome 15q11-q13, an imprinted region of the human genome. Despite their rarity, disorders such as PWS and AS can give focused insight into the role of genomic imprinting and imprinted genes in human development.     

The necdin gene is deleted in Prader-Willi syndrome and is imprinted in human and mouse

Human chromosome 15q11-q13 contains genes that are imprinted and expressed from only one parental allele. Prader-Willi syndrome (PWS) is due to the loss of expression of one or more paternally expressed genes on proximal human chromosome 15q, most often by deletion or maternal uniparental disomy. Several candidate genes and a putative imprinting centre have been identified in the deletion region. We report that the human necdin-encoding gene (NDN) is within the centromeric portion of the PWS deletion region, between the two imprinted genes ZNF127 and SNRPN. Murine necdin is a nuclear protein expressed exclusively in differentiated neurons in the brain. Necdin is postulated to govern the permanent arrest of cell growth of post-mitotic neurons during murine nervous system development. We have localized the mouse locus Ndn encoding necdin to chromosome 7 in a region of conserved synteny with human chromosome 15q11-q13, by genetic mapping in an interspecific backcross panel. Furthermore, we demonstrate that expression of Ndn is limited to the paternal allele in RNA from newborn mouse brain. Expression of NDN is detected in many human tissues, with highest levels of expression in brain and placenta. NDN is expressed exclusively from the paternally inherited allele in human fibroblasts. Loss of necdin gene expression may contribute to the disorder of brain development in individuals with PWS.      

Essential role for the Prader-Willi syndrome protein necdin in axonal outgrowth

Necdin and Magel2 are related proteins inactivated in Prader-Willi syndrome (PWS), a sporadic chromosomal deletion disorder. We demonstrate that necdin and Magel2 bind to and prevent proteasomal degradation of Fez1, a fasciculation and elongation protein implicated in axonal outgrowth and kinesin-mediated transport, and also bind to the Bardet-Biedl syndrome (BBS) protein BBS4 in co-transfected cells. The interactions among necdin, Magel2, Fez1 and BBS4 occur at or near centrosomes. Centrosomal or pericentriolar dysfunction has previously been implicated in BBS and may also be important in the features of PWS that overlap with BBS, such as learning disabilities, hypogonadism and obesity. Morphological abnormalities in axonal outgrowth and fasciculation manifest in several regions of the nervous system in necdin null mouse embryos, including axons of sympathetic, retinal ganglion cell, serotonergic and catecholaminergic neurons. These data demonstrate that necdin mediates intracellular processes essential for neurite outgrowth and that loss of necdin impinges on axonal outgrowth. We further suggest that loss of necdin contributes to the neurological phenotype of PWS, and raise the possibility that co-deletion of necdin and the related protein Magel2 may explain the lack of single gene mutations in PWS.     

Deconstructing Mendel: new paradigms in genetic mechanisms

As knowledge of the mechanisms of genetic action expands, this new information must be incorporated into the whole. The result is that old concepts are modified or deleted or new paradigms are created. The authors review advances in the understanding of traditional and nontraditional inheritance, including genomic imprinting and mitochondrial inheritance.     

Co-morbidity of complex genetic disorders and hypersomnias of central origin: lessons from the underlying neurobiology of wake and sleep

Appropriate wake and sleep cycles are important to physical well-being, and are modulated by neuronal networks in the brain. A variety of medical conditions can disrupt sleep or cause excessive daytime sleepiness. Clinical diagnostic classification schemes have historically lumped genetic disorders together into a category that considers the sleep dysfunction to be secondary to a medical condition. The unique nature of sleep endophenotypes that occur more frequently in particular genetic disorders has been underappreciated. Increased understanding of the pathophysiology of wake/sleep dysfunction in rare genetic disorders could inform studies of the neurological mechanisms that underlie more common forms of wake and sleep dysfunction. In this review, we highlight genetic developmental disorders in which sleep endophenotypes have been described, and then consider genetic neurodegenerative disorders with sleep characteristics that set them apart from the disruptions to sleep that are typically associated with aging and dementia.