|Overview of Rare Diseases Research Activities
NIA maintains a vigorous portfolio of research in the area of rare diseases and conditions. Much of the work in this area focuses on the progeroid syndromes, including the Werner, Cockayne, Hutchinson-Gilford, and Rothmund-Thomson syndromes that have implications for age-related diseases. However, investigators are actively conducting research into the underlying molecular mechanisms and clinical manifestations of a diverse array of other diseases or conditions that fit into the classification of rare disorders. These include Bloom syndrome (BS), Fanconi anemia (FA), ATRX syndrome, Ehlers-Danlos syndrome, and premature ovarian failure.
Cockayne syndrome (CS) is a rare human disease characterized by arrested postnatal growth, severe mental and growth retardation, microcephaly, progressive neurological and retinal degeneration, skeletal abnormalities, and a hypersensitivity to sunlight and results in premature aging and death. CS is an autosomal-recessive disorder that has been attributed to two genes—CSA and CSB. In recent experiments, NIA investigators have demonstrated that mutations in the CSB gene are the defect in DNA repair. The complex clinical phenotype of CS, however, suggests that DNA repair may not be the only defect. Studies have also demonstrated a significant transcription defect in CSB cells.
Recently, NIA scientists generated stable human cell lines with functional knockout of different regions of the CSB gene and demonstrated that the CSB protein plays a role in repair of oxidative DNA damage. Thus, this protein has several roles in DNA metabolism; it is involved in transcription, DNA repair, apoptosis, and chromatin assembly. Studies are now aimed at further structure/function analysis of CSB protein and aimed at further clarification of its function in these pathways.
The function of the CSB protein has also been investigated with microarray studies of gene expression. There are several genes that are underexpressed in mutated CS cells, and some of these confirm a substantial role for CSB protein in transcription and apoptosis.
In May 2004, the NIA, the NCI and the NIH ORD sponsored a 2-day workshop on Cockayne Syndrome and Related Disorders of DNA Repair and Transcription to discuss and address problems that impact patients with these diseases and to bring current research findings to clinicians and patients.
Werner syndrome (WS) is a recessive genetic disease characterized by early onset of many characteristics of normal aging, such as wrinkling of the skin, graying of the hair, cataracts, diabetes, and osteoporosis. The symptoms of WS begin to appear around the age of puberty, and most patients die before age 50. Because of the acceleration of aging in WS, the study of this disease may shed light on the degenerative processes that occur in normal aging.
The WS gene has been identified (WRN), and defects are characterized by karyotypic abnormalities including inversions, translocations, and chromosome losses. Research is ongoing to elucidate the role of WRN protein, WRNp, in pathways of DNA metabolism and to define the protein interactions of WRN, which will help to elucidate cellular processes necessary to maintain genomic stability.
Cells from WS patients grow more slowly and undergo senescence at an earlier population doubling than age-matched normal cells, possibly because these cells appear to lose the telomeric ends of their chromosomes at an accelerated rate. In general, WS cells have a high level of genomic instability, with increased amounts of DNA deletions, insertions, and rearrangements. These effects could be the result of defects in DNA repair, replication, and/or recombination, although the actual biochemical defect remains unknown. Investigators have made purified WRNp for use in a number of basic and complex biochemical assays and are pursuing several avenues to identify and characterize the biochemical defect in WS cells. Their observations of functional protein interactions suggest that WRN protein is involved in DNA repair processes, in the pathways of base excision repair and of recombination. Investigators have found that the WRNp participates in processes at the telomere end and helps unwind the specific DNA substrates that are found there and interacts with proteins localized to those regions. Thus, an important function of the WRN protein is to maintain and stabilize the telomere regions and to help repair DNA lesions situated there. Investigators’ ongoing and future studies will be directed toward elucidating the causes of the accelerated aging phenotype in WS, with hope that this knowledge can also be applied to our current understanding of both the aging of cells and organisms in general.
Hutchinson-Gilford Progeria Syndrome
Hutchinson-Gilford Progeria syndrome (HGPS) is a rare developmental disorder affecting most of the organ systems in a manner that mimics, to some extent, features of natural aging, but at a markedly accelerated rate. The individuals afflicted with this syndrome are clinically unaffected at birth, and the diagnosis is often not established until the second year of life. The diagnostic features include characteristic craniofacial disproportion, skin and hair abnormalities, loss of subcutaneous fatty tissue, and failure to thrive, resulting in short stature. These features give the patients a characteristic “old-like” appearance. Intellectual development is entirely normal. During the advancing years of the disease, the cardiovascular system becomes increasingly affected by atherosclerosis, and the patients die at an average age of 13 years from cardiovascular complications.
The NIA has continued to study HGPS because the identification of the culprit gene, designated as LMNA, has opened new avenues for research to explore the actual relationship between HGPS and normal aging. HGPS is a segmental progeria, and changes in lamin and nuclear structure might or might not happen during normal aging. In fact, while HGPS has been considered as a prototype of premature aging syndromes, the degree to which it truly recapitulates innate aging phenomena is still being studied.
The mode of inheritance, molecular basis, and pathogenic mechanisms of HGPS all remain elusive. Consequently, the NIA has an ongoing Program Announcement (PA-03-069) to encourage research on the biology of HGPS and other known laminopathies.
In April 2004, NIA co-sponsored, together with the Progeria Research Foundation, a 2-day workshop to explore the potential for stem cell transplantation in HGS. While the participants generally agreed that it was too early to apply such an approach to patients, NIA is considering ways to encourage transplantation research in premature aging mouse models that have been produced within the past few years.
Rothmund-Thomson and RAPADILINO Syndromes
Rothmund-Thomson syndrome (RTS) is a rare disease associated with genome instability; predisposition to cancer, skin, and skeletal abnormalities; and some features of premature aging. The disease is caused by mutation in the RECQL4 gene—the same RecQ family that includes the WRN and BLM proteins defective in Werner and Bloom syndromes, respectively. An RTS-related disease, termed the RAPADILINO syndrome, with a lower predisposition to cancer, was found to be caused by mutations in the same RECQL4 gene, however, the molecular mechanism of RTS and RAPADILINO syndromes is poorly understood.
The data suggest that RECQL4 may play a role in maintaining genomic stability and to understand the mechanism of this disease, NIA investigators have purified a human RECQL4 complex and identified its associated components; their work describes the first biochemical characterization of RECQL4 and its associated complex. The findings that RECQL4 is overexpressed and localized in the cytoplasm of multiple transformed cell lines are consistent with its role in maintaining genomic stability and suggest that it may be useful in cancer diagnosis. The discovery that RECQL4 is associated with UBR1 and UBR2, proteins that are ubiquitin ligases, is a hint to possible function; for example, the pathway in which these two proteins usually function is implicated in the regulation of chromosome stability and may be related to genomic instability of RTS patients.
FA is a recessively inherited disease characterized by congenital defects, bone marrow failure, and cancer susceptibility. Eight genes have been described that are mutated to cause FA; but many patients are not mutated in any of them and the mechanism underlying the FA pathway remains unclear, because most FA proteins lack recognizable structural features or any identifiable biochemical activity. Recent evidence suggests that FA proteins function in a DNA damage response pathway involving the proteins produced by the breast cancer susceptibility genes BRCA1 and BRCA2. A key step in that pathway is a modification of an FA protein, FANCD2. Five other FA proteins (FANCA, -C, -E, -F, and -G) have been found to interact with each other to form a multiprotein nuclear complex, the FA core complex. Investigators have purified the FA protein core complex and found that it contains four new components in addition to the five known FA proteins. One new component of this complex, termed PHF9, is defective in a cell line derived from an FA patient and therefore represents a novel FA gene (FANCL). The work identifies the first FA gene that encodes a product with a catalytic activity, and the discovery of PHF9/FANCL could provide a potential target for new therapeutic modalities. More generally, the Fanconi pathway is involved in the DNA repair mechanisms that are often implicated as disrupted in cancer and possibly in aging.
NIA investigators are now studying whether three other components of the FA core complex are also novel FA genes. Current data show that, indeed, a specific subunit of the complex is defective in FA complementation group B patients (the gene is named FANCB). X-linked inheritance in this complementation group has important consequences for genetic counseling of FA-B families. Being present as a single active copy and essential for a functional FA pathway, FANCB is a potentially vulnerable component of the cellular machinery that maintains genomic integrity.
BS is a rare human genetic disease in which patients exhibit growth retardation, immunodeficiency, infertility, photosensitivity, and predisposition to cancer. The gene defective in BS has recently been cloned (named BLM). The recombinant BLM protein, BLMp, has been shown to possess a helicase activity in vitro, suggesting that BS could be caused by a defect in a DNA metabolic reaction, such as replication or repair. Interestingly, the BLM gene belongs to the helicase family, like the genes mutated in Werner and Rothmund-Thomson syndromes. All three diseases have some common features, such as genetic instability and predisposition to cancer. To understand the molecular mechanism of these human diseases, investigators will isolate the protein complexes containing each gene product.
One isolated complex contains five of the FA complementation group proteins. FA resembles BS in genomic instability and cancer predisposition, but most of its gene products have no known biochemical activity and the molecular pathogenesis of the disease is poorly understood. This work by NIA investigators provides the first biochemical characterization of a multiprotein FA complex and suggests a connection between the BLM and FA pathways of genomic maintenance. The findings that FA proteins are part of a DNA-unwinding complex imply that FA proteins may participate in DNA repair.
ATRX syndrome represents a combination of alpha-thalassemia, mental retardation, and multiple associated developmental abnormalities. The gene defective in ATRX has been localized to the X chromosome and cloned. Mutations in the same gene also cause several other forms of syndromal X-linked mental retardation, and it has been hypothesized that ATRX could function in an ATP-dependent chromatin-remodeling complex and participate in regulation of gene expression. Investigators have recently found that ATRX is in a complex with transcription cofactor Daxx, and evidence supports that ATRX and Daxx are components of a novel ATP-dependent chromatin-remodeling complex. The defects in ATRX syndrome may result from inappropriate expression of genes controlled by this complex. Investigators have identified several sequence-specific transcription factors that co-purify with Daxx and are now studying if the genes regulated by these factors could be involved in ATRX syndrome.
Ehlers-Danlos Syndrome / Hereditary Connective Tissue Disorders
NIA investigators are examining the clinical and molecular effects of three well-known heritable disorders of connective tissue: Marfan syndrome, Ehlers-Danlos syndrome (EDS), and Stickler syndrome. Natural history data have been collected on subjects, including ophthalmologic, otolaryngologic, echocardiography, and rehabilitation medicine consultations. Investigators’ studies have documented newly recognized gastrointestinal complications of these disorders and that chronic musculoskeletal pain is a significant complication of both EDS and Stickler syndrome. Echocardiography analysis of patients with EDS demonstrated a 30-percent incidence of aortic root dilation in this group of patients. Investigators have compared the Berlin and Gent nosologies for Marfan syndrome and examined the efficacy of screening for dural ectasia (increase in the connective tissue lining the spine) in the diagnosis of Marfan syndrome. Investigators have analyzed the prevalence of spinal and hip abnormalities in Stickler syndrome and their relationship to chronic pain. Their studies documented an increased risk of femoral head fracture in children with Stickler syndrome. Investigators have developed proposed diagnostic criteria for Stickler syndrome based on their clinical and molecular studies in this population. Researchers have also identified a previously undescribed connective tissue disorder with features resembling Marfan syndrome, Stickler syndrome, and EDS.
Investigators are also involved in several projects aimed at understanding the potential roles of alternative and complementary medical practices in the care of persons with genetic conditions. Many persons with Hereditary Connective Tissue Disorders (HDCT) suffer from chronic musculoskeletal pain. NIA investigators are designing studies aimed at understanding whether there are fundamental differences in the neurobiology of patients with HDCT that contribute to chronic pain. Interventions designed to ameliorate chronic pain in this population include mindfulness-based stress reduction in the Ehlers-Danlos population and “dry needling” of myofacial trigger points in patients with several different disorders of connective tissue. Investigators are also attempting to understand the role of connective tissue in the mechanism of acupuncture and to understand whether variations of connective tissue seen in the HDCT influence the efficacy of acupuncture.
Premature Ovarian Failure
Premature ovarian failure (POF) is a common condition, affecting one to three percent of all women, in which early menopause could result from inadequate formation or maintenance of the pool of ovarian follicles. NIA investigators previously found that the blepharophimosis-ptosis-epicanthus inversus syndrome (BPES), a syndrome associated with eyelid malformation as well as POF, is caused by mutations in FOXL2, a transcription factor. This provided an entry point for the study of ovarian development and POF.
Studies of Foxl2 null mice point toward a new mechanism of POF. Mice lacking the gene recapitulate relevant features of human BPES: Males and females are small and show distinctive craniofacial morphology with upper eyelids absent. Furthermore, in mice as in humans, sterility is confined to females. More detailed analyses show that all major somatic cell lineages fail to develop around growing oocytes from the time of the first follicle formation. Thus, POF can arise from disrupted follicle development, with Foxl2 as a selective determinant of perinatal ovarian development.
Foxl2 disruption in mice provides the first model directly relevant to POF in humans, along with a route to genes selectively involved in the determination of the critical follicle pool. Such genes should include candidates for mutation in other instances of POF, where affected genes have been difficult to identify. In the long run, they may provide targets for therapeutic intervention to reverse ovarian failure.