Ciliopathy | |
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Eukaryotic cilium | |
Specialty | Medical genetics |
A ciliopathy is any genetic disorder that affects the cellular cilia or the cilia anchoring structures, the basal bodies, [1] or ciliary function. [2] Primary cilia are important in guiding the process of development, so abnormal ciliary function while an embryo is developing can lead to a set of malformations that can occur regardless of the particular genetic problem. [3] The similarity of the clinical features of these developmental disorders means that they form a recognizable cluster of syndromes, loosely attributed to abnormal ciliary function and hence called ciliopathies. Regardless of the actual genetic cause, it is clustering of a set of characteristic physiological features which define whether a syndrome is a ciliopathy.
Although ciliopathies are usually considered to involve proteins that localize to motile and/or immotile (primary) cilia or centrosomes, it is possible for ciliopathies to be associated with unexpected proteins such as XPNPEP3, which localizes to mitochondria but is believed to affect ciliary function through proteolytic cleavage of ciliary proteins. [4]
Significant advances in understanding the importance of cilia were made in the mid-1990s. For example, the discovery of the role of cilia in embryonic development, identification of ciliary defects in genetic disorders such as Polycystic kidney disease, Bardet–Biedl syndrome and Primary ciliary dyskinesia. [5] [6] However, the physiological role that this organelle plays in most tissues remains elusive. Additional studies of how ciliary dysfunction can lead to such severe disease and developmental pathologies is still a subject of current research. [7]
A wide variety of symptoms are potential clinical features of ciliopathy. The signs most exclusive to a ciliopathy, in descending order of exclusivity, are: [8] : 138
A case with polycystic ovary syndrome, multiple subcutaneous cysts, renal function impairment, Caroli disease and liver cirrhosis due to ciliopathy has been described. [9]
Phenotypes sometimes associated with ciliopathies can include: [8]
"In effect, the motile cilium is a nanomachine composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines." Cilia "function as mechano- or chemosensors and as a cellular global positioning system to detect changes in the surrounding environment." For example, ciliary signaling plays a role in the initiation of cellular replacement after cell damage. [11]
In addition to this sensory role mediating specific signaling cues, cilia play "a secretory role in which a soluble protein is released to have an effect downstream of the fluid flow" in epithelial cells, and can of course mediate fluid flow directly in the case of motile cilia. [1] Primary cilia in the retina play a role in transferring nourishment to the non-vascularized rod and cone cells from the pigment epithelial vascularized cells several micrometres behind the surface of the retina.
Signal transduction pathways involved include the Hedgehog signaling pathway and the Wnt signaling pathway. [12]
Dysfunctional cilia can lead to:
In organisms of normal health, cilia are critical for: [15]
"Just as different genes can contribute to similar diseases, so the same genes and families of genes can play a part in a range of different diseases." For example, in just two of the diseases caused by malfunctioning cilia, Meckel–Gruber syndrome and Bardet–Biedl syndrome, patients who carry mutations in genes associated with both diseases "have unique symptoms that are not seen in either condition alone." The genes linked to the two different conditions "interact with each other during development." Systems biologists are endeavoring to define functional modules containing multiple genes and then look at disorders whose phenotypes fit into such modules. [16]
A particular phenotype can overlap "considerably with several conditions (ciliopathies) in which primary cilia are also implicated in pathogenicity. One emerging aspect is the wide spectrum of ciliopathy gene mutations found within different diseases." [10]
Additionally, clinical presentations of patients with identical mutation can differ, suggesting the role of genetic modifiers. [17]
As of 2017, 187 ciliopathy associated genes have been confirmed, while the roles of further 241 candidate genes are still being investigated. [18]
A common way to identify ciliopathies such as ADPKD and ARPKD which have known genetic causes, is through linkage analysis direct mutation screening. [19] Other techniques, such as gene panels and whole-exome sequencing and whole genome sequencing can also be used to provide distinct advantages. [19] [20] Gene panels analyse specific sets of genes and can be more comprehensive than single gene or direct mutation screening. Whole-exome/genome sequencing can screen for heterozygous carriers, and detect novel/rare variations. [19] [21]
Mutations in the PKD1 and PKD2 genes which encode for polycystin-1 and polycistin-2 respectively are known to be causes of ADPKD, a ciliopathy that presents with the formation and growth of cysts in the kidneys, leading to renal failure. [22]
"The phenotypic parameters that define a ciliopathy may be used to both recognize the cellular basis of a number of genetic disorders and to facilitate the diagnosis and treatment of some diseases of unknown" cause. [8]
Condition | OMIM | Gene(s) | Systems/organs affected |
---|---|---|---|
Acrocallosal syndrome [23] | 200990 | KIF7 , GLI3 | |
Acromelic frontonasal dysostosis [23] | 603671 | ZSWIM6 | |
Arima syndrome [23] | 243910 | ||
Biemond syndrome [23] | 113400 | ||
COACH syndrome [23] | 216360 | TMEM67 , CC2D2A , RPGRIP1L | |
Conorenal syndrome [27] [23] | 266920 | ||
Greig cephalopolysyndactyly syndrome [23] | 175700 | GLI3 | |
Hydrolethalus syndrome [23] | 236680 | HYLS1 | |
Johanson–Blizzard syndrome [23] | 243800 | UBR1 | |
Mohr syndrome (oral-facial-digital syndrome type 2) [23] | 252100 | ||
Neu–Laxova syndrome [23] | 256520 | PHGDH , PSAT1 , PSPH | |
Opitz G/BBB syndrome [23] | 300000 | MID1 | |
Pallister–Hall syndrome [23] | 146510 | GLI3 | |
Papillorenal syndrome [23] | 120330 | PAX2 | |
Renal–hepatic–pancreatic dysplasia [23] | 208540 | NPHP3 | |
Varadi–Papp syndrome (oral-facial-digital syndrome type 6) [23] | 277170 |
In 1674-1677, the Dutch scientist Antonie van Leeuwenhoek changed humanity's perspective on the world with his discovery of "animalcules" in rainwater, along with their tiny appendages known as cilia today. It was marked as the first recorded observation of single-celled organisms and their locomotive structures. [30]
In the late 19th century, Karl Ernst von Baer's groundbreaking work in embryonic development laid the foundation for modern developmental biology. [31] Through meticulous observations, von Baer provided invaluable insights into tissue and organ formation during development, including the early stages of embryogenesis and the development of cilia-bearing tissues. [32] While von Baer may not have fully appreciated the significance of cilia at the time, his observations likely included their presence in embryonic tissues. Cilia - crucial for cell signaling, tissue development, and left-right asymmetry, are now recognized as ancient organelles with essential roles in development. [33] Von Baer's concept of embryonic recapitulation, despite refinement, underscores the evolutionary conservation of developmental processes, including ciliary function. Today, von Baer's legacy inspires ongoing research into embryology and developmental biology, particularly in understanding ciliary biology and its relevance to ciliopathies, where defects in ciliary structure or function lead to developmental disorder. [34]
Although non-motile or primary cilia were first described in 1898, they were largely ignored by biologists. However, microscopists continued to document their presence in the cells of most vertebrate organisms. The primary cilium was long considered—with few exceptions—to be a largely useless evolutionary vestige, a vestigial organelle. Recent research has revealed that cilia are essential to many of the body's organs. [35] These primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." [11]
Recent advances in mammalian genetic research have made possible the understanding of a molecular basis for a number of dysfunctional mechanisms in both motile and primary cilia structures of the cell. [36] A number of critical developmental signaling pathways essential to cellular development have been discovered. These are principally but not exclusively found in the non-motile or primary cilia. A number of common observable characteristics of mammalian genetic disorders and diseases are caused by ciliary dysgenesis and dysfunction. Once identified, these characteristics thus describe a set of hallmarks of a ciliopathy. [8]
Cilia have recently been implicated in a wide variety of human genetic diseases by "the discovery that numerous proteins involved in mammalian disease localize to the basal bodies and cilia." For example, in just a single area of human disease physiology, cystic renal disease, cilia-related genes and proteins have been identified to have causal effect in polycystic kidney disease, nephronophthisis, Senior–Løken syndrome type 5, orofaciodigital syndrome type 1 and Bardet–Biedl syndrome. [7]
The cilium is a membrane-bound organelle found on most types of eukaryotic cell. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projection that extends from the surface of the much larger cell body. Eukaryotic flagella found on sperm cells and many protozoans have a similar structure to motile cilia that enables swimming through liquids; they are longer than cilia and have a different undulating motion.
Joubert syndrome is a rare autosomal recessive genetic disorder that affects the cerebellum, an area of the brain that controls balance and coordination.
Exencephaly is a type of cephalic disorder wherein the brain is located outside of the skull. This condition is usually found in embryos as an early stage of anencephaly. As an exencephalic pregnancy progresses, the neural tissue gradually degenerates.
A basal body is a protein structure found at the base of a eukaryotic undulipodium. The basal body was named by Theodor Wilhelm Engelmann in 1880. It is formed from a centriole and several additional protein structures, and is, essentially, a modified centriole. The basal body serves as a nucleation site for the growth of the axoneme microtubules. Centrioles, from which basal bodies are derived, act as anchoring sites for proteins that in turn anchor microtubules, and are known as the microtubule organizing center (MTOC). These microtubules provide structure and facilitate movement of vesicles and organelles within many eukaryotic cells.
Ellis–Van Creveld syndrome is a rare genetic disorder of the skeletal dysplasia type.
In molecular biology, an axoneme, also called an axial filament, is the microtubule-based cytoskeletal structure that forms the core of a cilium or flagellum. Cilia and flagella are found on many cells, organisms, and microorganisms, to provide motility. The axoneme serves as the "skeleton" of these organelles, both giving support to the structure and, in some cases, the ability to bend. Though distinctions of function and length may be made between cilia and flagella, the internal structure of the axoneme is common to both.
Bardet–Biedl syndrome (BBS) is a ciliopathic human genetic disorder that produces many effects and affects many body systems. It is characterized by rod/cone dystrophy, polydactyly, central obesity, hypogonadism, and kidney dysfunction in some cases. Historically, slower mental processing has also been considered a principal symptom but is now not regarded as such.
Agenesis of the corpus callosum (ACC) is a rare birth defect in which there is a complete or partial absence of the corpus callosum. It occurs when the development of the corpus callosum, the band of white matter connecting the two hemispheres in the brain, in the embryo is disrupted. The result of this is that the fibers that would otherwise form the corpus callosum are instead longitudinally oriented along the ipsilateral ventricular wall and form structures called Probst bundles.
Intraflagellar transport (IFT) is a bidirectional motility along axoneme microtubules that is essential for the formation (ciliogenesis) and maintenance of most eukaryotic cilia and flagella. It is thought to be required to build all cilia that assemble within a membrane projection from the cell surface. Plasmodium falciparum cilia and the sperm flagella of Drosophila are examples of cilia that assemble in the cytoplasm and do not require IFT. The process of IFT involves movement of large protein complexes called IFT particles or trains from the cell body to the ciliary tip and followed by their return to the cell body. The outward or anterograde movement is powered by kinesin-2 while the inward or retrograde movement is powered by cytoplasmic dynein 2/1b. The IFT particles are composed of about 20 proteins organized in two subcomplexes called complex A and B.
Alström syndrome (AS), also called Alström–Hallgren syndrome, is a very rare autosomal recessive genetic disorder characterised by childhood obesity and multiple organ dysfunction. Symptoms include early-onset type 2 diabetes, cone-rod dystrophy resulting in blindness, sensorineural hearing loss and dilated cardiomyopathy. Endocrine disorders typically also occur, such as hypergonadotrophic hypogonadism and hypothyroidism, as well as acanthosis nigricans resulting from hyperinsulinemia. Developmental delay is seen in almost half of people with Alström syndrome.
Meckel-Gruber syndrome is a rare, lethal ciliopathic genetic disorder, characterized by renal cystic dysplasia, central nervous system malformations, polydactyly (postaxial), hepatic developmental defects, and pulmonary hypoplasia due to oligohydramnios. Meckel–Gruber syndrome is named for Johann Meckel and Georg Gruber.
Nephronophthisis is a genetic disorder of the kidneys which affects children. It is classified as a medullary cystic kidney disease. The disorder is inherited in an autosomal recessive fashion and, although rare, is the most common genetic cause of childhood kidney failure. It is a form of ciliopathy. Its incidence has been estimated to be 0.9 cases per million people in the United States, and 1 in 50,000 births in Canada.
Senior–Løken syndrome is a congenital eye disorder, first characterized in 1961. It is a rare, ciliopathic, autosomal recessive disorder characterized by juvenile nephronophthis and progressive eye disease.
Orofaciodigital syndrome 1 (OFD1), also called Papillon-Léage and Psaume syndrome, is an X-linked congenital disorder characterized by malformations of the face, oral cavity, and digits with polycystic kidney disease and variable involvement of the central nervous system.
Hydrolethalus syndrome (HLS) is a rare genetic disorder that causes improper fetal development, resulting in birth defects and, most commonly, stillbirth.
MORM syndrome is an autosomal recessive congenital disorder characterized by mental retardation, truncal obesity, retinal dystrophy, and micropenis". The disorder shares similar characteristics with Bardet–Biedl syndrome and Cohen syndrome, both of which are autosomal recessive genetic disorders. MORM syndrome can be distinguished from the above disorders because symptoms appear at a young age. The disorder is not dependent on sex of the offspring, both male and female offspring are equally likely to inherit the disorder.
A BBSome is a protein complex that operates in primary cilia biogenesis, homeostasis, and intraflagellar transport (IFT). The BBSome recognizes cargo proteins and signaling molecules like G-protein coupled receptors (GPCRs) on the ciliary membrane and helps transport them to and from the primary cilia. Primary cilia are nonmotile microtubule projections that function like antennae and are found in many types of cells. They receive various environmental signals to aid the cell in survival. They can detect photons by concentrating rhodopsin, a light receptor that converts photons into chemical signals, or odorants by concentrating olfactory receptors on the primary cilia surface. Primary cilia are also meaningful in cell development and signaling. They do not contain any way to make proteins within the primary cilia, so the BBSome aids in transporting essential proteins to, from, and within the cilia. Examples of cargo proteins that the BBSome is responsible for ferrying include smoothened, polycystic-1 (PC1), and several G-Protein coupled receptors (GPCRs) like somatostatin receptors (Sstr3), melanin-concentrating hormone receptor 1 (Mchr1), and neuropeptide Y2 receptor.
Ciliogenesis is defined as the building of the cell's antenna or extracellular fluid mediation mechanism. It includes the assembly and disassembly of the cilia during the cell cycle. Cilia are important appendages of cells and are involved in numerous activities such as cell signaling, processing developmental signals, and directing the flow of fluids such as mucus over and around cells. Due to the importance of these cell processes, defects in ciliogenesis can lead to numerous human diseases related to non-functioning cilia known as ciliopathies.
RVxP motif is a protein motif involved in localizing proteins into cilia.
Jeremy Reiter is an American developmental geneticist who is the chair of the Department of Biochemistry and Biophysics at the University of California, San Francisco (UCSF). He is holder of the Albert Bowers Endowed Chair. His research focuses on the cilium, particularly in understanding its role in cell signaling and its involvement in human diseases such as cancer, congenital disorders and obesity.