Scientists have reported a detailed description of how function-impairing mutations in polr1c and polr1d genes cause Treacher Collins syndrome (TCS). TCS is a rare congenital craniofacial development disorder that affects an estimated one in 50,000 live births.
These findings reveal that a unifying cellular and biochemical mechanism underlies the etiology and pathogenesis of Treacher Collins syndrome and its possible prevention, irrespective of the causative gene mutation.
Loss-of-function mutations in three human genes, TCOF1, POLR1C and POLR1D, have been implicated in TCS and are thought to be responsible for about 90 percent of the diagnoses of this congenital craniofacial condition.
Treacher Collins Syndrome Symptoms
The clinical manifestations of TCS include facial anomalies such as small jaws and cleft palate, hearing loss, and respiratory problems. Patients with TCS typically undergo multiple surgeries, but rarely are they fully corrective.
By uncovering a mechanism of action common to all three genes, Stowers scientists have advanced scientific understanding of TCS etiology and pathogenesis and identified possible new avenues for preventing or treating the birth defect.
This latest study from the laboratory of Stowers Investigator Paul Trainor, Ph.D., focused on Polr1c and Polr1d, whose roles as a genetic cause of TCS were revealed in a 2011 study of a small group of patients who had been diagnosed with TCS but who did not have the TCOF1 mutation. Unlike POLR1C and POLR1D, TCOF1 has been long recognized as a causative gene in TCS and as a result has been more extensively investigated.
The documentary above tells the story of Juliana Wetmore, a girl born without a face. By the time she was 11 years old, Juliana had been through 45 surgeries.
Patients with TCS are mostly characterized by these abnormalities:
Hypoplasia of the facial bones: Most described is an underdeveloped mandibular and zygomatic bone. This leads to a small and malformed jaw. Mandible hypoplasia can result in a malocclusion.
Ear anomalies: The external ear anomalies consist of small, rotated, or even absent auricles. Also symmetric, bilateral stenosis or atresia of the external auditory canals is described. In most cases, the ossicles and the middle ear cavity were dysmorphic. Inner ear malformations are rarely described. As a result of these abnormalities, a majority of the individuals with TCS have conductive hearing loss.
Most patients experience eye problems, varying from colobomata of the lower eyelids and aplasia of lid lashes to short, down-slanting palpebral fissures, and missing eyelashes.
Vision loss can occur and is associated with strabismus, refractive errors, and anisometropia.
Airway problems, which are often a result of mandibular hypoplasia, can occur.
The presence of an abnormally shaped skull is not distinctive for Treacher Collins syndrome, but brachycephaly with bitemporal narrowing is sometimes observed.
Usually, people with Treacher Collins syndrome grow to become functioning adults with normal intelligence. With proper management, life expectancy is approximately the same as in the general population. In some cases, the prognosis depends on the specific symptoms and severity in the affected person.
Polr1c And Polr1d In Craniofacial Development
Kristin Watt, Ph.D., lead author of the PLoS Genetics paper and postdoctoral scientist in the Trainor lab, said:
“Before we began the study, nothing was known about the role of Polr1c and Polr1d in craniofacial development. Using zebrafish as our animal model, we set out to explore the functional roles of polr1c and polr1d during embryogenesis and more specifically in craniofacial development."
Trainor, Watt and their collaborators compared the results of their findings on polr1c and polr1d with their and other labs' previous research results on Tcof1.
In all three loss-of-function models, the researchers found that the chain of cellular events that led to the TCS phenotype of abnormal craniofacial development originated in ribosomes, the cellular components that translate messenger RNA into proteins.
Like the Tcof1 gene, polr1c and polr1d mutations were found to perturb ribosome biogenesis, or production of ribosomes, which affects the generation and survival of progenitor neural crest cells, the precursors of craniofacial bone, cartilage and connective tissue.
In animal models of all three causative genes, the scientists determined that deficient ribosome biogenesis triggered a p53-dependent cell death mechanism in progenitor neural crest cells. As a result of the activation of the p53 gene, developing embryos no longer made the quantity of neural crest cells needed to properly form the craniofacial skeleton.
However, in the polr1c and polr1d models as in the Tcof1 models, Stowers scientists found that by experimentally blocking p53 activation, they could restore the neural crest cell population and thereby rescue the animal models' cranioskeletal cartilage.
Despite the rescue effect, Trainor said that he does not view the “guardian of the genome,” as the p53 gene is often called due to its ability to suppress cancer, as the basis of a potential therapy to prevent or reduce TCS during embryonic development. The p53 gene’s association with cancer makes inhibiting its function too risky, he said.
Enhancing Ribosomes In Treacher Collins Syndrome
A less risky and perhaps more effective target for the prevention or treatment of TCS could be enhancing ribosomes, Trainor said, because the loss-of-function mutations in all three causative genes involve ribosome RNA (rRNA) transcription. Polr1c and Polr1d, for example, are subunits of RNA polymerases I and III that are essential for ribosome biogenesis.
“Rather than blocking p53, a better approach may be to try to prevent TCS by treating the problem in ribosome biogenesis that triggers the activation of p53 and the loss of neural crest cells,” said Trainor.
In their research with zebrafish embryos, Trainor and collaborators also determined that polr1c and polr1d are spatiotemporally and dynamically expressed, particularly during craniofacial development. Furthermore, zebrafish embryos with the polr1c and polr1d loss-of-function mutations develop abnormalities in craniofacial cartilage development that mimic the clinical manifestations of TCS in patients.
[caption id=“attachment_80723” align=“aligncenter” width=“1200”] tp53 inhibition ameliorates cartilage anomalies in polr1d-/-mutant embryos in a dosage-dependent manner.
(A-D) Alcian blue staining of cartilage in an allelic series of polr1d and tp53 mutant embryos. Dosage-dependent improvement in cartilage development is particularly noticeable in the jaw (E-H), elements of the viscerocranium (I-L), and more specifically the ceratohyal (G-L). Abbreviations: M, Meckel’s cartilage; pq, palatoquadrate; ch, ceratohyal; cb, ceratobranchial. Scale bar = 200 μm.
Trainor said that he and his fellow researchers were surprised that mutations in polr1c and polr1d as well as Tcof1 specifically affected craniofacial development, because ribosome biogenesis occurs in every cell of the body. The mutation of a gene that is part of the ribosome complex would be expected to be detrimental to each of these cells, he said.
However, in the zebrafish models, the mutation appears to primarily affect progenitor neural crest cells. Trainor said that he and his team theorize that progenitor neural crest cells may be particularly sensitive to deficiencies in ribosome biogenesis during embryogenesis.
Thus, the study revealed new animal models for TCS: zebrafish with polr1c and polr1d loss-of-function mutations. Moreover, the existence of a common mechanism of action may simplify the research, particularly the search for a therapy to prevent or treat TCS.
Because of the similarities among the three causative genes, we may be able to develop creative ways of preventing TCS that will prove effective in at risk individuals who have one of the gene mutations, said Trainor, who has investigated the molecular origins and development of TCS and related craniofacial developmental disorders for 10 years.
The studies were funded by the Stowers Institute, the National Institute of Dental and Craniofacial Research of the National Institutes of Health, the National Institute of Child Health and Human Development of the NIH, the American Association of Anatomists, and the March of Dimes (#6-FY15-233).
Noack Watt KE, Achilleos A, Neben CL, Merrill AE, Trainor PA (2016) The Roles of RNA Polymerase I and III Subunits Polr1c and Polr1d in Craniofacial Development and in Zebrafish Models of Treacher Collins Syndrome PLoS Genet 12(7): e1006187. doi:10.1371/journal.pgen.1006187