Mice overexpressing genes from the 22q11 region deleted in velo-cardio-facial syndrome/DiGeorge syndrome have middle and inner ear defects

Otitis media: recent advances in otitis media vaccine development and model systems

Otitis media is an inflammatory disorder of the middle ear caused by airways-associated bacterial or viral infections. It is one of the most common childhood infections as globally more than 80% of children are diagnosed with acute otitis media by 3 years of age and it is a common reason for doctor's visits, antibiotics prescriptions, and surgery among children. Otitis media is a multifactorial disease with various genetic, immunologic, infectious, and environmental factors predisposing children to develop ear infections. Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis are the most common culprits responsible for acute otitis media. Despite the massive global disease burden, the pathogenesis of otitis media is still unclear and requires extensive future research. Antibiotics are the preferred treatment to cure middle ear infections, however, the antimicrobial resistance rate of common middle ear pathogens has increased considerably over the years. At present, pneumococcal and influenza vaccines are administered as a preventive measure against otitis media, nevertheless, these vaccines are only beneficial in preventing carriage and/or disease caused by vaccine serotypes. Otitis media caused by non-vaccine serotype pneumococci, non-typeable H. influenza, and M. catarrhalis remain an important healthcare burden. The development of multi-species vaccines is an arduous process but is required to reduce the global burden of this disease. Many novel vaccines against S. pneumoniae, non-typeable H. influenza, and M. catarrhalis are in preclinical trials. It is anticipated that these vaccines will lower the disease burden and provide better protection against otitis media. To study disease pathology the rat, mouse, and chinchilla are commonly used to induce experimental acute otitis media to test new therapeutics, including antibiotics and vaccines. Each of these models has its advantages and disadvantages, yet there is still a need to develop an improved animal model providing a better correlated mechanistic understanding of human middle ear infections, thereby underpinning the development of more effective otitis media therapeutics. This review provides an updated summary of current vaccines against otitis media, various animal models of otitis media, their limitations, and some future insights in this field providing a springboard in the development of new animal models and novel vaccines for otitis media.

Tbx1 and Foxi3 genetically interact in the pharyngeal pouch endoderm in a mouse model for 22q11.2 deletion syndrome

We investigated whether Tbx1, the gene for 22q11.2 deletion syndrome (22q11.2DS) and Foxi3, both required for segmentation of the pharyngeal apparatus (PA) to individual arches, genetically interact. We found that all Tbx1+/-;Foxi3+/- double heterozygous mouse embryos had thymus and parathyroid gland defects, similar to those in 22q11.2DS patients. We then examined Tbx1 and Foxi3 heterozygous, null as well as conditional Tbx1Cre and Sox172A-iCre/+ null mutant embryos. While Tbx1Cre/+;Foxi3f/f embryos had absent thymus and parathyroid glands, Foxi3-/- and Sox172A-iCre/+;Foxi3f/f endoderm conditional mutant embryos had in addition, interrupted aortic arch type B and retroesophageal origin of the right subclavian artery, which are all features of 22q11.2DS. Tbx1Cre/+;Foxi3f/f embryos had failed invagination of the third pharyngeal pouch with greatly reduced Gcm2 and Foxn1 expression, thereby explaining the absence of thymus and parathyroid glands. Immunofluorescence on tissue sections with E-cadherin and ZO-1 antibodies in wildtype mouse embryos at E8.5-E10.5, revealed that multilayers of epithelial cells form where cells are invaginating as a normal process. We noted that excessive multilayers formed in Foxi3-/-, Sox172A-iCre/+;Foxi3f/f as well as Tbx1 null mutant embryos where invagination should have occurred. Several genes expressed in the PA epithelia were downregulated in both Tbx1 and Foxi3 null mutant embryos including Notch pathway genes Jag1, Hes1, and Hey1, suggesting that they may, along with other genes, act downstream to explain the observed genetic interaction. We found Alcam and Fibronectin extracellular matrix proteins were reduced in expression in Foxi3 null but not Tbx1 null embryos, suggesting that some, but not all of the downstream mechanisms are shared.

Easy Stapes Subluxation in 22q11.2 Deletion Syndrome: A Clinical Capsule and Literature Review

OBJECTIVES

This report describes iatrogenic stapes subluxation in a 22q11 deletion syndrome patient and reviews the human and murine literature for evidence that these patients have stapes malformations. We aim to alert otologic surgeons regarding the possibility of stapes footplate abnormalities in 22q11 deletion patients.

PATIENT

An adult woman with known 22q11 deletion syndrome. Additionally, the literature review focused on other patients with 22q11 deletion syndrome as well as mouse models of this disorder.

INTERVENTIONS

A combination of diagnostic and therapeutic interventions were conducted consisting of middle ear exploration, removal of ossicular chain adhesions, and ultimately ossicular reconstruction.

RESULTS

The stapes footplate was poorly attached to the oval window in our patient. During removal of ossicular adhesions, the entire stapes subluxed requiring placement of a stapes prosthesis. The postoperative audiogram was similar to the preoperative audiogram. Literature review identified one other case of stapes subluxation in a patient with 22q11 deletion syndrome, and mouse models suggest that the stapes footplate has an abnormal connection to the oval window in those affected by 22q11 deletion syndrome.

CONCLUSIONS

Patients with 22q11 deletion syndrome have chronic middle ear pathology, and if middle ear exploration is undertaken, the surgeon should be aware that the stapes may have a weak attachment to the oval window. This could put the stapes at risk of injury and contribute to conductive hearing loss.

Heterozygous Mutations in TBX1 as a Cause of Isolated Hypoparathyroidism

CONTEXT

Most cases of autosomal dominant isolated hypoparathyroidism are caused by gain-of-function mutations in CASR or GNA11 or dominant negative mutations in GCM2 or PTH.

OBJECTIVE

To identify the genetic etiology for dominantly transmitted isolated hypoparathyroidism in two multigenerational families with 14 affected family members.

METHODS

We performed whole exome sequencing of DNA from two families and examined the consequences of mutations by minigene splicing assay.

RESULTS

We discovered disease-causing mutations in both families. A splice-altering mutation in TBX1 (c.1009+1G>C) leading to skipping of exon 8 (101 bp) was identified in 10 affected family members and five unaffected subjects of family A, indicating reduced penetrance for this point mutation. In a second family from France (family B), we identified another splice-altering mutation (c.1009+2T>C) adjacent to the mutation identified in family A that results in skipping of the same exon; two subjects in family B had isolated hypoparathyroidism, whereas a third subject manifested the clinical triad of the 22q11.2 deletion syndrome, indicative of variable expressivity.

CONCLUSIONS

We report evidence that heterozygous TBX1 mutations can cause isolated hypoparathyroidism. This study adds knowledge to the increasingly expanding list of causative and candidate genes in isolated hypoparathyroidism.

Anatomic Malformations of the Middle and Inner Ear in 22q11.2 Deletion Syndrome: Case Series and Literature Review

BACKGROUND AND PURPOSE

The 22q11.2 deletion syndrome is characterized by a heterogenic phenotype, including hearing loss. The underlying cause of hearing loss, especially sensorineural hearing loss, is not yet clear. Therefore, our objective was to describe anatomic malformations in the middle and inner ear in patients with 22q11.2 deletion syndrome.

MATERIALS AND METHODS

A retrospective case series was conducted in 2 tertiary referral centers. All patients with 22q11.2 deletion syndrome who had undergone CT or MR imaging of the temporal bones were included. Radiologic images were evaluated on predetermined parameters, including abnormalities of the ossicular chain, cochlea, semicircular canals, and vestibule.

RESULTS

There were 26 patients (52 ears) with a CT or MR imaging scan available. A dense stapes superstructure was found in 18 ears (36%), an incomplete partition type II was suspected in 12 cochleas (23%), the lateral semicircular canal was malformed with a small bony island in 17 ears (33%), and the lateral semicircular canal and vestibule were fused to a single cavity in 15 ears (29%).

CONCLUSIONS

Middle and inner ear abnormalities were frequently encountered in our cohort, including malformations of the lateral semicircular canal.

Early uneven ear input induces long-lasting differences in left-right motor function

How asymmetries in motor behavior become established normally or atypically in mammals remains unclear. An established model for motor asymmetry that is conserved across mammals can be obtained by experimentally inducing asymmetric striatal dopamine activity. However, the factors that can cause motor asymmetries in the absence of experimental manipulations to the brain remain unknown. Here, we show that mice with inner ear dysfunction display a robust left or right rotational preference, and this motor preference reflects an atypical asymmetry in cortico-striatal neurotransmission. By unilaterally targeting striatal activity with an antagonist of extracellular signal-regulated kinase (ERK), a downstream integrator of striatal neurotransmitter signaling, we can reverse or exaggerate rotational preference in these mice. By surgically biasing vestibular failure to one ear, we can dictate the direction of motor preference, illustrating the influence of uneven vestibular failure in establishing the outward asymmetries in motor preference. The inner ear–induced striatal asymmetries identified here intersect with non–ear-induced asymmetries previously linked to lateralized motor behavior across species and suggest that aspects of left–right brain function in mammals can be ontogenetically influenced by inner ear input. Consistent with inner ear input contributing to motor asymmetry, we also show that, in humans with normal ear function, the motor-dominant hemisphere, measured as handedness, is ipsilateral to the ear with weaker vestibular input.

Despite a long-standing fascination with asymmetries in left–right brain function, very little is known about the causes of functional brain asymmetry in mammals, which appear independent of the mechanisms that create anatomical asymmetries during development. Asymmetries in motor function are a common example and include preferred turning direction, handedness, and footedness. In this study, using mouse models, we establish a causal link between transient imbalances in degenerating inner ear function and the establishment of stable asymmetries in neural pathways that regulate motor activity and in motor behavior. Our study also suggests that shared mechanisms may underlie lateralized motor behaviors across mammalian species. For example, we show that in humans with normal ear function, the strength of the vestibular response from each ear in the forebrain correlates with asymmetric motor behavior, measured as handedness. In a broader sense, our study reveals a conceptually novel role for sensory input in shaping the asymmetric distribution of brain function, a process for which there is otherwise no clear mechanism.

Tbx1 and Jag1 act in concert to modulate the fate of neurosensory cells of the mouse otic vesicle

The domain within the otic vesicle (OV) known as the neurosensory domain (NSD), contains cells that will give rise to the hair and support cells of the otic sensory organs, as well as the neurons that form the cochleovestibular ganglion (CVG). The molecular dynamics that occur at the NSD boundary relative to adjacent OV cells is not well defined. The Tbx1 transcription factor gene expression pattern is complementary to the NSD, and inactivation results in expansion of the NSD and expression of the Notch ligand, Jag1 mapping, in part of the NSD. To shed light on the role of Jag1 in NSD development, as well as to test whether Tbx1 and Jag1 might genetically interact to regulate this process, we inactivated Jag1 within the Tbx1 expression domain using a knock-in Tbx1^Cre allele. We observed an enlarged neurogenic domain marked by a synergistic increase in expression of NeuroD and other proneural transcription factor genes in double Tbx1 and Jag1 conditional loss-of-function embryos. We noted that neuroblasts preferentially expanded across the medial-lateral axis and that an increase in cell proliferation could not account for this expansion, suggesting that there was a change in cell fate. We also found that inactivation of Jag1 with Tbx1^Cre resulted in failed development of the cristae and semicircular canals, as well as notably fewer hair cells in the ventral epithelium of the inner ear rudiment when inactivated on a Tbx1 null background, compared to Tbx1^Cre/− mutant embryos. We propose that loss of expression of Tbx1 and Jag1 within the Tbx1 expression domain tips the balance of cell fates in the NSD, resulting in an overproduction of neuroblasts at the expense of non-neural cells within the OV.

Summary: Normal dosages of Tbx1 and Jag1 are required to maintain a proper balance of cell types within the neurosensory domain of the otic vesicle to form the inner ear.

Transcriptome-wide comparison of the impact of Atoh1 and miR-183 family on pluripotent stem cells and multipotent otic progenitor cells

Over 5% of the global population suffers from disabling hearing loss caused by multiple factors including aging, noise exposure, genetic predisposition, or use of ototoxic drugs. Sensorineural hearing loss is often caused by the loss of sensory hair cells (HCs) of the inner ear. A barrier to hearing restoration after HC loss is the limited ability of mammalian auditory HCs to spontaneously regenerate. Understanding the molecular mechanisms orchestrating HC development is expected to facilitate cell replacement therapies. Multiple events are known to be essential for proper HC development including the expression of Atoh1 transcription factor and the miR-183 family. We have developed a series of vectors expressing the miR-183 family and/or Atoh1 that was used to transfect two different developmental cell models: pluripotent mouse embryonic stem cells (mESCs) and immortalized multipotent otic progenitor (iMOP) cells representing an advanced developmental stage. Transcriptome profiling of transfected cells show that the impact of Atoh1 is contextually dependent with more HC-specific effects on iMOP cells. miR-183 family expression in combination with Atoh1 not only appears to fine tune gene expression in favor of HC fate, but is also required for the expression of some HC-specific genes. Overall, the work provides novel insight into the combined role of Atoh1 and the miR-183 family during HC development that may ultimately inform strategies to promote HC regeneration or maintenance.

The Severity of Vestibular Dysfunction in Deafness as a Determinant of Comorbid Hyperactivity or Anxiety

Attention-deficit/hyperactivity disorder (ADHD) and anxiety-related disorders occur at rates 2-3 times higher in deaf compared with hearing children. Potential explanations for these elevated rates and the heterogeneity of behavioral disorders associated with deafness have usually focused on socio-environmental rather than biological effects. Children with the 22q11.2 deletion or duplication syndromes often display hearing loss and behavioral disorders, including ADHD and anxiety-related disorders. Here, we show that mouse mutants with either a gain or loss of function of the T-Box transcription factor gene, Tbx1, which lies within the 22q11.2 region and is responsible for most of the syndromic defects, exhibit inner ear defects and hyperactivity. Furthermore, we show that (1) inner ear dysfunction due to the tissue-specific loss of Tbx1 or Slc12a2, which encodes a sodium-potassium-chloride cotransporter and is also necessary for inner ear function, causes hyperactivity; (2) vestibular rather than auditory failure causes hyperactivity; and (3) the severity rather than the age of onset of vestibular dysfunction differentiates whether hyperactivity or anxiety co-occurs with inner ear dysfunction. Together, these findings highlight a biological link between inner ear dysfunction and behavioral disorders and how sensory abnormalities can contribute to the etiology of disorders traditionally considered of cerebral origin.SIGNIFICANCE STATEMENT This study examines the biological rather than socio-environmental reasons why hyperactivity and anxiety disorders occur at higher rates in deaf individuals. Using conditional genetic approaches in mice, the authors show that (1) inner ear dysfunction due to either Tbx1 or Slc12a2 mutations cause hyperactivity; (2) it is vestibular dysfunction, which frequently co-occurs with deafness but often remains undiagnosed, rather than auditory dysfunction that causes hyperactivity and anxiety-related symptoms; and (3) the severity of vestibular dysfunction can predict whether hyperactivity or anxiety coexist with inner ear dysfunction. These findings suggest a need to evaluate vestibular function in hearing impaired individuals, especially those who exhibit hyperactive and anxiety-related symptoms.

Prevalence of hearing loss and clinical otologic manifestations in patients with 22q11.2 deletion syndrome: A literature review

BACKGROUND

Hearing loss and otitis media are frequently reported in patients with 22q11.2 deletion syndrome.

OBJECTIVE OF REVIEW

Our objective was to review the current literature on the prevalence of hearing loss and otologic manifestations in patients with 22q11.2 deletion syndrome.

TYPE OF REVIEW

Systematic review.

SEARCH STRATEGY

We conducted a systematic search in PubMed and Embase combining the term "22q11.2 deletion syndrome" and synonyms with "hearing loss" and "otologic manifestations" and synonyms.

EVALUATION METHOD

We screened title/abstract and full text of all retrieved articles on pre-defined in- and exclusion criteria. The remaining articles were assessed on risk of bias. Outcome measures included the prevalence of hearing loss and otologic manifestations such as otitis media.

RESULTS

Our search yielded 558 unique studies of which a total of 25 articles were included for critical appraisal and data extraction. Twenty-one studies reported on hearing loss, and 21 studies on otologic manifestations. The prevalence of hearing loss varied from 6.0% to 60.3%, where in most studies conductive hearing loss was most prevalent. Rates of recurrent or chronic otitis media varied from 2.2% to 89.8%.

CONCLUSION

Although a very broad range in prevalences is reported in different studies, hearing loss and recurrent or chronic otitis media are frequently present in patients with 22q11.2 deletion syndrome. Regular check-ups and audiometric testing are advised in these patients.

Reduced dosage of β-catenin provides significant rescue of cardiac outflow tract anomalies in a Tbx1 conditional null mouse model of 22q11.2 deletion syndrome

The 22q11.2 deletion syndrome (22q11.2DS; velo-cardio-facial syndrome; DiGeorge syndrome) is a congenital anomaly disorder in which haploinsufficiency of TBX1, encoding a T-box transcription factor, is the major candidate for cardiac outflow tract (OFT) malformations. Inactivation of Tbx1 in the anterior heart field (AHF) mesoderm in the mouse results in premature expression of pro-differentiation genes and a persistent truncus arteriosus (PTA) in which septation does not form between the aorta and pulmonary trunk. Canonical Wnt/β-catenin has major roles in cardiac OFT development that may act upstream of Tbx1. Consistent with an antagonistic relationship, we found the opposite gene expression changes occurred in the AHF in β-catenin loss of function embryos compared to Tbx1 loss of function embryos, providing an opportunity to test for genetic rescue. When both alleles of Tbx1 and one allele of β-catenin were inactivated in the Mef2c-AHF-Cre domain, 61% of them (n = 34) showed partial or complete rescue of the PTA defect. Upregulated genes that were oppositely changed in expression in individual mutant embryos were normalized in significantly rescued embryos. Further, β-catenin was increased in expression when Tbx1 was inactivated, suggesting that there may be a negative feedback loop between canonical Wnt and Tbx1 in the AHF to allow the formation of the OFT. We suggest that alteration of this balance may contribute to variable expressivity in 22q11.2DS.

Identification of a Novel ENU-Induced Mutation in Mouse Tbx1 Linked to Human DiGeorge Syndrome

The patients with DiGeorge syndrome (DGS), caused by deletion containing dozens of genes in chromosome 22, often carry cardiovascular problem and hearing loss associated with chronic otitis media. Inside the deletion region, a transcription factor TBX1 was highly suspected. Furthermore, similar DGS phenotypes were found in the Tbx1 heterozygous knockout mice. Using ENU-induced mutagenesis and G1 dominant screening strategy, here we identified a nonsynonymous mutation p.W118R in T-box of TBX1, the DNA binding domain for transcription activity. The mutant mice showed deficiency of inner ear functions, including head tossing and circling, plus increased hearing threshold determined by audiometry. Therefore, our result further confirms the pathogenic basis of Tbx1 in DGS, points out the crucial role of DNA binding activity of TBX1 for the ear function, and provides additional animal model for studying the DGS disease mechanisms.

Otologic and audiologic findings in 22q11.2 deletion syndrome

Hearing loss is frequently present in the 22q11.2 deletion syndrome. Our aim was to describe the audiologic and otologic features of patients with 22q11.2 deletion syndrome. We conducted a retrospective cohort study in a single tertiary referral center. We reviewed medical files of all patients with 22q11.2 deletion syndrome who visited an otolaryngologist, plastic surgeon or speech therapist, for audiologic or otologic features. Hearing loss was defined as a pure tone average (of 0.5, 1, 2, and 4 kHz) of >20 decibel hearing level. Audiograms were available for 102 of 199 included patients, out of which 163 ears were measured in the required frquencies (0.5-4 kHz). Median age at time of most recent audiogram was 7 years (range 3-29 years). In 62 out of 163 ears (38%), hearing loss was present. Most ears had conductive hearing loss (n = 58) and 4 ears had mixed hearing loss. The severity of hearing loss was most frequently mild (pure tone average of ≤40 decibel hearing level). In 22.5% of ears, otitis media with effusion was observed at time of most recent audiogram. Age was not related to mean air conduction hearing thresholds or to otitis media with effusion (p = 0.43 and p = 0.11, respectively). In conclusion, hearing loss and otitis media are frequently present in patients with 22q11.2 deletion syndrome. Moreover, our results suggest that children with 22q11.2 deletion syndrome remain susceptible for otitis media as they age.

Prevalence of hearing loss in children with 22q11.2 deletion syndrome

OBJECTIVE

The purpose of this study was to determine the prevalence and characterize the types of hearing loss in pediatric patients with 22q11.2 deletion syndrome (22q11DS).

METHODS

Fifty-eight patients were identified via retrospective chart review performed of patients with 22q11DS between 1996 and 2014. Patient demographics, pertinent family history, associated comorbidities, and degree and type of hearing loss were gathered for each patient. A literature review of the National Library of Medicine's database with a focus on hearing loss and 22q11DS was performed.

RESULTS

22 patients (38%) were found to have hearing impairment: 68% with conductive hearing loss, 14% with sensorineural hearing loss, and 18% with mixed hearing loss. Patients with hearing loss regardless of type had a higher prevalence of developmental delay (55%), cleft palate (23%), articulation disorders (77%), and a greater need for tympanostomy tubes (73%) compared to patients with normal hearing. Temporal bone computed tomography scans of 5 patients revealed a variety of abnormalities in the middle and/or inner ears.

CONCLUSION

Hearing impairment occurs in up to 38% of 22q11DS patients of both conductive and sensorineural types, with the conductive type being the most common. These patients have a greater need for tympanostomy tubes and a higher prevalence of developmental delay and speech articulation disorders. Early hearing screening and treatment is warranted in this population.

22q11.2 Deletion Syndrome due to a Translocation t(6;22) in a Patient Conceived via in vitro Fertilization

We report on a patient conceived via in vitro fertilization (IVF) with a 22q11.2 deletion due to an unusual unbalanced translocation involving chromosomes 6 and 22 in a karyotype with 45 chromosomes. Cytogenomic studies showed that the patient has a 3.3-Mb deletion of chromosome 22q and a 0.4-Mb deletion of chromosome 6p, which resulted in haploinsufficiency of the genes responsible for the 22q11.2 deletion syndrome and also of the IRF4 gene, a member of the interferon regulatory factor family of transcription factors, which is expressed in the immune system cells. The rearrangement could be due to the manipulation of the embryo or as a sporadic event unrelated to IVF. Translocation involving chromosome 22 in a karyotype with 45 chromosomes is a rare event, with no previous reports involving chromosomes 6p and 22q.

A defect in early myogenesis causes Otitis media in two mouse models of 22q11.2 Deletion Syndrome

Otitis media (OM), the inflammation of the middle ear, is the most common disease and cause for surgery in infants worldwide. Chronic Otitis media with effusion (OME) often leads to conductive hearing loss and is a common feature of a number of craniofacial syndromes, such as 22q11.2 Deletion Syndrome (22q11.2DS). OM is more common in children because the more horizontal position of the Eustachian tube (ET) in infants limits or delays clearance of middle ear effusions. Some mouse models with OM have shown alterations in the morphology and angle of the ET. Here, we present a novel mechanism in which OM is caused not by a defect in the ET itself but in the muscles that control its function. Our results show that in two mouse models of 22q11.2DS (Df1/+ and Tbx1(+/-)) presenting with bi- or unilateral OME, the fourth pharyngeal arch-derived levator veli palatini muscles were hypoplastic, which was associated with an earlier altered pattern of MyoD expression. Importantly, in mice with unilateral OME, the side with the inflammation was associated with significantly smaller muscles than the contralateral unaffected ear. Functional tests examining ET patency confirmed a reduced clearing ability in the heterozygous mice. Our findings are also of clinical relevance as targeting hypoplastic muscles might present a novel preventative measure for reducing the high rates of OM in 22q11.2DS patients.

Segregating neural and mechanosensory fates in the developing ear: patterning, signaling, and transcriptional control

The vertebrate inner ear is composed of multiple sensory receptor epithelia, each of which is specialized for detection of sound, gravity, or angular acceleration. Each receptor epithelium contains mechanosensitive hair cells, which are connected to the brainstem by bipolar sensory neurons. Hair cells and their associated neurons are derived from the embryonic rudiment of the inner ear epithelium, but the precise spatial and temporal patterns of their generation, as well as the signals that coordinate these events, have only recently begun to be understood. Gene expression, lineage tracing, and mutant analyses suggest that both neurons and hair cells are generated from a common domain of neural and sensory competence in the embryonic inner ear rudiment. Members of the Shh, Wnt, and FGF families, together with retinoic acid signals, regulate transcription factor genes within the inner ear rudiment to establish the axial identity of the ear and regionalize neurogenic activity. Close-range signaling, such as that of the Notch pathway, specifies the fate of sensory regions and individual cell types. We also describe positive and negative interactions between basic helix-loop-helix and SoxB family transcription factors that specify either neuronal or sensory fates in a context-dependent manner. Finally, we review recent work on inner ear development in zebrafish, which demonstrates that the relative timing of neurogenesis and sensory epithelial formation is not phylogenetically constrained.

A novel FOXE1 mutation (R73S) in Bamforth-Lazarus syndrome causing increased thyroidal gene expression

BACKGROUND

Homozygous loss-of-function mutations in the FOXE1 gene have been reported in several patients with partial or complete Bamforth-Lazarus syndrome: congenital hypothyroidism (CH) with thyroid dysgenesis (usually athyreosis), cleft palate, spiky hair, with or without choanal atresia, and bifid epiglottis. Here, our objective was to evaluate potential functional consequences of a FOXE1 mutation in a patient with a similar clinical phenotype.

METHODS

FOXE1 was sequenced in eight patients with thyroid dysgenesis and cleft palate. Transient transfection was performed in HEK293 cells using the thyroglobulin (TG) and thyroid peroxidase (TPO) promoters in luciferase reporter plasmids to assess the functional impact of the FOXE1 mutations. Primary human thyrocytes transfected with wild type and mutant FOXE1 served to assess the impact of the mutation on endogenous TG and TPO expression.

RESULTS

We identified and characterized the function of a new homozygous FOXE1 missense mutation (p.R73S) in a boy with a typical phenotype (athyreosis, cleft palate, and partial choanal atresia). This new mutation located within the forkhead domain was inherited from the heterozygous healthy consanguineous parents. In vitro functional studies in HEK293 cells showed that this mutant gene enhanced the activity of the TG and TPO gene promoters (1.5-fold and 1.7-fold respectively vs. wild type FOXE1; p<0.05), unlike the five mutations previously reported in Bamforth-Lazarus syndrome. The gain-of-function effect of the FOXE1-p.R73S mutant gene was confirmed by an increase in endogenous TG production in primary human thyrocytes.

CONCLUSION

We identified a new homozygous FOXE1 mutation responsible for enhanced expression of the TG and TPO genes in a boy whose phenotype is similar to that reported previously in patients with loss-of-function FOXE1 mutations. This finding further delineates the role for FOXE1 in both thyroid and palate development, and shows that enhanced gene activity should be considered among the mechanisms underlying Bamforth-Lazarus syndrome.

Conditional and constitutive expression of a Tbx1-GFP fusion protein in mice

Background

Velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS) is caused by a 1.5-3 Mb microdeletion of chromosome 22q11.2, frequently referred to as 22q11.2 deletion syndrome (22q11DS). This region includes TBX1, a T-box transcription factor gene that contributes to the etiology of 22q11DS. The requirement for TBX1 in mammalian development is dosage-sensitive, such that loss-of-function (LOF) and gain-of-function (GOF) of TBX1 in both mice and humans results in disease relevant congenital malformations.

Results

To further gain insight into the role of Tbx1 in development, we have targeted the Rosa26 locus to generate a new GOF mouse model in which a Tbx1-GFP fusion protein is expressed conditionally using the Cre/LoxP system. Tbx1-GFP expression is driven by the endogenous Rosa26 promoter resulting in ectopic and persistent expression. Tbx1 is pivotal for proper ear and heart development; ectopic activation of Tbx1-GFP in the otic vesicle by Pax2-Cre and Foxg1-Cre represses neurogenesis and produces morphological defects of the inner ear. Overexpression of a single copy of Tbx1-GFP using Tbx1^Cre/+ was viable, while overexpression of both copies resulted in neonatal lethality with cardiac outflow tract defects. We have partially rescued inner ear and heart anomalies in Tbx1^Cre/- null embryos by expression of Tbx1-GFP.

Conclusions

We have generated a new mouse model to conditionally overexpress a GFP-tagged Tbx1 protein in vivo. This provides a useful tool to investigate in vivo direct downstream targets and protein binding partners of Tbx1.

Chronic overload of SEPT4, a parkin substrate that aggregates in Parkinson's disease, causes behavioral alterations but not neurodegeneration in mice

Background

In autosomal recessive early-onset Parkinsonism (PARK2), the pathogenetic process from the loss of function of a ubiquitin ligase parkin to the death of dopamine neurons remains unclear. A dominant hypothesis attributes the neurotoxicity to accumulated substrates that are exempt from parkin-mediated degradation. Parkin substrates include two septins; SEPT4/CDCrel-2 which coaggregates with α-synuclein as Lewy bodies in Parkinson’s disease, and its closest homolog SEPT5/CDCrel-1/PNUTL1 whose overload with viral vector can rapidly eliminate dopamine neurons in rats. However, chronic effects of pan-neural overload of septins have never been examined in mammals. To address this, we established a line of transgenic mice that express the largest gene product SEPT4^54kDa via the prion promoter in the entire brain.

Results

Histological examination and biochemical quantification of SEPT4-associated proteins including α-synuclein and the dopamine transporter in the nigrostriatal dopamine neurons found no significant difference between Sept4^Tg/+ and wild-type littermates. Thus, the hypothetical pathogenicity by the chronic overload of SEPT4 alone, if any, is insufficient to trigger neurodegenerative process in the mouse brain. Intriguingly, however, a systematic battery of behavioral tests revealed unexpected abnormalities in Sept4^Tg/+ mice that include consistent attenuation of voluntary activities in distinct behavioral paradigms and altered social behaviors.

Conclusions

Together, these data indicate that septin dysregulations commonly found in postmortem human brains with Parkinson’s disease, schizophrenia and bipolar disorders may be responsible for a subset of behavioral abnormalities in the patients.

Of mice and men: molecular genetics of congenital heart disease

Congenital heart disease (CHD) affects nearly 1 % of the population. It is a complex disease, which may be caused by multiple genetic and environmental factors. Studies in human genetics have led to the identification of more than 50 human genes, involved in isolated CHD or genetic syndromes, where CHD is part of the phenotype. Furthermore, mapping of genomic copy number variants and exome sequencing of CHD patients have led to the identification of a large number of candidate disease genes. Experiments in animal models, particularly in mice, have been used to verify human disease genes and to gain further insight into the molecular pathology behind CHD. The picture emerging from these studies suggest that genetic lesions associated with CHD affect a broad range of cellular signaling components, from ligands and receptors, across down-stream effector molecules to transcription factors and co-factors, including chromatin modifiers.

Understanding velocardiofacial syndrome: how recent discoveries can help you improve your patient outcomes

PURPOSE OF REVIEW

Improved recognition of velocardiofacial syndrome (VCFS) has led to increasing awareness of VCFS by otolaryngologists. Understanding the developmental biologic processes affected in VCFS patients will help improve treatment and outcomes. Advanced application of molecular labeling techniques has better outlined the role of T-Box transcription factor 1 (TBX1) as the primary genetic anomaly leading to VCFS. TBX1 plays multiple roles during branchial, cardiac, and craniofacial development and increased understanding of how these systems are affected by TBX1 mutations will improve patient outcomes. Furthermore, additional modifiers of TBX1 expression have been identified that may explain the variability of VCFS phenotypes. The phenotypic spectrum of VCFS may include cardiac anomalies, velopharyngeal insufficiency, aberrant calcium metabolism, and immune dysfunction. Recent interest has focused on the cognitive and neuropsychiatric manifestations of VCFS. Improved understanding of the biology of VCFS associated mutations has the potential to improve therapeutic outcomes.

RECENT FINDINGS

This article will discuss recent developmental biologic understanding of the role of TBX1 and genetic modifiers generating the phenotypic variability seen in VCFS patients. Special attention is given to advances in the realms of immunodeficiency, hypocalcemia, cardiac and arterial patterning anomalies, velopharyngeal insufficiency, as well as cognitive and psychiatric problems.

SUMMARY

Enhanced understanding of the multiple systems affected by TBX1 mutations will result in improved patient outcomes and improved family education. Future research will lead to improved detection of potential targets for gene therapy and change the way physicians counsel families and treat patients.

Biallelic expression of Tbx1 protects the embryo from developmental defects caused by increased receptor tyrosine kinase signaling

BACKGROUND

22q11.2 deletion syndrome (22q11DS) is the most common microdeletion syndrome in humans, characterized by cardiovascular defects such as interrupted aortic arch, outflow tract defects, thymus and parathyroid hypo- or aplasia, and cleft palate. Heterozygosity of Tbx1, the mouse homolog of the candidate TBX1 gene, results in mild defects dependent on genetic background, whereas complete inactivation results in severe malformations in multiple tissues.

RESULTS

The loss of function of two Sprouty genes, which encode feedback antagonists of receptor tyrosine kinase (RTK) signaling, phenocopy many defects associated with 22q11DS in the mouse. The stepwise reduction of Sprouty gene dosage resulted in different phenotypes emerging at specific steps, suggesting that the threshold up to which a given developmental process can tolerate increased RTK signaling is different. Tbx1 heterozygosity significantly exacerbated the severity of all these defects, which correlated with a substantial increase in RTK signaling.

CONCLUSIONS

Our findings suggest that TBX1 functions as an essential component of a mechanism that protects the embryo against perturbations in RTK signaling that may lead to developmental defects characteristic of 22q11DS. We propose that genetic factors that enhance RTK signaling ought to be considered as potential genetic modifiers of this syndrome.

Stem cell therapy for the inner ear: recent advances and future directions

In vertebrates, perception of sound, motion, and balance is mediated through mechanosensory hair cells located within the inner ear. In mammals, hair cells are only generated during a short period of embryonic development. As a result, loss of hair cells as a consequence of injury, disease, or genetic mutation, leads to permanent sensory deficits. At present, cochlear implantation is the only option for profound hearing loss. However, outcomes are still variable and even the best implant cannot provide the acuity of a biological ear. The recent emergence of stem cell technology has the potential to open new approaches for hair cell regeneration. The goal of this review is to summarize the current state of inner ear stem cell research from a viewpoint of its clinical application for inner ear disorders to illustrate how complementary studies have the potential to promote and refine stem cell therapies for inner ear diseases. The review initially discusses our current understanding of the genetic pathways that regulate hair cell formation from inner ear progenitors during normal development. Subsequent sections discuss the possible use of endogenous inner ear stem cells to induce repair as well as the initial studies aimed at transplanting stem cells into the ear.

MicroRNA-182 regulates otocyst-derived cell differentiation and targets T-box1 gene

BACKGROUND

Recently, in vitro and in vivo models have identified that microRNAs (miRNAs), which are extensively expressed in the inner ear, play important roles in inner ear development and function. However, the function of miRNA in vertebrate tissue is not well understood.

RESULTS

The current study used an in vitro model of embryonic mouse inner ear in a stem/progenitor cell culture to demonstrate that: 1) miR-182 is expressed during differentiation of inner ear stem/progenitor cell into a hair cell-like fate, 2) ectopic miR-182 promotes inner ear stem/progenitor cell differentiation into a hair cell-like fate, and 3) the function of miR-182 may be associated with its putative target Tbx1, a transcription factors that have been implicated in inner ear development and hair cell fate.

CONCLUSIONS

Our findings suggest that miR-182 could regulate inner ear progenitor cell differentiation and that miRNAs are important regulators of hair cell differentiation, providing new targets for hair cell repair.

A noncoding point mutation of Zeb1 causes multiple developmental malformations and obesity in Twirler mice

Heterozygous Twirler (Tw) mice develop obesity and circling behavior associated with malformations of the inner ear, whereas homozygous Tw mice have cleft palate and die shortly after birth. Zeb1 is a zinc finger protein that contributes to mesenchymal cell fate by repression of genes whose expression defines epithelial cell identity. This developmental pathway is disrupted in inner ears of Tw/Tw mice. The purpose of our study was to comprehensively characterize the Twirler phenotype and to identify the causative mutation. The Tw/+ inner ear phenotype includes irregularities of the semicircular canals, abnormal utricular otoconia, a shortened cochlear duct, and hearing loss, whereas Tw/Tw ears are severely malformed with barely recognizable anatomy. Tw/+ mice have obesity associated with insulin-resistance and have lymphoid organ hypoplasia. We identified a noncoding nucleotide substitution, c.58+181G>A, in the first intron of the Tw allele of Zeb1 (Zeb1^Tw). A knockin mouse model of c.58+181G>A recapitulated the Tw phenotype, whereas a wild-type knockin control did not, confirming the mutation as pathogenic. c.58+181G>A does not affect splicing but disrupts a predicted site for Myb protein binding, which we confirmed in vitro. In comparison, homozygosity for a targeted deletion of exon 1 of mouse Zeb1, Zeb1^ΔEx1, is associated with a subtle abnormality of the lateral semicircular canal that is different than those in Tw mice. Expression analyses of E13.5 Twirler and Zeb1^ΔEx1 ears confirm that Zeb1^ΔEx1 is a null allele, whereas Zeb1^Tw RNA is expressed at increased levels in comparison to wild-type Zeb1. We conclude that a noncoding point mutation of Zeb1 acts via a gain-of-function to disrupt regulation of Zeb1^Tw expression, epithelial-mesenchymal cell fate or interactions, and structural development of the inner ear in Twirler mice. This is a novel mechanism underlying disorders of hearing or balance.

Twirler (Tw) mice have a combination of abnormalities that includes cleft palate, malformations of the inner ear, hearing loss, vestibular dysfunction, obesity, and lymphoid hypoplasia. In this study, we show that the underlying mutation affects the Zeb1 gene. Zeb1 was already known to encode a protein normally expressed in mesenchymal cells, where it represses expression of genes that are uniquely expressed in epithelial cells. The Tw mutation is a rare example of a single-nucleotide substitution in a region of a gene that does not encode protein, promoter, or splice sites, so we engineered a mouse model with the mutation that confirmed its causative role. The Tw mutation disrupts a consensus DNA binding site sequence for the Myb family of regulatory proteins. We conclude that this mutation leads to abnormal expression of Zeb1, structural malformations of the inner ear, and a loss of hearing and balance function. A similar mechanism may underlie other features of Twirler, such as obesity and cleft palate.

Mature middle and inner ears express Chd7 and exhibit distinctive pathologies in a mouse model of CHARGE syndrome

Heterozygous mutations in the gene encoding chromodomain-DNA-binding-protein 7 (CHD7) cause CHARGE syndrome, a multiple anomaly condition which includes vestibular dysfunction and hearing loss. Mice with heterozygous Chd7 mutations exhibit semicircular canal dysgenesis and abnormal inner ear neurogenesis, and are an excellent model of CHARGE syndrome. Here we characterized Chd7 expression in mature middle and inner ears, analyzed morphological features of mutant ears and tested whether Chd7 mutant mice have altered responses to noise exposure and correlated those responses to inner and middle ear structure. We found that Chd7 is highly expressed in mature inner and outer hair cells, spiral ganglion neurons, vestibular sensory epithelia and middle ear ossicles. There were no obvious defects in individual hair cell morphology by prestin immunostaining or scanning electron microscopy, and cochlear innervation appeared normal in Chd7(Gt)(/+) mice. Hearing thresholds by auditory brainstem response (ABR) testing were elevated at 4 and 16 kHz in Chd7(Gt)(/+) mice, and there were reduced distortion product otoacoustic emissions (DPOAE). Exposure of Chd7(Gt)(/+) mice to broadband noise resulted in variable degrees of hair cell loss which inversely correlated with severity of stapedial defects. The degrees of hair cell loss and threshold shifts after noise exposure were more severe in wild type mice than in mutants. Together, these data indicate that Chd7(Gt)(/+) mice have combined conductive and sensorineural hearing loss, correlating with changes in both middle and inner ears.

Transient retinoic acid signaling confers anterior-posterior polarity to the inner ear

Vertebrate hearing and balance are based in complex asymmetries of inner ear structure. Here, we identify retinoic acid (RA) as an extrinsic signal that acts directly on the ear rudiment to affect its compartmentalization along the anterior-posterior axis. A rostrocaudal wave of RA activity, generated by tissues surrounding the nascent ear, induces distinct responses from anterior and posterior halves of the inner ear rudiment. Prolonged response to RA by posterior otic tissue correlates with Tbx1 transcription and formation of mostly nonsensory inner ear structures. By contrast, anterior otic tissue displays only a brief response to RA and forms neuronal elements and most sensory structures of the inner ear.

Canonical Wnt signaling modulates Tbx1, Eya1, and Six1 expression, restricting neurogenesis in the otic vesicle

To understand the mechanism by which canonical Wnt signaling sets boundaries for pattern formation in the otic vesicle (OV), we examined Tbx1 and Eya1-Six1 downstream of activated beta-catenin. Tbx1, the gene for velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS), is essential for inner ear development where it promotes Bmp4 and Otx1 expression and restricts neurogenesis. Using floxed beta-catenin gain-of-function (GOF) and loss-of-function (LOF) alleles, we found Tbx1 expression was down-regulated and maintained/enhanced in the two mouse mutants, respectively. Bmp4 was ectopically expressed and Otx1 was lost in beta-catenin GOF mutants. Normally, inactivation of Tbx1 causes expanded neurogenesis, but expression of NeuroD was down-regulated in beta-catenin GOF mutants. To explain this paradox, Eya1 and Six1, genes for branchio-oto-renal (BOR) syndrome were down-regulated in the OV of beta-catenin GOF mutants independently of Tbx1. Overall, this work helps explain the mechanism by which Wnt signaling modulates transcription factors required for neurogenesis and patterning of the OV.

A symphony of inner ear developmental control genes

The inner ear is one of the most complex and detailed organs in the vertebrate body and provides us with the priceless ability to hear and perceive linear and angular acceleration (hence maintain balance). The development and morphogenesis of the inner ear from an ectodermal thickening into distinct auditory and vestibular components depends upon precise temporally and spatially coordinated gene expression patterns and well orchestrated signaling cascades within the otic vesicle and upon cellular movements and interactions with surrounding tissues. Gene loss of function analysis in mice has identified homeobox genes along with other transcription and secreted factors as crucial regulators of inner ear morphogenesis and development. While otic induction seems dependent upon fibroblast growth factors, morphogenesis of the otic vesicle into the distinct vestibular and auditory components appears to be clearly dependent upon the activities of a number of homeobox transcription factors. The Pax2 paired-homeobox gene is crucial for the specification of the ventral otic vesicle derived auditory structures and the Dlx5 and Dlx6 homeobox genes play a major role in specification of the dorsally derived vestibular structures. Some Micro RNAs have also been recently identified which play a crucial role in the inner ear formation.

Supportive evidence for reduced expression of GNB1L in schizophrenia

BACKGROUND

Chromosome 22q11 deletion syndrome (22q11DS) increases the risk of development of schizophrenia more than 10 times compared with that of the general population, indicating that haploinsufficiency of a subset of the more than 20 genes contained in the 22q11DS region could increase the risk of schizophrenia. In the present study, we screened for genes located in the 22q11DS region that are expressed at lower levels in postmortem prefrontal cortex of patients with schizophrenia than in those of controls.

METHODS

Gene expression was screened by Illumina Human-6 Expression BeadChip arrays and confirmed by real-time reverse transcription-polymerase chain reaction assays and Western blot analysis.

RESULTS

Expression of GNB1L was lower in patients with schizophrenia than in control subjects in both Australian (10 schizophrenia cases and 10 controls) and Japanese (43 schizophrenia cases and 11 controls) brain samples. TBX1 could not be evaluated due to its low expression levels. Expression levels of the other genes were not significantly lower in patients with schizophrenia than in control subjects. Association analysis of tag single-nucleotide polymorphisms in the GNB1L gene region did not confirm excess homozygosity in 1918 Japanese schizophrenia cases and 1909 Japanese controls. Haloperidol treatment for 50 weeks increased Gnb1l gene expression in prefrontal cortex of mice.

CONCLUSIONS

Taken together with the impaired prepulse inhibition observed in heterozygous Gnb1l knockout mice reported by the previous study, the present findings support assertions that GNB1L is one of the genes in the 22q11DS region responsible for increasing the risk of schizophrenia.

Tourette syndrome and klippel-feil anomaly in a child with chromosome 22q11 duplication

This is the first case description of the association of Klippel-Feil Syndrome (KFS), Tourette Syndrome (TS), Motor Stereotypies, and Obsessive Compulsive Behavior, with chromosome 22q11.2 Duplication Syndrome (22q11DupS). Neuropsychiatric symptoms in persons with 22q11.2 deletion, including obsessive compulsiveness, anxiety, hyperactivity, and one prior case report of TS, have been attributed to low copy number effects on Catechol-O-Methyltransferase (COMT). However, the present unique case of 22q11DupS and TS suggests a more complex relationship, either for low- or high-COMT activity, or for other genes at this locus.

Analysis of prepulse inhibition in mouse lines overexpressing 22q11.2 orthologues

Animal models have been useful in elucidating the genetic basis of the cognitive and behavioural phenotypes associated with the 22q11.2 microdeletions. Loss-of-function models have implicated a number of genes as playing a role in prepulse inhibition (PPI) of the startle response. Here, we report the generation and initial analysis of bacterial artificial chromosome (BAC) transgenic (Tg) mice, overexpressing genes from within the 22q11.2 locus. We used engineered BAC constructs to generate Tg lines and quantitative RT-PCR to assess levels of gene expression in each line. We assessed PPI and open-field activity in mice from two low copy number lines. In Tg-1, a line overexpressing Prodh and Vpreb2, PPI was significantly increased at prepulse levels of 78 dB and 82 dB while no differences were found in activity measures. By contrast, no significant differences were found in PPI testing of the Tg-2 line overexpressing Zdhhc8, Ranbp1, Htf9c, T10, Arvcf and Comt. Taken together with previous loss-of-function reports, these findings suggest that Prodh has a key role in modulating the degree of sensorimotor gating in mice and possibly in humans and provide additional support for an important role of this pathway in modulating behavioural deficits associated with genomic gains or losses at 22q11.2.

Mouse models for human otitis media

Otitis media (OM) remains the most common childhood disease and its annual costs exceed $5 billion. Its potential for permanent hearing impairment also emphasizes the need to better understand and manage this disease. The pathogenesis of OM is multifactorial and includes infectious pathogens, anatomy, immunologic status, genetic predisposition, and environment. Recent progress in mouse model development is helping to elucidate the respective roles of these factors and to significantly contribute toward efforts of OM prevention and control. Genetic predisposition is recognized as an important factor in OM and increasing numbers of mouse models are helping to uncover the potential genetic bases for human OM. Furthermore, the completion of the mouse genome sequence has offered a powerful set of tools for investigating gene function and is generating a rich resource of mouse mutants for studying the genetic factors underlying OM.

Mouse models of otitis media

PURPOSE OF REVIEW

Otitis media is one of the most prevalent inflammatory diseases in the pediatric population. The personal and societal costs for otitis media are significant. Problems arising from antibiotic use have led to considerable animal research efforts to better understand the mechanisms of acute otitis media and to develop new strategies for its prevention and treatment.

RECENT FINDINGS

Various animal models induce acute otitis media from a variety of interventions, including direct injection of whole bacteria or their products into the middle ear. The mouse model has begun to emerge as a model for otitis media. The mouse affords many advantages for in-vivo research, including ease of genetic manipulation, availability of numerous inbred and transgenic strains, and an extensively studied immune system. Experimental reagents for cellular and molecular studies are widely available for the mouse. The mouse is an excellent model for investigating the genetics and molecular bases for otitis media due to the extensive understanding of the mouse genome.

SUMMARY

With the increased availability of knockout and transgenic mice, and the large amount of data to indicate that human disease is accurately modeled in the mouse, the mouse model is increasingly becoming a model of choice.

How much is too much? Phenotypic consequences of Rai1 overexpression in mice

The retinoic acid induced 1 (RAI1) gene when deleted or mutated results in Smith-Magenis syndrome (SMS), while duplication of 17p11.2, including RAI1, results in the dup(17)(p11.2) syndrome characterized by mental retardation, growth and developmental delays, and hyperactivity. Mouse models for these human syndromes may help define critical roles for RAI1 in mammalian development and homeostasis that otherwise cannot be deduced from patient studies. A mouse model for duplication, Dp(11)17+, involving Rai1 has been reported. However, this mutant was engineered on a mixed genetic background confounding phenotypic effects due to possible modifier genes. We have therefore created and evaluated mice with a graded series of four (hemizygous) and six (homozygous) copies of Rai1, and overexpressing Rai1>1.5-fold and >2-fold, respectively. Data show that Rai1-transgenic mice have growth retardation, increased locomotor activity, and abnormal anxiety-related behavior compared to wild-type littermates. Rai1-transgenic mice also have an altered gait with short strides and long sways, impaired ability on a cage-top hang test, decreased forelimb grip strength, and a dominant social behavior. Further, analyses of homozygous transgenic mice revealed a dosage-dependent exacerbation of the phenotype, including extreme growth retardation, severe neurological deficits, and increased hyperactivity. Our results show that Rai1 dosage has major consequences on molecular processes involved in growth, development, and neurological and behavioral functions, thus providing evidence for several dosage-thresholds for phenotypic manifestations causing dup(17)(p11.2) syndrome or SMS in humans.

Identification of downstream genetic pathways of Tbx1 in the second heart field

Tbx1, a T-box transcription factor, and an important gene for velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS) in humans, causes outflow tract (OFT) heart defects when inactivated in the mouse. Tbx1 is expressed in the second heart field (SHF) and is required in this tissue for OFT development. To identify Tbx1 regulated genetic pathways in the SHF, we performed gene expression profiling of the caudal pharyngeal region in Tbx1(-/-) and wild type embryos. Isl1, a key marker for the SHF, as well as Hod and Nkx2-6, were downregulated in Tbx1(-/-) mutants, while genes required for cardiac morphogenesis, such as Raldh2, Gata4, and Tbx5, as well as a subset of muscle contractile genes, signifying myocardial differentiation, were ectopically expressed. Pan-mesodermal ablation of Tbx1 resulted in similar gene expression changes, suggesting cell-autonomous roles of Tbx1 in regulating these genes. Opposite expression changes concomitant with SHF-derived cardiac defects occurred in TBX1 gain-of-function mutants, indicating that appropriate levels of Tbx1 are required for heart development. When taken together, our studies show that Tbx1 acts upstream in a genetic network that positively regulates SHF cell proliferation and negatively regulates differentiation, cell-autonomously in the caudal pharyngeal region.

Human TBX1 missense mutations cause gain of function resulting in the same phenotype as 22q11.2 deletions

Deletion 22q11.2 syndrome is the most frequent known microdeletion syndrome and is associated with a highly variable phenotype, including DiGeorge and Shprintzen (velocardiofacial) syndromes. Although haploinsufficiency of the T-box transcription factor gene TBX1 is thought to cause the phenotype, to date, only four different point mutations in TBX1 have been reported in association with six of the major features of 22q11.2 deletion syndrome. Although, for the two truncating mutations, loss of function was previously shown, the pathomechanism of the missense mutations remains unknown. We report a novel heterozygous missense mutation, H194Q, in a familial case of Shprintzen syndrome and show that this and the two previously reported missense mutations result in gain of function, possibly through stabilization of the protein dimer DNA complex. We therefore conclude that TBX1 gain-of-function mutations can result in the same phenotypic spectrum as haploinsufficiency caused by loss-of-function mutations or deletions.

Regulation of cell fate in the sensory epithelia of the inner ear

The sensory epithelia of the inner ear contain mechanosensory hair cells and non-sensory supporting cells. Both classes of cell are heterogeneous, with phenotypes varying both between and within epithelia. The specification of individual cells as distinct types of hair cell or supporting cell is regulated through intra- and extracellular signalling pathways that have been poorly understood. However, new methodologies have resulted in significant steps forward in our understanding of the molecular pathways that direct cells towards these cell fates.

Mouse models as a tool to unravel the genetic basis for human otitis media

The pathogenesis of otitis media (OM) is multifactorial and includes infection, anatomical factors, immunologic status, genetic predisposition, and environmental factors. OM remains the most common cause of hearing impairment in childhood. Genetic predisposition is increasingly recognized as an important factor. The completion of the mouse genome sequence has offered a powerful basket of tools for investigating gene function and can expect to generate a rich resource of mouse mutants for the elucidation of genetic factors underlying OM. We review the literature and discuss recent progresses in developing mouse models and using mouse models to uncover the genetic basis for human OM.

Tissue-specific roles of Tbx1 in the development of the outer, middle and inner ear, defective in 22q11DS patients

Most 22q11.2 deletion syndrome (22q11DS) patients have middle and outer ear anomalies, whereas some have inner ear malformations. Tbx1, a gene hemizygously deleted in 22q11DS patients and required for ear development, is expressed in multiple tissues during embryogenesis. To determine the role of Tbx1 in the first pharyngeal pouch (PPI) in forming outer and middle ears, we tissue-specifically inactivated the gene using Foxg1-Cre. In the conditional mutants, PPI failed to outgrow, preventing the middle ear bone condensations from forming. Tbx1 was also inactivated in the otic vesicle (OV), resulting in the failure of inner ear sensory organ formation, and in duplication of the cochleovestibular ganglion (CVG). Consistent with the anatomical defects, the sensory genes, Otx1 and Bmp4 were downregulated, whereas the CVG genes, Fgf3 and NeuroD, were upregulated. To delineate Tbx1 cell-autonomous roles, a more selective ablation, exclusively in the OV, was performed using Pax2-Cre. In contrast to the Foxg1-Cre mutants, Pax2-Cre conditional mutant mice survived to adulthood and had normal outer and middle ears but had the same inner ear defects as the Tbx1 null mice, with the same gene expression changes. These results demonstrate that Tbx1 has non-cell autonomous roles in PPI in the formation of outer and middle ears and cell-autonomous roles in the OV. Periotic mesenchymal markers, Prx2 and Brn4 were normal in both conditional mutants, whereas they were diminished in Tbx1-/- embryos. Thus, Tbx1 in the surrounding mesenchyme in both sets of conditional mutants cannot suppress the defects in the OV that occur in the null mutants.

T-box genes in vertebrate development

The myriad developmental roles served by the T-box family of transcription factor genes defy easy categorization. Present in all metazoans, the T-box genes are involved in early embryonic cell fate decisions, regulation of the development of extraembryonic structures, embryonic patterning, and many aspects of organogenesis. They are unusual in displaying dosage sensitivity in most instances. In humans, mutations in T-box genes are responsible for developmental dysmorphic syndromes, and several T-box genes have been implicated in neoplastic processes. T-box transcription factors function in many different signaling pathways, notably bone morphogenetic protein and fibroblast growth factor pathways. The few downstream target genes that have been identified indicate a wide range of downstream effectors.

Inactivation of Tbx1 in the pharyngeal endoderm results in 22q11DS malformations

The 22q11 deletion (22q11DS; velo-cardio-facial syndrome/DiGeorge syndrome) is characterized by defects in the derivatives of the pharyngeal apparatus. Mouse genetic studies have identified Tbx1, a member of the T-box family of transcription factors, as being responsible for the physical malformations of the syndrome. Mice heterozygous for a null mutation in Tbx1 have mild anomalies, whereas homozygous Tbx1 mutants die at birth with severe defects in the derivatives of the pharyngeal apparatus, including cleft palate, thymus gland aplasia and cardiac outflow tract malformations. Tbx1 is expressed in the splanchnic mesenchyme, the pharyngeal endoderm (PE) and in the core mesoderm of the pharyngeal apparatus. Tissue interactions between the epithelia and mesenchyme of the arches are required for development of the pharyngeal apparatus; the precise role of Tbx1 in each tissue is not known. To assess the role of Tbx1 in the PE, a conditional allele of Tbx1 was generated using the Cre/loxP system. Foxg1-Cre was used to drive PE-specific ablation of Tbx1. Conditional null mutants survived embryogenesis, but died in the neonatal period with malformations identical to the defects observed in Tbx1 homozygous null mutants. The abnormalities appear to be secondary to failed outgrowth of the pharyngeal pouches. These results show that Tbx1 in the PE is required for the patterning and development of the pharyngeal apparatus, thereby disrupting the formation of its derivative structures.

A 200-kb region of human chromosome 22q11.2 confers antipsychotic-responsive behavioral abnormalities in mice

Human chromosome 22q11.2 has been implicated in various behavioral abnormalities, including schizophrenia and other neuropsychiatric/behavioral disorders. However, the specific genes within 22q11.2 that contribute to these disorders are still poorly understood. Here, we show that an approximately 200-kb segment of human 22q11.2 causes specific behavioral abnormalities in mice. Mice that overexpress an approximately 200-kb region of human 22q11.2, containing CDCrel, GP1Bbeta, TBX1, and WDR14, exhibited spontaneous sensitization of hyperactivity and a lack of habituation. These effects were ameliorated by antipsychotic drugs. The transgenic mice were also impaired in nesting behavior. Although Tbx1 has been shown to be responsible for many physical defects associated with 22q11.2 haploinsufficiency, Tbx1 heterozygous mice did not display these behavioral abnormalities. Our results show that the approximately 200-kb region of 22q11.2 contains a gene(s) responsible for behavioral abnormalities and suggest that distinct genetic components within 22q11.2 mediate physical and behavioral abnormalities.

Tbx1 is required for proper neural crest migration and to stabilize spatial patterns during middle and inner ear development

Tbx1 belongs to the family of T-box containing transcription factors. In humans, TBX1 is implicated in the etiology of the DiGeorge syndrome. Inactivation of the Tbx1 gene in mice produces a variety of malformations including abnormal branching of the heart outflow tract, deficiencies in the branchial arch derivatives, agenesis of pharyngeal glands and abnormal development of the auditory system. We analyze here the middle and inner ear phenotypes of the Tbx1 null mice. The middle ear is strongly affected. Its skeletal components are malformed to varying degrees, some being slightly hypoplastic and others completely absent. However, a seemingly normal-looking tympanic membrane can still be recognized. Middle ear anomalies are associated with other skeletal deficiencies in the branchial arch-derived skeleton. These phenotypes derive from a combination of the failure of the posterior branchial arches to develop and the misrouting of neural crest cells. The inner ears of Tbx1(-/-) animals are hypoplastic. No vestibular or cochlear structures are detectable, but the endolymphatic duct, the cochleovestibular ganglia and residual sensory patches are still identifiable. Molecular analyses revealed a seemingly normal spatial distribution of a variety of patterning markers in the otic vesicles of Tbx1 null mutants at E9.0. However, 1 day later, several of these markers presented altered domains of expression in the otocysts of these mutant embryos, suggesting that Tbx1 is not required for the establishment of spatial patterns in the otocyst, but rather for their maintenance. The inability of the Tbx1(-/-) embryos to keep properly segregated functional domains in the otocyst is likely the cause of the strong inner ear phenotypes observed in these mutants.