The molecular basis of X-linked deafness type 3 (DFN3) in two sporadic cases: identification of a somatic mosaicism for a POU3F4 missense mutation

Clinical and Molecular Aspects Associated with Defects in the Transcription Factor POU3F4: A Review

X-linked deafness (DFNX) is estimated to account for up to 2% of cases of hereditary hearing loss and occurs in both syndromic and non-syndromic forms. POU3F4 is the gene most commonly associated with X-linked deafness (DFNX2, DFN3) and accounts for about 50% of the cases of X-linked non-syndromic hearing loss. This gene codes for a transcription factor of the POU family that plays a major role in the development of the middle and inner ear. The clinical features of POU3F4-related hearing loss include a pathognomonic malformation of the inner ear defined as incomplete partition of the cochlea type 3 (IP-III). Often, a perilymphatic gusher is observed upon stapedectomy during surgery, possibly as a consequence of an incomplete separation of the cochlea from the internal auditory canal. Here we present an overview of the pathogenic gene variants of POU3F4 reported in the literature and discuss the associated clinical features, including hearing loss combined with additional phenotypes such as cognitive and motor developmental delays. Research on the transcriptional targets of POU3F4 in the ear and brain is in its early stages and is expected to greatly advance our understanding of the pathophysiology of POU3F4-linked hearing loss.

Considering gene therapy to protect from X-linked deafness DFNX2 and associated neurodevelopmental disorders

Mutations and deletions in the gene or upstream of the gene encoding the POU3F4 transcription factor cause X-linked progressive deafness DFNX2 and additional neurodevelopmental disorders in humans. Hearing loss can be purely sensorineural or mixed, that is, with both conductive and sensorineural components. Affected males show anatomical abnormalities of the inner ear, which are jointly defined as incomplete partition type III. Current approaches to improve hearing and speech skills of DFNX2 patients do not seem to be fully effective. Owing to inner ear malformations, cochlear implantation is surgically difficult and may predispose towards severe complications. Even in cases where implantation is safely performed, hearing and speech outcomes remain highly variable among patients. Mouse models for DFNX2 deafness revealed that sensorineural loss could arise from a dysfunction of spiral ligament fibrocytes in the lateral wall of the cochlea, which leads to reduced endocochlear potential. Highly positive endocochlear potential is critical for sensory hair cell mechanotransduction and hearing. In this context, here, we propose to develop a therapeutic approach in male Pou3f4 -/y mice based on an adeno-associated viral (AAV) vector-mediated gene transfer in cochlear spiral ligament fibrocytes. Among a broad range of AAV vectors, AAV7 was found to show a strong tropism for the spiral ligament. Thus, we suggest that an AAV7-mediated delivery of Pou3f4 complementary DNA in the spiral ligament of Pou3f4 -/y mice could represent an attractive strategy to prevent fibrocyte degeneration and to restore normal cochlear functions and properties, including a positive endocochlear potential, before hearing loss progresses to profound deafness.

The noncoding genome and hearing loss

The age of sequencing has provided unprecedented insights into the human genome. The coding region of the genome comprises nearly 20,000 genes, of which approximately 4000 are associated with human disease. Beyond the protein-coding genome, which accounts for only 3% of the genome, lies a vast pool of regulatory elements in the form of promoters, enhancers, RNA species, and other intricate elements. These features undoubtably influence human health and disease, and as a result, a great deal of effort is currently being invested in deciphering their identity and mechanism. While a paucity of material has caused a lag in identifying these elements in the inner ear, the emergence of technologies for dealing with a minimal number of cells now has the field working overtime to catch up. Studies on microRNAs (miRNAs), long non-coding RNAs (lncRNAs), methylation, histone modifications, and more are ongoing. A number of microRNAs and other noncoding elements are known to be associated with hearing impairment and there is promise that regulatory elements will serve as future tools and targets of therapeutics and diagnostics. This review covers the current state of the field and considers future directions for the noncoding genome and implications for hearing loss.

Study of complex structural variations of X-linked deafness-2 based on single-molecule sequencing

X-linked deafness-2 (DFNX2) is cochlear incomplete partition type III (IP-III), one of inner ear malformations characterized by an abnormally wide opening in the bone separating the basal turn of the cochlea from the internal auditory canal, fixation of the stapes and cerebrospinal fluid (CSF) gusher upon stapedectomy or cochleostomy. The causative gene of DFNX2 was POU3F4. To investigate the genetic causes of DFNX2 and compare the efficiency of different sequencing methods, 12 unrelated patients were enrolled in the present study. Targeted next-generation sequencing (NGS) and long-read sequencing were used to analyze the genetic etiology of DFNX2. Six variants of POU3F4 were identified in this cohort by NGS. Three patients with a negative diagnosis based on NGS were enrolled in further long-read sequencing. Two of them were all found to carry structural variations (SVs) on chromosome X, consisting of an 870-kb deletion (DEL) at upstream of POU3F4 and an 8-Mb inversion (INV). The 870-kb DEL may have been due to non-homologous end joining (NHEJ), while non-allelic homologous recombination (NAHR) within a single chromatid may have accounted for the 8-Mb INV. Common POU3F4 mutations in DFNX2 included point mutations, small insertions and deletions (INDELs), and exon mutations, which can be detected by Sanger sequencing and NGS. Single-molecule long-read sequencing constitutes an additional and valuable method for accurate detection of pathogenic SVs in IP-III patients with negative NGS results.

A New Pathogenic Variant in POU3F4 Causing Deafness Due to an Incomplete Partition of the Cochlea Paved the Way for Innovative Surgery

Incomplete partition type III (IP-III) is a relatively rare inner ear malformation that has been associated with a POU3F4 gene mutation. The IP-III anomaly is mainly characterized by incomplete separation of the modiolus of the cochlea from the internal auditory canal. We describe a 71-year-old woman with profound sensorineural hearing loss diagnosed with an IP-III of the cochlea that underwent cochlear implantation. Via targeted sequencing with a non-syndromic gene panel, we identified a heterozygous c.934G > C p. (Ala31Pro) pathogenic variant in the POU3F4 gene that has not been reported previously. IP-III of the cochlea is challenging for cochlear implant surgery for two main reasons: liquor cerebrospinalis gusher and electrode misplacement. Surgically, it may be better to opt for a shorter array because it is less likely for misplacement with the electrode in a false route. Secondly, the surgeon has to consider the insertion angles of cochlear access very strictly to avoid misplacement along the inner ear canal. Genetic results in well describes genotype-phenotype correlations are a strong clinical tool and as in this case guided surgical planning and robotic execution.

Uncovering the secreted signals and transcription factors regulating the development of mammalian middle ear ossicles

The mammalian middle ear comprises a chain of ossicles, the malleus, incus, and stapes that act as an impedance matching device during the transmission of sound from the tympanic membrane to the inner ear. These ossicles are derived from cranial neural crest cells that undergo endochondral ossification and subsequently differentiate into their final functional forms. Defects that occur during middle ear development can result in conductive hearing loss. In this review, we summarize studies describing the crucial roles played by signaling molecules such as sonic hedgehog, bone morphogenetic proteins, fibroblast growth factors, notch ligands, and chemokines during the differentiation of neural crest into the middle ear ossicles. In addition to these cell-extrinsic signals, we also discuss studies on the function of transcription factor genes such as Foxi3, Tbx1, Bapx1, Pou3f4, and Gsc in regulating the development and morphology of the middle ear ossicles.

A rapid improved multiplex ligation detection reaction method for the identification of gene mutations in hereditary hearing loss

Hearing loss (HL) is a common sensory disorder. More than half of HL cases can be attributed to genetic causes. There is no effective therapy for genetic HL at present, early diagnosis to reduce the incidence of genetic HL is important for clinical intervention in genetic HL. Previous studies have identified 111 nonsyndromic hearing loss genes. The most frequently mutated genes identified in NSHL patients in China include GJB2, SLC26A4, and the mitochondrial gene MT-RNR1. It is important to develop HL gene panels in Chinese population, which allow for etiologic diagnosis of both SHL and NSHL. In this study, a total of 220 unrelated Han Chinese patients with bilateral progressive SNHL and 50 unrelated healthy controls were performed Single nucleotide polymorphism (SNP) genotyping using an improved multiplex ligation detection reaction (iMLDR) technique, is to simultaneously detect a total of 32 mutations in ten HL genes, covering all currently characterized mutations involved in the etiology of nonsyndromic or syndromic hearing loss in the Chinese population. The 49 positive samples with known mutations were successfully detected using the iMLDR Technique. For 171 SNHL patients, gene variants were found in 57 cases (33.33%), among which, 30 patients carried mutations in GJB2, 14 patients carried mutations in SLC26A4, seven patients carried mutations in GJB3, and six patients carried mutations in MT-RNR1. The molecular etiology of deafness was confirmed in 12.9% (22/171) of patients carried homozygous variants. These results were verified by Sanger sequencing, indicating that the sensitivity and specificity of the iMLDR technique was 100%. We believe that the implementation of this population-specific technology at an efficient clinical level would have great value in HL diagnosis and treatment.

Clinical and molecular characterization of POU3F4 mutations in multiple DFNX2 Chinese families

Background

Many X-linked non-syndromic hearing loss (HL) cases are caused by various mutations in the POU domain class 3 transcription factor 4 (POU3F4) gene. This study aimed to identify allelic variants of this gene in two Chinese families displaying X-linked inheritance deafness-2 (DFNX2) and one sporadic case with indefinite inheritance pattern.

Methods

Direct DNA sequencing of the POU3F4 gene was performed in these families and in 100 Chinese individuals with normal hearing.

Results

There are characteristic imaging findings in DFNX2 Chinese families with POU3F4 mutations. The temporal bone computed tomography (CT) images of patients with DFNX2 are characterized by a thickened stapes footplate, hypoplasia of the cochlear base, absence of the bony modiolus, and dilated internal acoustic meatus (IAM) as well as by abnormally wide communication between the IAM and the basal turn of the cochlea. We identified three causative mutations in POU3F4 for three probands and their extended families. In family 1468, we observed a novel deletion mutation, c.973delT, which is predicted to result in a p.Trp325Gly amino acid frameshift. In family 2741, the mutation c.927delCTC was identified, which is predicted to result in the deletion of serine at position 310. In both families, the mutations were located in the POU homeodomain and are predicted to truncate the C-terminus of the POU domain. In the third family, a novel de novo transversion mutation (c.669 T > A) was identified in a 5-year-old boy that resulted in a nonsense mutation (p.Tyr223*). The mutation created a new stop codon and is predicted to result in a truncated POU3F4 protein.

Conclusions

Based on characteristic radiological findings and clinical features, POU3F4 gene mutation analysis will increase the success rate of stapes operations and cochlear implantations, and improve molecular diagnosis, genetic counseling, and knowledge of the molecular epidemiology of HL among patients with DFNX2.

A POU3F4 Mutation Causes Nonsyndromic Hearing Loss in a Chinese X-linked Recessive Family

Background:

The molecular genetic research showed the association between X-linked hearing loss and mutations in POU3F4. This research aimed to identify a POU3F4 mutation in a nonsyndromic X-linked recessive hearing loss family.

Methods:

A series of clinical evaluations including medical history, otologic examinations, family history, audiologic testing, and a high-resolution computed tomography scan were performed for each patient. Bidirectional sequencing was carried out for all polymerase chain reaction products of the samples. Moreover, 834 controls with normal hearing were also tested.

Results:

The pedigree showed X-linkage recessive inheritance pattern, and pathogenic mutation (c.499C>T) was identified in the proband and his family member, which led to a premature termination prior to the entire POU domains. This mutation co-segregated with hearing loss in this family. No mutation of POU3F4 gene was found in 834 controls.

Conclusions:

A nonsense mutation is identified in a family displaying the pedigree consistent with X-linked recessive pattern in POU3F4 gene. In addition, we may provide molecular diagnosis and genetic counseling for this family.

Facial nerve stimulation following cochlear implantation for X-linked stapes gusher syndrome leading to identification of a novel POU3F4 mutation

We report a case of a nine-year-old male who presented with facial nerve stimulation four years after cochlear implantation. Computed tomography was performed revealing a dilated internal auditory meatus and the cochlear implant electrode was found to be protruding into the fallopian canal at the level of the geniculate ganglion. Subsequent genetic analysis demonstrated X-linked deafness type 2 (DFNX2) caused by a novel c.769C > T nucleotide change in the POU domain, class 3, transcription factor 4 gene (POU3F4). Inactivation of electrodes 1 and 19-21 successfully abated facial nerve stimulation.

A novel mutation in POU3F4 in a Chinese family with X-linked non-syndromic hearing loss

Objective

Based on the clinical manifestations of a hearing loss patient, the POU3F4 gene was tested for diagnosis of etiology.

Methods

A comprehensive physical examination was performed on the proband to exclude abnormalities of other organs, and detailed audiological testing and temporal bone CT scan were also performed. Genomic DNA was extracted using the proband's peripheral blood leukocytes. Polymerase chain reactions (PCR) were performed in the coding sequence of the POU3F4 gene. Direct DNA sequencing was subsequently applied to screen the entire coding region of the POU3F4 gene.

Results

The proband had severe sensorineural hearing loss. Temporal CT showed bilateral cochlear incomplete partition, vestibule dysplasia, internal auditory canal fundus expansion, and cochlear interlink with the internal auditory canal fundus. A novel mutation (c.530C > A (p.S177X)) in the POU3F4 gene was found in this patient, creating an new stop codon and was predicted to result in a truncated protein lacking normal POU3F4 transcription factor function.

Conclusion

Through analysis of the POU3F4 gene and clinical manifestations in the patient, we conclude that a novel mutation may have resulted in a premature stop codon, contributing to the mutation of POU3F4 gene.

De novo mutation in X-linked hearing loss-associated POU3F4 in a sporadic case of congenital hearing loss

OBJECTIVES

In this report, we present a male patient with no family history of hearing loss, in whom we identified a novel de novo mutation in the POU3F4 gene.

METHODS

One hundred ninety-four (194) Japanese subjects from unrelated and nonconsanguineous families were enrolled in this study. We used targeted genomic enrichment and massively parallel sequencing of all known nonsyndromic hearing loss genes for identifying the genetic causes of hearing loss.

RESULTS

A novel de novo frameshift mutation of POU3F4 to c.727_728insA (p.N244KfsX26) was identified. The patient was a 7-year-old male with congenital progressive hearing loss and inner ear deformity. Although the patient had received a cochlear implant, auditory skills were still limited. The patient also exhibited developmental delays similar to those previously associated with POU3F4 mutation.

CONCLUSION

This is the first report of a mutation in POU3F4 causing hearing loss in a Japanese patient without a family history of hearing loss. This study underscores the importance of comprehensive genetic testing of patients with hearing loss for providing accurate prognostic information and guiding the optimal management of patient rehabilitation.

X-linked hearing loss: two gene mutation examples provide generalizable implications for clinical care

PURPOSE

To describe the inheritance patterns and auditory phenotype features of 3 Canadian families with mutations in 2 X-linked "deafness" genes (DFNX).

METHOD

Audiological, medical, and family histories were collected and family members interviewed to compare hearing thresholds and case histories between cases with mutations in SMPX versus POU3F4.

RESULTS

The family pedigrees reveal characteristic X-linked inheritance patterns. Phenotypic features associated with the SMPX (DFNX4) mutation include early onset in males with rapid progression from mild and flat to sloping sensorineural loss, with highly variable onset and hearing loss severity in females. In contrast, phenotypic features associated with the POU3F4 (DFNX2) mutation are characterized by an early onset, mixed hearing loss with fluctuation in males, and a normal hearing phenotype reported for females.

CONCLUSIONS

The study shows how this unique inheritance pattern and both gender and mutation-specific phenotype variations can alert audiologists to the presence of X-linked genetic etiologies in their clinical practice. By incorporating this knowledge into clinical decision making, audiologists can facilitate the early identification of X-linked hearing loss and contribute to the effective team management of affected families.

Inherited hearing loss: molecular genetics and diagnostic testing

BACKGROUND

Hearing loss is a clinically and genetically heterogeneous condition with major medical and social consequences. It affects up to 8% of the general population.

OBJECTIVE

This review recapitulates the principles of auditory physiology and the molecular basis of hearing loss, outlines the main types of non-syndromic and syndromic deafness by mode of inheritance, and provides an overview of current clinically available genetic testing.

METHODS

This paper reviews the literature on auditory physiology and on genes, associated with hearing loss, for which genetic testing is presently offered.

RESULTS/CONCLUSION

The advent of molecular diagnostic assays for hereditary hearing loss permits earlier detection of the underlying causes, facilitates appropriate interventions, and is expected to generate the data necessary for more specific genotype-phenotype correlations.

Identification of a novel mutation in POU3F4 for prenatal diagnosis in a Chinese family with X-linked nonsyndromic hearing loss

We present the clinical and genetic findings for a Chinese family with X-linked non-syndromic hearing loss in which the affected males showed congenital profound sensorineural hearing impairment. In two affected brothers, the computer tomography of temporal bone showed bilateral dilation of the internal auditory canal with fistulous communication between the lateral canal and the basal cochlear turn, which is consistent with the typical DFNX2 phenotype. A missense mutation (c.647G→A) in the POU3F4 gene caused a substitution from glycine to glutamic acid at position 216 (p.G216E), and this mutation was found to consistently cosegregate with the deafness phenotype in the family. The mutation resulted in the loss of function of the POU3F4 by decreasing the affinity between the protein and DNA, as shown in silico by the structural analysis. Prenatal diagnosis of pregnant proband of this family revealed the c.647G→A mutation in DNA extracted from the amniotic fluid surrounding the fetus. The appropriate use of genetic testing and prenatal diagnosis plays a key role in reducing the recurrence of genetic defects in high-risk families.

Cochlear implantation in children with congenital X-linked deafness due to novel mutations in POU3F4 gene

OBJECTIVES

We report novel mutations in the POU3F4 gene resulting in congenital X-linked deafness DFN3, and describe the results of cochlear implantation in 4 boys (3 siblings) followed for an average of 3.5 years.

METHODS

The diagnosis of DFN3 was made in infant boys on the basis of the radiologic criteria of an underdeveloped modiolus, a wide cochlear fossette, and the presence of all cochlear turns. The POU3F4 gene was sequenced. A standard, transmastoid, facial recess approach was used for cochlear implantation. A lumbar drain was placed before the operation.

RESULTS

The identified mutations in the POU3F4 gene were novel (p.R167X in the 3 siblings) or recently reported (p.S310del). A high-flow cerebrospinal fluid leak through the cochleostomy was encountered in each patient and was ultimately controlled. Although the implants functioned properly, the auditory perceptual abilities did not progress past sound detection in the 3 siblings, or past closed-set word identification in the non-sibling, who achieved better speech perception with contralateral amplification. Three boys (2 siblings) show signs of other learning disorders; 1 boy was too young for a complete assessment.

CONCLUSIONS

Preoperative gene mutation analysis in DFN3 patients who are considering cochlear implantation may help in long-term counseling and in avoidance of postoperative complications. Limited auditory perception and language acquisition may result. Amplification may sometimes be a better alternative than cochlear implantation, despite the severity of the hearing loss.

Multiple enhancers located in a 1-Mb region upstream of POU3F4 promote expression during inner ear development and may be required for hearing

POU3F4 encodes a POU-domain transcription factor required for inner ear development. Defects in POU3F4 function are associated with X-linked deafness type 3 (DFN3). Multiple deletions affecting up to ~900-kb upstream of POU3F4 are found in DFN3 patients, suggesting the presence of essential POU3F4 enhancers in this region. Recently, an inner ear enhancer was reported that is absent in most DFN3 patients with upstream deletions. However, two indications suggest that additional enhancers in the POU3F4 upstream region are required for POU3F4 function during inner ear development. First, there is at least one DFN3 deletion that does not eliminate the reported enhancer. Second, the expression pattern driven by this enhancer does not fully recapitulate Pou3f4 expression in the inner ear. Here, we screened a 1-Mb region upstream of the POU3F4 gene for additional cis-regulatory elements and searched for novel DFN3 mutations in the identified POU3F4 enhancers. We found several novel enhancers for otic vesicle expression. Some of these also drive expression in kidney, pancreas and brain, tissues that are known to express Pou3f4. In addition, we report a new and smallest deletion identified so far in a DFN3 family which eliminates 3.9 kb, comprising almost exclusively the previous reported inner ear enhancer. We suggest that multiple enhancers control the expression of Pou3f4 in the inner ear and these may contribute to the phenotype observed in DFN3 patients. In addition, the novel deletion demonstrates that the previous reported enhancer, although not sufficient, is essential for POU3F4 function during inner ear development.

Deletion of and novel missense mutation in POU3F4 in 2 families segregating X-linked nonsyndromic deafness

OBJECTIVE

To analyze the physical manifestations and genetic features of 2 families segregating X-linked deafness, which is most commonly reported to be caused by mutations of the POU domain gene POU3F4 at the DFN3 locus.

DESIGN

Computed tomographic study of the temporal bone in probands from each family, followed by mutation screening and deletion mapping of POU3F4 in family members.

SETTING

Two midwestern genetics clinics.

PARTICIPANTS

Two families with X-linked deafness.

MAIN OUTCOME MEASURES

Anomalies of the inner ear in the probands; results of gene mapping and severity and effects of hearing loss in the family members.

RESULTS

In the first family, a large deletion was identified that includes POU3F4 and extends upstream approximately 530 kilobases; in the second family, a novel serine-to-leucine (S228L) amino acid mutation was identified in the POU-specific domain of POU3F4. Both the deletion and the missense mutation segregate with the clinical phenotype and are causally related to the deafness in these families.

CONCLUSIONS

Deafness related to the POU3F4 gene is associated with dilation of the internal auditory canal and a spectrum of other temporal bone anomalies that range in severity from mild to severe dysplasia of the cochlea and semicircular canals. The consequence of these anomalies is a congenital mixed hearing loss, the sensorineural component of which progresses over time. Affected males can also present with vestibular dysfunction that is associated with delayed developmental motor milestones. Intrafamilial variability occurs.

Nuclear and mitochondrial genes mutated in nonsyndromic impaired hearing

Half of the cases with congenital impaired hearing are hereditary (HIH). HIH may occur as part of a multisystem disease (syndromic HIH) or as disorder restricted to the ear and vestibular system (nonsyndromic HIH). Since nonsyndromic HIH is almost exclusively caused by cochlear defects, affected patients suffer from sensorineural hearing loss. One percent of the total human genes, i.e. 300-500, are estimated to cause syndromic and nonsyndromic HIH. Of these, approximately 120 genes have been cloned thus far, approximately 80 for syndromic HIH and 42 for nonsyndromic HIH. In the majority of the cases, HIH manifests before (prelingual), and rarely after (postlingual) development of speech. Prelingual, nonsyndromic HIH follows an autosomal recessive trait (75-80%), an autosomal dominant trait (10-20%), an X-chromosomal, recessive trait (1-5%), or is maternally inherited (0-20%). Postlingual nonsyndromic HIH usually follows an autosomal dominant trait. Of the 41 mutated genes that cause nonsyndromic HIH, 15 cause autosomal dominant HIH, 15 autosomal recessive HIH, 6 both autosomal dominant and recessive HIH, 2 X-linked HIH, and 3 maternally inherited HIH. Mutations in a single gene may not only cause autosomal dominant, nonsyndromic HIH, but also autosomal recessive, nonsyndromic HIH (GJB2, GJB6, MYO6, MYO7A, TECTA, TMC1), and even syndromic HIH (CDH23, COL11A2, DPP1, DSPP, GJB2, GJB3, GJB6, MYO7A, MYH9, PCDH15, POU3F4, SLC26A4, USH1C, WFS1). Different mutations in the same gene may cause variable phenotypes within a family and between families. Most cases of recessive HIH result from mutations in a single locus, but an increasing number of disorders is recognized, in which mutations in two different genes (GJB2/GJB6, TECTA/KCNQ4), or two different mutations in a single allele (GJB2) are involved. This overview focuses on recent advances in the genetic background of nonsyndromic HIH.

Somatic and germinal mosaicism for the steroid sulfatase gene deletion in a steroid sulfatase deficiency carrier

Steroid sulfatase deficiency results in X-linked ichthyosis, an inborn error of metabolism in which the principal molecular defect is the complete deletion of the steroid sulfatase gene and flanking markers. Mosaicism for the steroid sulfatase gene has not yet been reported in X-linked ichthyosis. In this study we describe an X-linked ichthyosis patient with complete deletion of the steroid sulfatase gene and his mother with somatic and germinal mosaicism for this molecular defect. The family (X-linked ichthyosis patient, grandmother, mother, and sister) was analyzed through steroid sulfatase enzyme assay, polymerase chain reaction, DNA markers, and fluorescence in situ hybridization of the steroid sulfatase gene. Steroid sulfatase activity was undetectable in the X-linked ichthyosis patient, very low in the mother, and normal in the grandmother and sister. The X-linked ichthyosis patient showed a 2 Mb deletion of the steroid sulfatase gene and flanking regions from 5'DXS1139 to 3'DXF22S1. The mother showed one copy of the steroid sulfatase gene in 98.5% of oral cells and in 80% of leukocytes. The grandmother and sister showed two copies of the steroid sulfatase gene. The origin of the X chromosome with the deletion of the steroid sulfatase gene corresponded to the grandfather of the proband. We report the first case of somatic and germinal mosaicism of the steroid sulfatase gene in an X-linked ichthyosis carrier and propose DNA slippage as the most plausible mechanism in the genesis of this mosaicism.

Missense mutations of human homeoboxes: A review

The homeodomain (encoded by the homeobox) is the DNA-binding domain of a large variety of transcriptional regulators involved in controlling cell fate decisions and development. Mutations of homeobox-containing genes cause several diseases in humans. A variety of missense mutations giving rise to human diseases have been described. These mutations are an excellent model to better understand homeodomain molecular functions. To this end, homeobox missense mutations giving rise to human diseases are reviewed. Seventy-four independent homeobox mutations have been observed in 17 different genes. In the same genes, 30 missense mutations outside the homeobox have been observed, indicating that the homeodomain is more easily affected by single amino acids changes than the rest of the protein. Most missense mutations have dominant effects. Several data indicate that dominance is mostly due to haploinsufficiency. Among proteins having the homeodomain as the only DNA-binding domain, three "hot spot" regions can be delineated: 1) at codon encoding for Arg5; 2) at codon encoding for Arg31; and 3) at codons encoding for amino acids of recognition helix. In the latter, mutations at codons encoding for Arg residues at positions 52 and 53 are prevalent. In the recognition helix, Arg residues at positions 52 and 53 establish contacts with phosphates in the DNA backbone. Missense mutations of amino acids that contribute to sequence discrimination (such as those at positions 50 and 54) are present only in a minority of cases. Similar data have been obtained when missense mutations of proteins possessing an additional DNA-binding domain have been analyzed. The only exception is observed in the POU1F1 (PIT1) homeodomain, in which Arg58 is a "hot spot" for mutations, but is not involved in DNA recognition.

The role of POU domain proteins in the regulation of mammalian pituitary and nervous system development

POU domain proteins represent a subfamily of homeodomain-containing transcription factors that are expressed in many animal orders in a number of distinct regions in the developing and adult organism. In mammals, the expression profiles of these factors have suggested roles for class I, class III, and class IV POU domain proteins in the development, maintenance, and function of the endocrine and nervous systems. The genetic characterizations of the functions of these proteins during the generation, differentiation, and maturation of cells comprising these tissues have revealed a requirement for the individual actions of these transcription factors in the development of various elements of the anterior pituitary, the brain, and the somatosensory, vestibular/cochlear, and visual systems.

Structural basis of an embryonically lethal single Ala --> Thr mutation in the vnd/NK-2 homeodomain

The structural and DNA binding behavior is described for an analog of the vnd/NK-2 homeodomain, which contains a single amino acid residue alanine to threonine replacement in position 35 of the homeodomain. Multidimensional nuclear magnetic resonance, circular dichroism, and electrophoretic gel retardation assays were carried out on recombinant 80-aa residue proteins that encompass the wild-type and mutant homeodomains. The mutant A35T vnd/NK-2 homeodomain is unable to adopt a folded conformation free in solution at temperatures down to -5 degreesC in contrast to the behavior of the corresponding wild-type vnd/NK-2 homeodomain, which is folded into a functional three-dimensional structure below 25 degreesC. The A35T vnd/NK-2 binds specifically to the vnd/NK-2 target DNA sequence, but with an affinity that is 50-fold lower than that of the wild-type homeodomain. Although the three-dimensional structure of the mutant A35T vnd/NK-2 in the DNA bound state shows characteristic helix-turn-helix behavior similar to that of the wild-type homeodomain, a notable structural deviation in the mutant A35T analog is observed for the amide proton of leucine-40. The wild-type homeodomain forms an unusual i,i-5 hydrogen bond with the backbone amide oxygen of residue 35. In the A35T mutant this amide proton resonance is shifted upfield by 1.27 ppm relative to the resonance frequency for the wild-type analog, thereby indicating a significant alteration of this i,i-5 hydrogen bond.

Genetic causes of hearing loss

In the past year, genes involved in the branchio-oto-renal and Treacher-Collins syndromes were cloned. Myosin 7A, a gene previously implicated in Usher syndrome type 1B, was also found to be mutated in non-syndromic hearing loss. Likewise, linkage studies in Pendred syndrome and Usher syndrome type 1D suggest that allelic mutations can cause syndromic and non-syndromic forms of deafness. In patients with X-linked deafness type 3, a hotspot for deletions was found 900 kb proximal to the causal gene POU3F4. Most importantly, the connexin 26 gene is mutated in approximately 50% of all recessive deafness families, enabling early diagnosis and carrier detection.