Waardenburg syndrome type 3 (Klein-Waardenburg syndrome) segregating with a heterozygous deletion in the paired box domain of PAX3: a simple variant or a true syndrome?

Loss of Pax3 causes reduction of melanocytes in the developing mouse cochlea

Cochlear melanocytes are intermediate cells in the stria vascularis that generate endocochlear potentials required for auditory function. Human PAX3 mutations cause Waardenburg syndrome and abnormalities of skin and retinal melanocytes, manifested as congenital hearing loss (~ 70%) and hypopigmentation of skin, hair and eyes. However, the underlying mechanism of hearing loss remains unclear. Cochlear melanocytes in the stria vascularis originated from Pax3-traced melanoblasts and Plp1-traced Schwann cell precursors, both of which derive from neural crest cells. Here, using a Pax3-Cre knock-in mouse that allows lineage tracing of Pax3-expressing cells and disruption of Pax3, we found that Pax3 deficiency causes foreshortened cochlea, malformed vestibular apparatus, and neural tube defects. Lineage tracing and in situ hybridization show that Pax3+ derivatives contribute to S100+, Kir4.1+ and Dct+ melanocytes (intermediate cells) in the developing stria vascularis, all of which are significantly diminished in Pax3 mutant animals. Taken together, these results suggest that Pax3 is required for the development of neural crest cell-derived cochlear melanocytes, whose absence may contribute to congenital hearing loss of Waardenburg syndrome in humans.

Genetic insights, disease mechanisms, and biological therapeutics for Waardenburg syndrome

Waardenburg syndrome (WS), also known as auditory-pigmentary syndrome, is the most common cause of syndromic hearing loss (HL), which accounts for approximately 2-5% of all patients with congenital hearing loss. WS is classified into four subtypes depending on the clinical phenotypes. Currently, pathogenic mutations of PAX3, MITF, SOX10, EDN3, EDNRB or SNAI2 are associated with different subtypes of WS. Although supportive techniques like hearing aids, cochlear implants, or other assistive listening devices can alleviate the HL symptom, there is no cure for WS to date. Recently major progress has been achieved in preclinical studies of genetic HL in animal models, including gene delivery and stem cell replacement therapies. This review focuses on the current understandings of pathogenic mechanisms and potential biological therapeutic approaches for HL in WS, providing strategies and directions for implementing WS biological therapies, as well as possible problems to be faced, in the future.

Biallelic variants in PAX3 cause Klein syndrome

Waardenburg syndrome is a group of genetic conditions that can cause hearing loss and pigmentation deficiency of the hair, skin, and eyes. Klein-Waardenburg syndrome (Waardenburg syndrome type 3 ) represents a distinct presentation of Waardenburg syndrome type 1 and includes musculoskeletal abnormalities in addition to dystopia canthorum hearing loss and pigmentary changes. Heterozygous or homozygous variants in the PAX3 gene cause Klein-Waardenburg syndrome. Here we report on a new severely affected child, with a homozygous PAX3 variant (c.251C>T; p.Ser84Phe), review the features of the syndrome, and propose a new classification. The designation of Waardenburg syndrome should be given only to patients with monoallelic pathogenic variants in PAX3 whether or not musculoskeletal abnormalities are present. Patients with biallelic PAX3 variants should be outlined as a distinct group and designated Klein syndrome. This article is protected by copyright. All rights reserved.

Case Report: A Novel Gross Deletion in PAX3 (10.26 kb) Identified in a Chinese Family With Waardenburg Syndrome by Third-Generation Sequencing

Waardenburg syndrome (WS) is a group of autosomal-dominant hereditary conditions with a global incidence of 1/42,000. WS can be categorized into at least four types: WS1–4, and these are characterized by heterochromia iridis, white forelock, prominent nasal root, dystopia canthorum, hypertrichosis of the medial part of the eyebrows, and deaf-mutism. WS3 is extremely rare, with a unique phenotype (upper limb abnormality). Heterozygous mutations of PAX3 are commonly associated with WS1, whereas partial or total deletions of PAX3 are often observed in WS3 cases. Deletions, together with insertions, translocations, inversions, mobile elements, tandem duplications, and complexes, constitute structural variants (SVs), which can be fully and accurately detected by third-generation sequencing (TGS), a new generation of high-throughput DNA sequencing technology. In this study, after failing to identify the causative gene by Sanger sequencing, SNP-array, and whole-exome sequencing (WES), we finally detected a heterozygous gross deletion of PAX3 (10.26kb, chr2: 223153899-223164405) in a WS family by TGS. Our description would enrich the genetic map of WS and help us to further understand this disease. Our findings also demonstrated the value of TGS in clinical genetics researches.

Waardenburg Syndrome Type-II in Twin Siblings: An Unusual Audio-Pigmentary Disorder

Waardenburg syndrome (WS) is an interesting inherited audio-pigmentary disorder. The syndrome shows no gender, racial, or ethnic predilection. This unique disorder is characterized by pigmentary abnormalities, deafness, and neural crest-derived tissue defect. WS can be recognized by some specific clinical features that appear after birth; not all affected individuals possess all the clinical features. It has four clinical sub types based on the mutant gene and characteristic morphology. These morphological features are broad nasal root, white forelock, the difference in the colour of eyes, congenital leukoderma, and sensorineural deafness. We report an interesting case of WS in twin boys who fulfill the criteria of WS-II. Our cases have four major criteria (white forelock, heterochromia, sensorineural hearing loss, first degree relative with WS), and 1 minor criterion to establish the diagnosis of WS-II. Most clinical features of WS-II except sensorineural deafness are benign and do not need any intervention but severe deafness can be a serious problem. The current report is unique and is a rare case of WS in twin infants. We present this case for its rarity, relative paucity of literature, and also to emphasize the clinical presentation of this extremely rare disease in twins.

Phenotypic diversity and genetic complexity of PAX3-related Waardenburg syndrome

Waardenburg syndrome subtypes 1 and 3 are caused by pathogenic variants in PAX3. We investigated 12 individuals from four unrelated families clinically diagnosed with Waardenburg syndrome type 1/3. Novel pathogenic variants identified in PAX3 included single nucleotide variants (c.166C>T, c.829C>T), a 2-base pair deletion (c.366_367delAA) and a multi-exonic deletion. Two novel variants, c.166C>T and c.829C>T and a previously reported variant, c.256A>T in PAX3 were evaluated for their nuclear localization and ability to activate MITF promoter. The coexistence of two subtypes of Waardenburg syndrome with pathogenic variants in PAX3 and EDNRB was seen in one of the affected individuals. Multiple genetic diagnoses of Waardenburg syndrome type 3 and autosomal recessive deafness 1A was identified in an individual. We also review the phenotypic and genomic spectrum of individuals with PAX3-related Waardenburg syndrome reported in the literature.

Hearing loss in Waardenburg syndrome: a systematic review

Waardenburg syndrome (WS) is a rare genetic disorder characterized by hearing loss (HL) and pigment disturbances of hair, skin and iris. Classifications exist based on phenotype and genotype. The auditory phenotype is inconsistently reported among the different Waardenburg types and causal genes, urging the need for an up-to-date literature overview on this particular topic. We performed a systematic review in search for articles describing auditory features in WS patients along with the associated genotype. Prevalences of HL were calculated and correlated with the different types and genes of WS. Seventy-three articles were included, describing 417 individual patients. HL was found in 71.0% and was predominantly bilateral and sensorineural. Prevalence of HL among the different clinical types significantly differed (WS1: 52.3%, WS2: 91.6%, WS3: 57.1%, WS4: 83.5%). Mutations in SOX10 (96.5%), MITF (89.6%) and SNAI2 (100%) are more frequently associated with hearing impairment than other mutations. Of interest, the distinct disease-causing genes are able to better predict the auditory phenotype compared with different clinical types of WS. Consequently, it is important to confirm the clinical diagnosis of WS with molecular analysis in order to optimally inform patients about the risk of HL.

Pax genes: regulators of lineage specification and progenitor cell maintenance

Pax genes encode a family of transcription factors that orchestrate complex processes of lineage determination in the developing embryo. Their key role is to specify and maintain progenitor cells through use of complex molecular mechanisms such as alternate RNA splice forms and gene activation or inhibition in conjunction with protein co-factors. The significance of Pax genes in development is highlighted by abnormalities that arise from the expression of mutant Pax genes. Here, we review the molecular functions of Pax genes during development and detail the regulatory mechanisms by which they specify and maintain progenitor cells across various tissue lineages. We also discuss mechanistic insights into the roles of Pax genes in regeneration and in adult diseases, including cancer.

Spectrum of novel mutations found in Waardenburg syndrome types 1 and 2: implications for molecular genetic diagnostics

OBJECTIVES

Till date, mutations in the genes PAX3 and MITF have been described in Waardenburg syndrome (WS), which is clinically characterised by congenital hearing loss and pigmentation anomalies. Our study intended to determine the frequency of mutations and deletions in these genes, to assess the clinical phenotype in detail and to identify rational priorities for molecular genetic diagnostics procedures.

DESIGN

Prospective analysis.

PATIENTS

19 Caucasian patients with typical features of WS underwent stepwise investigation of PAX3 and MITF. When point mutations and small insertions/deletions were excluded by direct sequencing, copy number analysis by multiplex ligation-dependent probe amplification was performed to detect larger deletions and duplications. Clinical data and photographs were collected to facilitate genotype-phenotype analyses.

SETTING

All analyses were performed in a large German laboratory specialised in genetic diagnostics.

RESULTS

15 novel and 4 previously published heterozygous mutations in PAX3 and MITF were identified. Of these, six were large deletions or duplications that were only detectable by copy number analysis. All patients with PAX3 mutations had typical phenotype of WS with dystopia canthorum (WS1), whereas patients with MITF gene mutations presented without dystopia canthorum (WS2). In addition, one patient with bilateral hearing loss and blue eyes with iris stroma dysplasia had a de novo missense mutation (p.Arg217Ile) in MITF. MITF 3-bp deletions at amino acid position 217 have previously been described in patients with Tietz syndrome (TS), a clinical entity with hearing loss and generalised hypopigmentation.

CONCLUSIONS

On the basis of these findings, we conclude that sequencing and copy number analysis of both PAX3 and MITF have to be recommended in the routine molecular diagnostic setting for patients, WS1 and WS2. Furthermore, our genotype-phenotype analyses indicate that WS2 and TS correspond to a clinical spectrum that is influenced by MITF mutation type and position.

Tetraphocomelia with the Waardenburg syndrome and multiple malformations

A male infant delivered spontaneously at the 29th week of pregnancy had gross tetraphocomelia and features of the Waardenburg syndrome. There were no relevant factors in the pregnancy nor family history. It is possible that microdeletions or contiguous gene defects are involved in the pathogenesis of these malformations.

Pigmentation PAX-ways: the role of Pax3 in melanogenesis, melanocyte stem cell maintenance, and disease

Transcription factors initiate programs of gene expression and are catalysts in downstream molecular cascades that modulate a variety of cellular processes. Pax3 is a transcription factor that is important in the melanocyte and influences melanocytic proliferation, resistance to apoptosis, migration, lineage specificity and differentiation. In this review, we focus on Pax3 and the molecular pathways that Pax3 is a part of during melanogenesis and in the melanocyte stem cell. These roles of Pax3 are emphasized during the development of diseases and syndromes resulting from either too much or too little Pax3 function. Due to its key task in melanocyte stem cells and tumors, the Pax3 pathway may provide an ideal target for either stem cell or cancer therapies.

Sensorineural deafness, distinctive facial features, and abnormal cranial bones: a new variant of Waardenburg syndrome?

The Waardenburg syndromes (WS) account for approximately 2% of congenital sensorineural deafness. This heterogeneous group of diseases currently can be categorized into four major subtypes (WS types 1-4) on the basis of characteristic clinical features. Multiple genes have been implicated in WS, and mutations in some genes can cause more than one WS subtype. In addition to eye, hair, and skin pigmentary abnormalities, dystopia canthorum and broad nasal bridge are seen in WS type 1. Mutations in the PAX3 gene are responsible for the condition in the majority of these patients. In addition, mutations in PAX3 have been found in WS type 3 that is distinguished by musculoskeletal abnormalities, and in a family with a rare subtype of WS, craniofacial-deafness-hand syndrome (CDHS), characterized by dysmorphic facial features, hand abnormalities, and absent or hypoplastic nasal and wrist bones. Here we describe a woman who shares some, but not all features of WS type 3 and CDHS, and who also has abnormal cranial bones. All sinuses were hypoplastic, and the cochlea were small. No sequence alteration in PAX3 was found. These observations broaden the clinical range of WS and suggest there may be genetic heterogeneity even within the CDHS subtype.

Homozygous and heterozygous inheritance of PAX3 mutations causes different types of Waardenburg syndrome

Type I Waardenburg syndrome (WS-I) is an auditory-pigmentary syndrome caused by heterozygous loss of function mutations in the PAX3 gene. Klein-Waardenburg syndrome (WS-III) is a very rare condition and represents an extreme presentation of WS-I, additionally associated with musculoskeletal abnormalities. We present an 18-months old Turkish child with typical Klein-Waardenburg syndrome (WS) including dystopia canthorum, partial albinism, and upper-limb defects. The child was born to a consanguineous couple and both parents had WS-I. We screened the entire coding region of the PAX3 gene for mutations and identified a novel missense mutation, Y90H, within the paired box domain of PAX3. Both parents were heterozygous for the mutation and the proposita was homozygous. This is the third report of a homozygous PAX3 mutation causing the WS-III phenotype. Molecular analysis of four additional Turkish families with variable clinical expression of WS-I identified two missense mutations, one splice-site mutation, and one small insertion in the PAX3 gene.