Screening of MITF and SOX10 Regulatory Regions in Waardenburg Syndrome Type 2

Waardenburg syndrome (WS) is a rare auditory-pigmentary disorder that exhibits varying combinations of sensorineural hearing loss and pigmentation defects. Four subtypes are clinically defined based on the presence or absence of additional symptoms. WS type 2 (WS2) can result from mutations within the MITF or SOX10 genes; however, 70% of WS2 cases remain unexplained at the molecular level, suggesting that other genes might be involved and/or that mutations within the known genes escaped previous screenings. The recent identification of a deletion encompassing three of the SOX10 regulatory elements in a patient presenting with another WS subtype, WS4, defined by its association with Hirschsprung disease, led us to search for deletions and point mutations within the MITF and SOX10 regulatory elements in 28 yet unexplained WS2 cases. Two nucleotide variations were identified: one in close proximity to the MITF distal enhancer (MDE) and one within the U1 SOX10 enhancer. Functional analyses argued against a pathogenic effect of these variations, suggesting that mutations within regulatory elements of WS genes are not a major cause of this neurocristopathy.     

Delineating the role of MITF isoforms in pigmentation and tissue homeostasis

MITF, a gene that is mutated in familial melanoma and Waardenburg syndrome, encodes multiple isoforms expressed from alternative promoters that share common coding exons but have unique amino termini. It is not completely understood how these isoforms influence pigmentation in different tissues and how the expression of these independent isoforms of MITF is regulated. Here, we show that melanocytes express two isoforms of MITF, MITF‐A and MITF‐M. The expression of MITF‐A is partially regulated by a newly identified retinoid enhancer element located upstream of the MITF‐A promoter. Mitf‐A knockout mice have only subtle changes in melanin accumulation in the hair and reduced Tyr expression in the eye. In contrast, Mitf‐M‐null mice have enlarged kidneys, lack neural crest‐derived melanocytes in the skin, choroid, and iris stroma, yet maintain pigmentation within the retinal pigment epithelium and iris pigment epithelium of the eye. Taken together, these studies identify a critical role for MITF‐M in melanocytes, a minor role for MITF‐A in regulating pigmentation in the hair and Tyr expression in the eye, and a novel role for MITF‐M in size control of the kidney.     

Novel mutations of PAX3, MITF, and SOX10 genes in Chinese patients with type I or type II Waardenburg syndrome

Waardenburg syndrome (WS) is a rare disorder characterized by distinctive facial features, pigment disturbances, and sensorineural deafness. There are four WS subtypes. WS1 is mostly caused by PAX3 mutations, while MITF, SNAI2, and SOX10 mutations are associated with WS2. More than 100 different disease-causing mutations have been reported in many ethnic groups, but the data from Chinese patients with WS remains poor. Herein we report 18 patients from 15 Chinese WS families, in which five cases were diagnosed as WS1 and the remaining as WS2. Clinical evaluation revealed intense phenotypic variability in Chinese WS patients. Heterochromia iridis and sensorineural hearing loss were the most frequent features (100% and 88.9%, respectively) of the two subtypes. Many brown freckles on normal skin could be a special subtype of cutaneous pigment disturbances in Chinese WS patients. PAX3, MITF, SNAI2, and SOX10 genes mutations were screened for in all the patients. A total of nine mutations in 11 families were identified and seven of them were novel. The SOX10 mutations in WS2 were first discovered in the Chinese population, with an estimated frequency similar to that of MITF mutations, implying SOX10 is an important pathogenic gene in Chinese WS2 cases and should be considered for first-step analysis in WS2, as well as MITF.     

Mouse models for four types of Waardenburg syndrome

Waardenburg syndrome (WS) is an auditory-pigmentary syndrome caused by a deficiency of melanocytes and other neural crest-derived cells. Depending on a variety of symptoms associated with the auditory-pigmentary symptoms, WS is classified into four types: WS type 1 (WS1), WS2, WS3, and WS4. Six genes contributing to this syndrome--PAX3, SOX10, MITF, SLUG, EDN3 and EDNRB--have been cloned so far, all of them necessary for normal development of melanocytes. Mutant mice with coat color anomalies were helpful in identifying these genes, although the phenotypes of these mice did not necessarily perfectly match those of the four types of WS. Here we describe mice with mutations of murine homologs of WS genes and verify their suitability as models for WS with special interest in the cochlear disorder. The mice include splotch (Sp), microphthalmia (mi), Slugh-/-, WS4, JF1, lethal-spotting (ls), and Dominant megacolon (Dom). The influence of genetic background on the phenotypes of mice mutated in homologs of WS genes is also addressed. Finally, possible interactions among the six WS gene products are discussed.