A Nonsense MITF Mutation Associated with Autosomal Dominant Hearing Loss and Intra-Family Clinical Heterogeneity

Objectives: Hearing loss is the most common sensory disorder in humans. Approximately 60% of cases of congenital hearing loss have a genetic aetiology. Consequently, the genetic study would significantly improve the diagnostic performance of these hearing impairments avoiding unnecessary tests and helping to establish the evolution, prognosis and treatment of the disease. These studies are of great relevance in cases of syndromic hearing loss. Methods: Using an own designed panel of next generation sequencing (NGS), we studied the case of a two year old boy, with a congenital bilateral NSHL detected through neonatal hearing screening. The patient did not present any other associated pathology. As the hearing loss was profound and bilateral, it was decided to place a cochlear implant in both ears when he was one year old. Results: The analysis revealed the presence of the heterozygous mutation c.1066C>T, p. (Arg356*), in the MITF (NM_198159.2) gene which was confirmed by Sanger sequencing. The segregation study of the mutation in the MITF gene was performed in the family and only the father showed the same mutation with a different phenotype. Conclusion: We describe the clinical spectrum in a family with MITF mutation. They are a clear example of incomplete intrafamilial penetrance and variable expressivity already reported in other cases with MITF mutations.

Waardenburg syndrome, Pendred syndrome and Usher syndromes among others.
Waardenburg syndrome (WS) is an autosomal genetic condition that exhibits a very high penetrance rate, a variable expressivity of clinical features and heterogeneity of loci and alleles. Its main clinical features are congenital hearing loss and pigmentation abnormalities (including depigmented patches of the skin and hair, heterochromia and/or hypopigmentation of the irides). The syndrome is genetically heterogeneous, having been classified into four groups based on clinical grounds [2]. WS type 1 is characterized by eyes that appear widely spaced, congenital hearing loss, and patchy pigment disturbances of the iris, hair and skin. In contrast to WS type 1 (WS1), WS2 patients do not show dystopia cantorum (eyelid anomaly; lateral displacement of the inner canthi of the eye). The phenotype of WS3 and WS4 patients is similar to WS1 but with additional hypoplasia of limb muscles (WS3) or Hirschprung's disease (WS4) [3].
WS2 is an autosomal dominant syndrome characterized by congenital neurosensorial hearing loss (NSHL), pigmentary abnormalities of the eyes, hair and skin and the absence of dystopia cantorum, the lateral displacement of the ocular inner canthi, which is seen in some other forms of Waardenburg syndrome [4].

WS2 is caused by heterozygous pathogenic variants in genes MITF
and SOX10, which together explain nearly 30% of cases [5]. We report the case of a two years old boy with profound NSHL leading to bilateral cochlear implant, which showed a truncating mutation in MITF. In order to make a diagnosis, it was decided to carry out a genetic study of the patient. Written informed consent was obtained from the index case and other family members. Molecular study was performed, after DNA extraction, through next generation    (Figure 2). It should be noted that the genetic study could not be carried out on the paternal grandparents of the index case, because they had died a few years before. Therefore, it was not possible to know if the mutation was inherited or de novo.

Discussion
Hearing loss (HL) is one of the most common sensory defects birth in developed countries and is clinically and genetically heterogeneous. There are at least 30 autosomal loci for nonsyndromic hearing impairment and more than 400 disorders in which hearing impairment is a characteristic of the syndrome. A recent study in pigs suggests that MITF gene mutation only affects hair cells of the saccule but has no effect on other vestibular organs. The study also indicated that the survival of cochlear and saccular hair cells was dependent on the potassium release from the cochlear vascular stria, but hair cells of the utricle and semicircular canals were independent on stria vascular [9]. Our patients had hearing loss typical of the cochlear impairment described in WS2. There were no associated vestibular disorders.
Hearing loss and hypopigmentation and in Waardenburg syndrome are thought to be the results of an abnormality of melanocyte differentiation caused by MITF mutations [10].
Our index patient had retinal hypopigmentation in addition to hearing loss. The father did not present any alterations due to hypopigmentation. García-Llorca et al. [11] described in mice how MITF mutations affect both types of pigment cells in the eye: neural crest-derived melanocytes and neuroepithelium-derived RPE cells [11][12][13][14]. MITF promotes RPE differentiation, regulates the proliferation of the RPE during development and consequently their retinal function [14]. This study provides more evidence that a functional RPE is important for normal photoreceptor function.
Thus, mutations in genes expressed in the RPE such as MITF and OTX2, can have profound effects on retinal structure and function, as a consequence of alterations in the RPE [15]. A three-generation family from North of Iran with the same mutation c.1066 C > T, with 9 people with WS2 and 4 normal carriers was reported [17]. We have not found any genetic alterations in all living relatives, but the genetic study was not possible in the paternal grandparents because they were dead at the time of the study, so it could not be ruled out that the mutation was de novo or inherited. In summary, this family is a clear example of incomplete intrafamilial penetrance and variable expressivity already reported in other cases with MITF mutations. We would like to highlight the importance of analyzing this gene in patients who only present with hearing loss.