Newborn hearing screening and genetic testing in 8974 Brazilian neonates

Abstract

Objective

An early diagnosis has been a priority in the audiological practice. Identifying hearing loss until 3 months old through Universal Newborn Hearing Screening and intervention before 6 months old, minimize the impact of auditory loss in the health and communication development of these children. However, in the clinical practice, despite the help of the risk indicators in the audiological and etiological diagnosis, the integrated services have come up against the challenge of determining the causes of auditory loss, bearing in mind that approximately 50% of the subjects who have congenital loss do not show risk factors in their clinical history. The current research aims introduce together etiologic and audiological diagnosis of newborns.

Methods

We eluted dried blood spots from paper and performed genetic testing for 35delG mutation in 8974 newborns that were also screened for transient otoacoustic emissions (TOAE). In addition, the A1555G and A827G mutations in the MTRNR1 mitochondrial gene were screened in all newborns.

Results

We have found 17 individuals who failed in TOAE. Among them, we detected 4 homozygous newborns for 35delG mutation and 3 individuals with A827G mutation in the MTRNR1 mitochondrial gene. The frequency of 35delG carriers was 0.94% [84/8974]. In all 17 individuals who failed in OAE no other mutation besides those mentioned above was found.

Conclusions

The results greatly contribute to the public health area indicating the etiologic diagnosis, allowing family counseling as well as the early rehabilitation treatment or surgical intervention. Over time that will help to reduce the costs of rehabilitation considerably.

Introduction

In developed countries approximately 1 in 1000 children is born with hearing loss and half of them can be attributed to genetic factors. There is no official data about the prevalence of deafness in Brazil ranging 2–7 per 1000 depending on the region, and most of cases of hearing loss are due to environmental factors [1].

Brazil is under a process of demographic transformation that has been leading to a gradual aging of the population, decline of fertility, reduction of mortality level, changes in the causes of death, although increasing those typical among elderly population. Therefore the proportion of genetic causes in this country tends to increase as a result of improvements in health care.

The Brazilian population is unevenly distributed in 5 main geographic regions: North, Northeast, Midwest, Southeast and South, comprehending 26 states. Universal newborn hearing screening is available at 188 health care centers belonging to 22 states.

The increasing number of health care centers is quite significant, in spite of the difficulties of the health care systems. We have to point out Brazil has a legislation on performing the neonatal hearing screening. On the other hand, these laws seem to have very weak influences on the creation of these referred services. The improvement of new screening programs is, as a matter of fact, the result of high awareness of the problem, in the high credibility because of the results and access to new techniques and equipment.

Therefore, these items are responsible for raising the number of children with hearing loss who can take advantages with the early diagnosis identification.

The Joint Committee on Infant Hearing [2] recommends all infants should have access to hearing screening using a physiologic measure at no later than 1 month of age. All infants who do not pass the initial hearing screening and the subsequent rescreening should have appropriate audiological and medical evaluations to confirm the presence of hearing loss at no later than 3 months of age. Every infant with confirmed hearing loss and/or middle ear dysfunction should be referred for otologic and other medical evaluation. The purpose of these evaluations is to determine the etiology of hearing loss, to identify related physical conditions, and to provide recommendations for medical/surgical treatment as well as referral for other services. Essential components of the medical evaluation include clinical history, family history of childhood-onset permanent hearing loss, identification of syndromes associated with early- or late-onset permanent hearing loss, physical examination, indicated radiologic and laboratory studies (including genetic testing).

As we referred before, in developed countries about 50% of the causes of isolated deafness have a genetic origin. In Brazil most of cases are due to environmental factors, such as congenital infections (mainly rubella), perinatal anoxia, kernicterus and meningitis [3].

It is estimated that more than 400 loci may contribute to syndromic deafness and it is believed that almost 100 genes are involved with non-syndromic deafness. Half of newborns with severe-to-profound or profound congenital autosomal recessive non-syndromic hearing loss have mutations in the GJB2 gene which encodes connexin 26 protein [4].

Therefore, the most remarkable and clinically significant discovery has been the finding that mutations involving a single gene, GJB2, are the most common cause of hereditary deafness in many populations. Interestingly, there is one specific mutation, namely 35delG that accounts for the majority of mutations detected. The estimated frequency of 35delG in the general population can be as high as 1 in 35 individuals [5]. In a first study in Brazil the carrier frequency of 35delG in randomly selected neonates was 0.97% [6]. Mutations in the GJB2 gene were found in 22% of Brazilian families presenting at least one individual with non-syndromic deafness [7].

The carrier frequency of 35delG mutation in 307 unrelated Brazilian individuals with three different ethnic origins was previously studied, despite potential admixture of the Brazilian population. In 100 whites of European origin the carrier frequency was 2%. In 100 Afro-Brazilians from Salvador, State of Bahia, northeast region, classified according to skin color in the medial part of the arm, hair color and texture, and shape of the nose and lips, and also individuals who reported absence of any other ethnic group in all four grandparents, the carrier frequency was 1%. In the Japanese descendents with the first and second generation of Japanese immigrants the 35delG was not found [8].

Also in Brazil testing for GJB2 gene is the first step for determining the cause of hearing loss. The screening for mutations in GJB2 gene, specially the mutation 35delG, should be a routine investigation either for familial or sporadic affected individuals with congenital or prelingual hearing impairment [9].

On the other hand, a number of distinct mutations in the mitochondrial DNA (mtDNA) have been associated with both syndromic and non-syndromic forms of hearing impairment [10]. A particular mitochondrial mutation, an A to G transition at position 1555 in MTRNR1 gene have been associated with aminoglycoside-induced and non-syndromic deafness in many families from different ethnic backgrounds [11], [12], [13].

In the developed countries, the aminoglycoside antibiotics are mainly used in the treatment of hospitalized patients with aerobic Gram-negative bacterial infections, particularly in patients with chronic infections [14]. However, in developing countries aminoglycosides are used even for relative minor infections.

In the absence of aminoglycosides, the A1555G mutation was responsible for a clinical phenotype that ranges from severe congenital deafness to moderate progressive hearing loss with later onset, or to completely normal hearing [15], [16]. Also, a Chinese family with non-syndromic hearing loss with a maternal inheritance pattern revealed in the molecular analysis the homoplasmic mtDNA mutation A827G in the MTRNR1 gene [17].

Some authors have suggested the possibility that the alteration of the tertiary or quaternary structure of this ribosomal RNA by the mitochondrial mutations may lead to mitochondrial dysfunction, thereby playing a role in the pathogenesis of hearing loss and/or aminoglycoside hypersensitivity [18].

The aim of this study was to screen newborns for hearing impairment using audiological and genetic testing in order to amplify the spectrum of hearing impairment detection at birth and to reduce the time for etiological diagnostic.