Academic journal article Genetic Counseling

Deafness on the Island of Providencia - Colombia: Different Etiology, Different Genetic Counseling

Academic journal article Genetic Counseling

Deafness on the Island of Providencia - Colombia: Different Etiology, Different Genetic Counseling

Article excerpt

Summary: Deafness on the island of Providencia - Colombia: different etiology, different genetic counseling: Providencia is a small island located in the Caribbean Ocean, northwest of Colombia with an unusually high frequency of individuals with hearing loss (5 in 1 ,000) is present. The hearing loss in the island was characterized as non-syndromic autosomal recessive deafness accounting for 47% (8/17) of the deaf population, Waardenburg Syndrome (deafness associated with pigmentary anomalies) for 29% (5/17), and the remaining 24% (4/17) are cases of sporadic non-syndromic deafness. For appropriate genetic counseling a complete pedigree of families with deaf individuals was constructed. The 35delG mutation in GJB2 gene, which encodes connexin 26 (Cx26), is responsible for the deafness observed in the 8 individuals with autosomal recessive non-syndromic hearing loss. The deaf individuals with Waardenburg Syndrome and the sporadic cases did not have this mutation. Therefore, we present here an atypical case of an isolated community with at least two different genetic etiologies for deafness: non-syndromic genetic deafness caused by the 35delG mutation in the GJB2 gene and deafness associated with Waardenburg Syndrome not related to GJB2.

In a small and isolated population, it is feasible to assume that the deafness is caused by the same factor; however, Providencia is an atypical case. Therefore, it is extremely important to define the exact etiology of deafness in each case, since different etiologies require different genetic counseling.

Key-words: Providencia Island - Colombia - Non-syndromic recessive deafness - Waardenburg syndrome - GJB2 - Genetic counseling - Etiology of deafness.

INTRODUCTION

Congenital hearing impairment affects 1 in 1,000 infants in North America, genetic factors accounting for more than half of these cases. Non-syndromic deafness represents about 70% of the hereditary group, whereas syndromic deafness accounts for 30% (6, 7, 1 1). In Colombia, a study performed in 16 educational institutions for the deaf demonstrated that genetic factors were responsible for 34% of the deafness found in this population (22, 23). The hereditary hearing loss homepage: http://webh01.ua.ac.be/hhh, recompiles data from over 40 deafness genes identified as responsible for syndromic and non-syndromic hereditary deafness. Mutations in the GJB2 (OMIM * 121011) gene encoding connexin 26 accounts for a high number of the non-syndromic deafness DFNA3 (OMIM #601544) and DFNBl (OMIM #220290) worldwide. Specifically, the 35delG mutation which was initially found in the Mediterranean population (27) and then in New Zealand/Australia, France, and Great Britain (4). Since then, the 35delG mutation has been shown to have an increased frequency (28.4% - 40%) in Caucasian populations such as Czech Republic, France, Greece, Italy/Spain, Lebanon, Morocco, Slovania, Tunisia, and Turkey (1).

Waardenburg syndrome (WS) is a rare disease characterized by sensorineural deafness associated with pigmentary anomalies in skin, hair and eyes (26). WS is autosomically dominant inherited, and presents itself with a variable clinical expression, even within members of the same family. In Colombia, WS was estimated to account for 5.38% of the institutionalized deaf population (20). The syndrome is also genetically heterogeneous, with the Waardenburg Index (WI), a composite measure of the inter inner canthal, inter pupillary, and inter outer canthal distances used as the key diagnostic feature to discriminate between WS Type I (WI > 2.07) and WS type II (WI < 1 .87) (5). Mutations in the PAX3 (OMIM *606597) gene are responsible for WS type I (OMIM #193500) and WS type III (OMIM # 148820; WS type I and upper limb abnormalities), mutations in the MITF (OMIM * 156845) gene are responsible for WS type II (OMIM #193510), and mutations in the EDNRB (OMIM *131244), SOXlO (OMIM * 602229) and EDN3 (OMIM * 1 3 1 242) genes are responsible for WS type IV (OMIM #277580; WS with Hirschsprung disease). …

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