Does epidermal thickening explain GJB2 high carrier frequency and heterozygote advantage?

Abstract

In the post-human-genome area, the challenge is to derive details of heritable variation in relation to how human variation reflects adaptation to the different environments. Heterozygote advantage represents a superior genetic adaptation presumably explaining the presence of the allele at frequencies above those expected from a simple replacement of a homozygous lethal allele by mutation alone. A successful adaptation requires natural selection acting on that part of the body that makes a difference in survival. There is ongoing discussion as to what extent inherited diseases may be phenotypically balanced by a selective heterozygous advantage. We know for sure that sickle cell anemia and some other red blood cell disorders are selected for by malaria,1 whereas a selective carrier advantage for cystic fibrosis has been recently shown.2 For other common inherited diseases counterbalancing mechanisms have been only hypothesized. Congenital hearing loss accounts for about 1 in 1000 infants and approximately 80% of the cases are inherited as an autosomal recessive trait. Connexin 26 (GJB2) gene, a component of a gap junction, is a major gene for these forms, and a large proportion of recessive cases are due to mutations in this gene.3 Different common population-specific mutations have been so far described.4, 5 Among them, mutation 35delG explains up to 70% of the alleles detected in Caucasians, with carrier frequencies ranging from 1/30 in the Mediterranean area to 1/70 in northern Europe.5 The high carrier frequency of the GJB2 mutations in many ethnic groups strongly suggests that there may be a heterozygous advantage. Why GJB2 carriers are so frequent? Quite recently, it has been reported that carriers of R134W allele, present in some African countries such as Ghana,6, 7 show a thicker epidermis than wild-type ones.8 Moreover, in vitro studies have further supported this finding showing that cells expressing the R134W allele: (a) form a significantly thicker epidermis in an organotypic co-culture skin model, (b) show an increased migration, (c) are significantly less susceptible to cellular invasion by the enteric pathogen Shigella flexneri than wild-type cells.9 Another study has shown that the transfection of some deafness-associated mutant constructs resulted in a statistically significant reduction in cell death when compared with wild-type ones.10 Thus, researchers hypothesized that an increased cell survival may explain a thicker epidermis due to an extended terminal differentiation program leading to an improved barrier against infection. Moreover, some specific GJB2 mutated alleles lead to skin diseases in which abnormal thickening of the skin is present.11 If epidermal thickening is the phenomenon underlying heterozygote advantage, should we be able to find an increased thickness of the epidermis in GJB2 carriers? To answer this question and possibly to explain the increased 35delG carrier frequency, we have developed an accurate sonographic protocol to measure epidermal thickness able to discriminate between normal controls and GJB2 carriers.12 Table 1 report the data on epidermal thickness in a series of 240 Italian normal controls (134 females and 106 males) aged from 20 to 86 years. They have been then compared with those obtained in a series of fifty-four 35delG obligate carriers (29 females and 25 males). Results of the analysis are shown in Figure 1, in which two regression lines are calculated for each of the two groups (normal controls and carriers). A clear tendency to thickening along with age is present in both sexes (P<2.0e−16), being more relevant in males. Two clusters are easily distinguishable, one defined by controls and one by carriers. An ANCOVA analysis showed that three variables (GJB2 status, age and gender) explain up to 75% of epidermal thickness variability (P<2.1e−16). Removing the GJB2 status, the impact of the remaining two variables (ie, gender and age) is of only 26%. The difference between controls and GJB2 carriers cannot be explained with any social or occupational reason (ie, lifestyle, job and so on).