No abstract

Keywordsfatty aldehyde dehydrogenase; ichthyosis; mutation; Sjögren-Larsson syndrome Sjögren-Larsson syndrome (SLS; OMIM 270200) is an autosomal recessive disorder characterized by the presence of pruritic ichthyosis, mental retardation, spastic diplegia or tetraplegia, retinal perimacular 'glistening white dots' and photophobia. 1 SLS is caused by mutations in the ALDH3A2 gene, which codes for fatty aldehyde dehydrogenase (FALDH), a microsomal enzyme that catalyses the oxidation of medium-and long-chain aliphatic aldehydes. 2 More than 70 mutations have been identified in this disease.3 Most mutations are unique to each SLS family, but several common mutations have been reported among patients from Europe and the Middle East. In the first molecular genetic analysis of a cohort of patients with SLS from Brazil, we now report a common disease-causing ALDH3A2 mutation, delineate its associated phenotypic spectrum and describe a diagnostic screening test using restriction enzyme digestion. Case and methodsNine patients with SLS from three apparently unrelated Brazilian kindreds native to the south and southeastern part of the country were studied. Patients 1, 2 and 3 were previously reported 4 and were born to consanguineous parents; patients 4, 5 and 6 are sisters and patients 7 and 8 are their cousins. Patient 9 was also born to first cousins. Blood specimens, Total genomic DNA was extracted from blood using standard phenol-chloroform methods. ALDH3A2 exons and their flanking sequences were amplified by polymerase chain reaction (PCR) and sequenced. 2 To detect the c.1108-1G → C mutation more conveniently, exon 8 was digested with DdeI according to the manufacturer's instructions and restriction products separated by agarose gel electrophoresis. ALDH3A2 haplotypes were determined using four intragenic single nucleotide polymorphisms as described. 2 FALDH enzyme activity was measured in cultured skin fibroblasts. 5Results and discussion Biochemical and molecular characterizationAll patients had <10% of normal FALDH enzyme activity in cultured skin fibroblasts ( Table 1). Sequencing of the ALDH3A2 gene revealed a homozygous c.1108-1G → C mutation in intron 7 of all of the affected individuals. Their parents were heterozygous for the mutation, except for the parents of patient 9 who were not tested. The c.1108-1G → C mutation abolishes a DdeI restriction enzyme cut site in the wild-type gene. DdeI digestion of a 275-bp PCR product of exon 8 and its flanking intron 7 sequence yielded a 181-bp fragment from normal control subjects, but a 263-bp fragment from the patients with SLS (Fig. 1b). This constitutes a convenient screening test for the mutation. Haplotype analysis of the ALDH3A2 gene showed that patients from each family were homozygous for haplo-type 1 (see Rizzo et al.2).The c.1108-1G → C mutation alters the splice acceptor site at the junction of intron 7 and exon 8. Like many exons, exon 8 does not end between codons, but rather ends with part of a codon, which is completed when exon 8 is spliced to exon 9...

Hereditary hearing loss is a complex disorder that involves a large number of genes. In developed countries, 1 in 1,000 children is born with deafness severe enough to require special education services, and about 60% of the cases of isolated deafness have a genetic origin. Although more than 100 genes for hearing loss are known currently, only a few are routinely tested in the clinical practice. In this study, we present our findings from the molecular diagnostic screening of the GJB2 and GJB3 genes, del(GJB6-D13S1,830) and del(GJB6-D13S1,854) deletions in the GJB6 gene, Q829X mutation in the otoferlin gene (OTOF) and, the A1,555G and A7,445G mutations in the mitochondrial genome over an 8-year period. Mutations analysis in the previously mentioned genes and mutations was performed on 645 unrelated Brazilian patients with hearing loss who fell into two different testing groups. Different mutations in the GJB2 gene were responsible for most of cases studied, but deletions in the GJB6 gene as well as mitochondrial mutations were also found. While most cases of hearing loss in this country are due to environmental factors, the genetic etiology of deafness will increasingly be determined as more genetic tests become available.

Mutations in the GJB2 gene are a major cause of congenital deafness. One specific mutation, the 35delG mutation, has accounted for most of the GJB2 mutations detected in European populations and is one of the most frequent disease mutations identified so far. We evaluated the frequency of the 35delG mutation in DNA samples from Brazilians of European, Asian, and African ancestry. All DNA samples were screened for the 35delG mutation using an allele-specific PCR. This study shows that the frequency of a common mutation (35delG) is significantly lower in nonEuropean populations.

Mutations in the GJB2 gene, which encodes the protein connexin 26, are a major cause of autosomal recessive deafness. The most frequent mutation, 35delG, has a carrier frequency as high as 4% in some countries, and this frequency varies in different ethnic groups. Most of the Brazilian population results from interethnic crosses of people from three continents (European, African, and Amerindian), and the proportion of each varies according to the geographical region of the country. To verify if the different ethnic composition of Brazilian regions leads to variable 35delG carrier frequencies, we performed the screening of the 35delG mutation using DNA from dried-blood filter paper samples obtained from 1,856 newborns from 10 cities in different regions. The 35delG mutation was found in 25 individuals (1.35%), indicating an overall carrier frequency of 1:74. This frequency was 1:47 in the north, 1:64 in the southeast, 1:85 in the south and 1:124 in the northeast, but these differences were not significant. The overall frequency of the 35delG allele was estimated as 0.0067, and comparison between expected and observed genotype frequencies indicates that the population is in Hardy-Weinberg equilibrium.