Triple-A Syndrome in Morocco: Founder Effect, Age Estimation of the AAAS c.1331+1G>A Variant, and Implications for Genetic Diagnosis

Abstract

Introduction: Triple-A syndrome (Triple-A) is an autosomal recessive disorder characterized by alacrimia, achalasia, and adrenal insufficiency. Several variants on the AAAS gene have been described, and some variants are clustered in particular geographical areas, such as the c.1331+1G>A variant which is very frequent in North Africa. Here, we describe the genetic features of Triple-A in a series of unrelated families from Morocco.

Methods: Screening for the AAAS c.1331+1G>A variant was performed by direct sequencing or by PCR-RFLP. Haplotype analysis using Single Tandem Repeat (STR) markers flanking AAAS gene was performed in order to evaluate the founder effect and estimate the age of the c.1331+1G>A variant.

Results: Seven unrelated families with ten individuals clinically diagnosed with Triple-A were evaluated for sequence variations in the AAAS gene. The median age at diagnosis was 3 years, with a range between 2 and 11 years. Molecular analysis revealed that all patients were homozygous for the c.1331+1G>A variant. This variant was not found in 200 healthy controls, indicating that carriers are very rare in the general Moroccan population. Subsequently, STR marker analysis revealed a founder effect and that the most recent common ancestor of Triple-A patients in Morocco would have lived 125 years ago.

Conclusion: This is the largest series of Triple-A in Morocco. The same AAAS c.1331+1G>A variant was found in all patients, suggesting a founder effect in Morocco which was subsequently confirmed by microsatellite marker analysis. Therefore, this variant should be systematically investigated to diagnose Triple-A in Morocco.

Keywords: AAAS gene; Allgrove syndrome; BRO Biobank; Carrier frequency; Founder effect; Triple-A syndrome; c.1331+1G > A variant.

Fig. 2.

Screening for the AAAS c.1331+1G>A variant and STR markers genotyping. a Electropherograms of the between exon 14 and intron 14 of the AAAS gene showing the c.1331+1G>A variant in the homozygous state in patients with Triple-A (left) and in the heterozygous state in parents and some siblings of affected individuals (right). Site of the variant is indicated. b Schematic genomic organization of the of the AAAS gene on chromosome 12q13.13, showing the STR markers used for haplotype analysis and the location of the c.1331+1G>A variant within the AAAS gene.

Fig. 1.

Pedigree and haplotype analysis of the studied families. The position of the AAAS gene is between STR markers D12S96 and D12S1604. Haplotypes of the closely linked STR markers on chromosome 12q13.13 are shown beneath each symbol. Different shaded or colored bars indicate segregating chromosomal segments and show regions of crossover. The smallest disease-associated haplotype is shown in black.