|Number of repeats||Counts (alleles)||Percentage|
Overview of repeats
There are four polyalanine tracts in the ARX gene, two of which are in the exon 2 and are associated with multiple syndromes. Healthy individuals have 12 repeats of alanine in one of these tracts (ARX_2) and the most frequent mutation found there, an eight polyalanine expansion, is linked to Partington syndrome (PRTS), X-linked mental retardation with or without seizures (MRXARX) and the developmental and epileptic encephalopathy-1 (DEE1) (Bienvenu et al., 2002; Strømme et al., 2002). In all of these patients, a 24 bp duplication of alanine, resulting in 20 repeat allele was observed (Bienvenu et al., 2002; Strømme et al., 2002). Moreover, a longer, 33 bp insertion in the same tract has been also described in two brothers, stretching an allele to 23 repeats (Demos et al., 2009).
The longer tract (ARX_1) has a length of 16 alanines and has been associated also with MRXARX and DEE1. Patients in one family with the MRXARX syndrome had 2 repeat expansion, resulting in 18 residues of alanine (Bienvenu et al., 2002). In addition, alleles with the length of 23 and 25 repeats have been reported as well (Kato et al., 2007; Strømme et al., 2002).