Further studies confirmed a loss of one copy each of the FMR1, FMR2, and IDS genes (which are mutated in Fragile X syndrome, FRAXE syndrome, and Hunter syndrome, respectively).
The folate-sensitive fragile site FRAXE is located in proximal Xq28 of the human X chromosome and lies approximately 600 kb distal to the fragile X syndrome (FRAXA) fragile site at Xq27.3.
To date four types of trinucleotide repeat expansions have been identified: (1) long cytosine-guanine-guanine (CGG) repeats in the two fragile X syndromes (FRAXA and FRAXE), (2) long cytosine-thymine-guanine (CTG) repeat expansions in myotonic dystrophy, (3) long guanine-adenine-adenine repeat expansions in Friedreich's ataxia and (4) short cytosine-adenine-guanine repeat expansions (CAG) which are implicated in eight neurodegenerative disorders and are the focus of this review.
To gain insight into the FRAXE phenotype, the clinical details of the identified FRAXE male plus three other FRAXE individuals identified through previous referrals for fragile X syndrome testing are presented.