Figure 1.

Utrophin and dystrophin genes. Dystrophin (DMD) and utrophin (UTRN) genes with associated promoters and exons. Full-length dystrophin transcription is controlled by 3 promoters (brain, muscle, Purkinje) [91,127,128]. Four alternate promoters result in short isoforms dp260, dp140, dp116, and dp71 [86,129,130]. An IRES is found in exon 5. Dp427 is the main variant produced in muscle [127]. The two isoforms of utrophin (A and B) have distinct promoters and 5ʹUTR regions [131,132]. Utrophin A is expressed primarily at the myotendinous and neuromuscular junction in mature skeletal muscle, in neurons, astrocytes as well as in the brain (choroid plexus and pia mater), whereas utrophin B is primarily localized in vasculature [131–135] The utrophin A transcript contains an IRES between nucleotides 71 and 152 in its 5ʹUTR [22]. The utrophin B promoter is located within the second intron of the utrophin gene resulting in a unique 5ʹ end [131,132]. Three short utrophin isoforms Up140, G-Utrophin and Up71 were reported similar to the dystrophin gene [129,130,136]