What is the role of "utrophin" immunofluorescent staining in muscle biopsy evaluation?

We received a muscle biopsy recently from a 5-year-old girl who was likely a dystrophin mutation carrier. Her muscle biopsy displayed a patchwork pattern of dystrophin positive and dystrophin negative muscle fibers. Utrophin was upregulated in fibers deficient for dystrophin.

So, what is utrophin? It is the autosomal homologue of dystrophin. To quote the Dubowitz muscle biopsy text: "[S]arcolemmal labelling of utrophin peaks at about 20 weeks of gestation. After about 26 weeks of gestation and in mature muscle utrophin is no longer seen on the sarcolemma and it is confined to blood vessels and neuromuscular junctions.... In the absence of dystrophin, or reduced dystrophin, as in Duchenne and Becker muscular dystrophy, and in inflammatory myopathies, sarcolemmal utrophin is prominent." So it appears that utrophin is upregulated to take the place of dystrophin when the latter is not sufficiently present or is abnormal. It serves as a confirmatory positive assay when one sees reduced dystrophin in a biopsy. The fact that it disappears during gestation suggests the possibility that it might be a phylogenetically more ancient protein than is dystrophin. In any case, utrophin may play a therapeutic role since muscular dystrophy patients have an intact utrophin gene that could theoretically be upregulated to take the place of dystrophin. At least one animal model suggests this possibility.

Reference:
Dubowitz V and Sewry CA. Muscle Biopsy: A Practical Approach (3rd Edition). Saunders Elsevier (2007) p 207-8.