Abstract
Background
The macrolide sirolimus (rapamycin) selectively blocks translation of mRNAs containing
a terminal 5′ oligopyrimidine (TOP) tract by altering the activity of mammalian target
of rapamycin (mTOR) and inhibiting downstream mTOR pathway components involved in
TOP mRNA translation. The skin disorder pachyonychia congenita (PC) is caused by mutations
in the inducible keratins (K) including K6a, K6b, K16 and K17. Published sequence
data suggest the 5′ untranslated regions of K6a and K6b mRNAs contain 5′ TOP motifs
and therefore may be sensitive to rapamycin treatment.
Objective
Determine if mTOR inhibitors (rapamycin, temsirolimus or everolimus) are viable drug
candidates for treatment of PC and other disorders caused by inappropriate expression
of K6a and K6b.
Methods
5′ RACE analysis was used to map the transcriptional start sites for K5, K6a, K6b,
K14, K16 and K17. The sensitivity of these keratins to mTOR inhibitors was determined
by Western and qPCR analysis following treatment of a human HaCaT keratinocyte cell
line with rapamycin, temsirolimus or everolimus. A small off-label study was undertaken
using orally administered rapamycin in three PC patients and the effects were monitored
by clinical examination, photography, a validated Dermatology Life Quality Index (DLQI)
and a pain and activity diary.
Results
Sequence comparison and 5′ RACE analysis of the 5′ untranslated regions of K6a and
K6b revealed putative TOP regulatory elements. Treatment of a human HaCaT keratinocyte
cell line with mTOR inhibitors (rapamycin, temsirolimus or everolimus) resulted in
selective K6a repression. Furthermore, treatment of this HaCaT cell line with siRNAs
targeting components of the mTOR pathway altered the levels of K6a expression. To
test the ability of rapamycin to ameliorate PC symptoms, an off-label study was conducted.
PC patient clinical responses to oral rapamycin showed a therapeutic response in callus
character as well as subjective improvement. Of particular note, rapamycin greatly
reduced the presence of painful cutaneous thromboses after reaching therapeutic serum
levels. The well-known rapamycin side effects led to the early withdrawal of all of
the patients from the study.
Conclusion
Rapamycin selectively blocks K6a expression in human keratinocytes. The improvement
of symptoms in PC patients following rapamycin treatment suggests rapamycin (or rapamycin
analogs) may be a therapeutic option, particularly if topical formulations can be
developed that avoid the side effects associated with systemic administration.
Keywords
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Article info
Publication history
Published online: July 12, 2010
Accepted:
July 15,
2009
Received in revised form:
July 9,
2009
Received:
June 10,
2009
Identification
Copyright
© 2009 Japanese Society for Investigative Dermatology. Published by Elsevier Inc. All rights reserved.
