Abstract
The hair of all mammals consists of terminally differentiated cells that undergo a specialized form of apoptosis called cornification. While DNA is destroyed during cornification, the extent to which RNA is lost is unknown. Here we find that multiple types of RNA are incompletely degraded after hair shaft formation in both mouse and human. Notably, mRNAs and short regulatory microRNAs (miRNAs) are stable in the hair as far as 10 cm from the scalp. To better characterize the post-apoptotic RNAs that escape degradation in the hair, we performed sequencing (RNA-seq) on RNA isolated from hair shafts pooled from several individuals. This hair shaft RNA library, which encompasses different hair types, genders, and populations, revealed 7,193 mRNAs, 449 miRNAs and thousands of unannotated transcripts that remain in the post-apoptotic hair. A comparison of the hair shaft RNA library to that of viable keratinocytes revealed surprisingly similar patterns of gene coverage and indicates that degradation of RNA is highly inefficient during apoptosis of hair lineages. The generation of a hair shaft RNA library could be used as months of accumulated transcriptional history useful for retrospective detection of disease, drug response and environmental exposure.
Highlights
Apoptosis is a cellular program utilized by multicellular organisms to eliminate cells during development or in surveillance of foreign or abnormal cells altered by viral infection or neoplastic transformation [1,2]
Much of the strength of the hair shaft and sheath come from intermediate filament proteins called keratins and keratin-associated proteins (KRTAPs) which become crosslinked by several enzymes during terminal differentiation [13,14]
The transcriptional profiles of living hair follicle-specific miRNAs and mRNAs have been previously examined [23,24]. Based on these expression profiles, we examined whether miRNAs and mRNAs expressed in the hair follicle could be detected in the external hair shaft and tested whether non-hair shaft transcripts were excluded
Summary
Apoptosis is a cellular program utilized by multicellular organisms to eliminate cells during development or in surveillance of foreign or abnormal cells altered by viral infection or neoplastic transformation [1,2]. Ultrastructural studies demonstrate that hair follicle cells undergo a specialized form of apoptosis called cornification during terminal differentiation [6]. During this process, the nuclear membrane is lost, and the chromatin becomes less coarse. Evidence of DNA damage as detected by direct end-labeling of nicked DNA and indirect double-strand break activity have been identified during early and late stages of hair differentiation [4,7,8] Skinspecific endonucleases such as DNase1L2 target genomic DNA during cornification [3,9], and in the absence of DNase1L2, nuclear DNA persists in the hair and causes hair fragility [10]. Much of the strength of the hair shaft and sheath come from intermediate filament proteins called keratins and keratin-associated proteins (KRTAPs) which become crosslinked by several enzymes during terminal differentiation [13,14]
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