Hair follicles are among the most dynamic mini-organs in the mammalian body. Each follicle cycles repeatedly through phases of vigorous growth, destruction, and rest, and each new cycle depends on stem cells resident in a region of the follicle called the bulge exiting their quiescent state at precisely the right moment. Once activated, these cells must proliferate, migrate downward, and differentiate into the multiple lineages that construct the follicle and its associated structures. This process demands an extraordinary degree of coordination, and it has long been clear that transcription factors, proteins that switch specific genes on or off, are central to establishing the identity of each cell type. What has remained murky is how cells ensure that the right proteins are actually synthesized in the right amounts once their messenger RNAs have been made, a question of post-transcriptional control that the new study addresses directly.

The answer lies in a chemical modification known as N7-methylguanosine, abbreviated m7G, in which a methyl group is attached to the nitrogen at position seven of a guanosine base. This modification has attracted growing attention in recent years as one of several RNA marks, alongside the better-known m6A modification, that influence the fate and function of RNA molecules inside cells. METTL1, working together with its partner protein WDR4, is the key methyltransferase responsible for installing m7G marks. The modification is best characterized on transfer RNAs, the adaptor molecules of protein synthesis, where it stabilizes the tRNA structure and supports efficient translation. But m7G can also be deposited internally within messenger RNAs, and it is this internal messenger RNA methylation that turned out to be central to the hair follicle story.

To probe the role of METTL1 in hair biology, the researchers created mice in which the Mettl1 gene could be deleted specifically in keratinocytes. The consequences were dramatic. Animals lacking the enzyme in these cells displayed severe dysplasia of hair follicle development, meaning the follicles formed abnormally and failed to achieve their normal architecture. Regeneration was also impaired, with the follicles unable to cycle properly through subsequent rounds of growth. In addition, the team observed disruptions in keratinocyte adhesion, the process by which these cells stick to one another to maintain the structural integrity of the epidermis and follicle. These defects point to METTL1 as a gene whose loss undermines both the construction of the follicle and its ongoing maintenance across cycles.

The complementary experiment strengthened the conclusion. When the researchers introduced a keratinocyte-specific knock-in that increased Mettl1 activity, hair regeneration accelerated. This gain-of-function result demonstrates that the enzyme is not merely permissive for hair growth but is genuinely rate-limiting: more METTL1 means faster regrowth, at least under the conditions of the experiment. Together, the loss-of-function and gain-of-function data establish a bidirectional relationship between METTL1 levels and the tempo of the hair follicle cycle, a relationship that suggests the enzyme could in principle be targeted to modulate hair growth in pathological settings, although the authors are careful to frame the work as mechanistic biology rather than as a therapeutic demonstration.

Mechanistically, the study traced the effects of METTL1 loss to a specific transcription factor with a well-established role in hair biology: HOXC13. This homeobox protein is known to govern the expression of genes required for hair shaft formation and follicle differentiation. The researchers found that when METTL1 is absent, HOXC13 messenger RNA becomes destabilized and its levels fall. The destabilization is attributed to the loss of internal m7G modification on the HOXC13 transcript itself, implying that the methyl mark normally protects or otherwise supports the messenger RNA, allowing it to persist long enough to be translated into protein. Without the mark, the transcript decays, HOXC13 protein levels drop, and the genetic program that depends on HOXC13 falters.

Downstream of HOXC13, the study mapped a signaling axis that connects the RNA modification to the observable defects in the follicle. HOXC13 normally promotes the expression of FOXN1, a transcription factor famous for its role in hair and immune development, and FOXN1 in turn supports the expression of DSG4, a desmoglein, which is a calcium-dependent adhesion molecule of the cadherin family concentrated in the hair shaft and inner root sheath. DSG4 provides the intercellular adhesion that keeps differentiating keratinocytes properly glued together as they build the hair shaft. When METTL1 is lost, HOXC13 messenger RNA decays, FOXN1 declines, and DSG4 falls with it, explaining both the differentiation defects and the adhesion disruptions seen in the knockout animals. The chain from RNA mark to mRNA stability to transcription factor to adhesion molecule constitutes a complete mechanistic pathway linking a chemical modification to tissue architecture.

The conceptual significance of the work extends beyond hair. Biologists have long distinguished between transcriptional regulation, which determines which genes are read into RNA, and post-transcriptional regulation, which determines what happens to those RNA molecules afterward. The new findings show that an RNA modification enzyme can sit upstream of a transcription factor, effectively using post-transcriptional control to tune a transcriptional circuit. In this arrangement, METTL1 does not directly change which genes are transcribed; instead, it safeguards the messenger RNA of a master regulator, ensuring that the transcriptional program for hair differentiation is executed at the right intensity. The authors describe this as an essential regulatory layer of RNA modification in hair follicle morphogenesis and cycling, one that operates through precise post-transcriptional control of a key transcriptional circuit to ensure structural integrity and timely regeneration.

For the broader field of RNA modification biology, the study adds hair follicle cycling to the growing list of developmental and physiological processes controlled by m7G methylation. It also highlights the specificity of the mechanism: although METTL1 modifies many transfer RNAs, the authors found that its effects on hair follicle biology run, at least in part, through internal modification of a single messenger RNA target. This kind of transcript-selective action suggests that m7G marks may function as individualized tuning knobs on particular messages rather than as a blanket adjustment of global protein synthesis, a distinction that will likely shape future experiments in other tissues.

Practical implications remain speculative but tantalizing. Hair loss disorders affect a large fraction of the human population, and most current treatments act indirectly on the follicle cycle. A pathway that demonstrably accelerates regeneration when its enzyme is boosted, and that acts through defined molecular intermediaries, offers a fresh set of potential targets. At the same time, the severe developmental defects caused by METTL1 loss in keratinocytes caution that the pathway is deeply embedded in normal epithelial biology, and any intervention would need to respect its broader roles. For now, the study stands as a vivid demonstration that the fate of a hair, and perhaps of many other tissues, can hinge on a methyl group attached to a single guanosine deep inside a messenger RNA molecule.

Subject of Research: METTL1-mediated m7G RNA modification and its regulation of the hair follicle cycle through the HOXC13/FOXN1/DSG4 axis

Article Title: METTL1-mediated m 7 G modification regulates hair follicle cycle via the HOXC13/FOXN1/DSG4 axis

Article References: Gan, X., Li, Q., Xiong, Q., Huang, D., Liu, Z., Yin, Q., Jiang, S., Matsubara, T., Kokabu, S., Yan, W., & Yuan, Q. (2026). METTL1-mediated m7G modification regulates hair follicle cycle via the HOXC13/FOXN1/DSG4 axis. PLOS Genetics, 22(9), e1012307. https://doi.org/10.1371/journal.pgen.1012307

Image Credits: AI Generated

DOI: 10.1371/journal.pgen.1012307

Keywords: METTL1, m7G modification, RNA methylation, hair follicle cycle, keratinocytes, HOXC13, FOXN1, DSG4, post-transcriptional regulation, hair regeneration, PLOS Genetics, epigenetics

Hoxc13 belongs to the Hox family of homeobox transcription factors, master regulators of embryonic patterning that specify body plans along the head-to-tail axis. Within this family, Hoxc13 has long been recognized as a critical controller of integumentary appendages, the skin-derived structures that include hair, nails, and claws. Previous work by the same team had shown that during mammalian evolution, the Hoxc13 protein acquired a characteristic polyglycine repeat insertion that distinguishes it from its non-mammalian homologs, the related proteins found in birds, reptiles, and other vertebrates. The new study set out to test whether this small structural addition actually changes what the protein can do at the genome-wide level, and whether that change could plausibly have contributed to the emergence of hair.

To answer this question, the researchers employed an elegant experimental design built around two versions of the Hoxc13 protein from cashmere goats. The first, designated Hoxc13-W, is the wild-type protein carrying the intact polyglycine repeat. The second, Hoxc13-S, is a shortened variant in which the polyglycine fragment has been deleted, mimicking the ancestral, non-mammalian form of the protein. Both proteins were synthesized using an in vitro expression system, allowing the team to compare their DNA-binding behavior directly without the confounding effects of living cells. This reductionist approach isolates the intrinsic DNA-binding capacity of each protein variant, a critical consideration when the goal is to attribute functional differences to a specific structural feature.

The centerpiece of the study was DNA affinity purification sequencing, or DAP-seq, a technique that uses purified proteins to fish out genomic DNA fragments they can bind, which are then identified by high-throughput sequencing. The results were striking. Hoxc13-W, the version with the polyglycine repeat, identified 12,679 binding peaks across the goat genome. Hoxc13-S, lacking the insertion, identified only 3,634 peaks. Even more telling was the overlap: just 243 peaks were shared between the two proteins. In other words, the vast majority of genomic sites bound by the wild-type protein were not recognized by the polyglycine-deleted variant, and vice versa. A single short amino acid insertion had fundamentally rewired the DNA-binding landscape of this transcription factor, transforming it from a relatively restricted regulator into one with a vastly expanded reach.

Functional enrichment analysis of the genes associated with these binding peaks revealed why this expansion matters for hair. The targets of Hoxc13-W were specifically enriched in pathways governing hair follicle morphogenesis and cycling, the developmental programs that build follicles and drive them through their growth, regression, and resting phases. By contrast, the targets of Hoxc13-S clustered primarily in the axon guidance pathway, a set of genes involved in steering neuronal growth cones rather than in skin biology. This shift in target preference suggests that the polyglycine insertion did not merely add more binding sites but redirected the protein toward an entirely different set of biological functions, aligning its regulatory activity with the demands of building hair-producing skin.

One of the most consequential findings concerned the Wnt signaling pathway, a cascade of intercellular communication that plays a central and well-documented role in hair follicle development, stem cell activation, and the hair growth cycle. Within this pathway, Hoxc13-W bound 30 genes, while Hoxc13-S bound only 5. The sixfold difference in Wnt pathway coverage implies that the polyglycine-bearing form of Hoxc13 can exert far broader control over one of the most important signaling networks in hair biology. Because Wnt signaling is repeatedly recruited at multiple stages of follicle morphogenesis and regeneration, an expanded capacity to regulate Wnt pathway genes could have provided mammalian ancestors with finer and more powerful control over hair production.

The study also probed how the polyglycine repeat affects the recognition of cis-regulatory motifs, the short DNA sequences to which transcription factors attach. Motif analysis demonstrated that the polyglycine fragment substantially influenced the motifs associated with Hoxc13 binding beyond the core Hoxc13-binding motif, which has the consensus sequence 5′-ATAAA-3′. This suggests that the insertion does not simply strengthen or weaken binding to the same sequence; rather, it changes the sequence context in which the protein operates, likely through cooperation with other DNA-bound factors or through altered preferences for flanking sequence features. Such changes in motif usage can ripple through regulatory networks, opening or closing access to large suites of target genes across the genome.

Molecular docking analysis added a structural dimension to the story. The computational modeling indicated that the polyglycine fragment alters the N-terminal structure of the Hoxc13 protein, the region involved in protein-protein interactions. Glycine is the smallest amino acid and confers exceptional conformational flexibility, so a repeat of glycine residues can act as a flexible hinge or spacer within a protein. By reshaping the N-terminal architecture, the insertion appears to affect how Hoxc13 interacts with partner proteins, which in turn influences how it engages DNA in a cellular context, since many transcription factors bind DNA effectively only as part of larger protein complexes. The polyglycine repeat, in this view, functions as a gain-of-function modification that enhances both the DNA-binding breadth and the interaction capacity of the protein.

Taken together, the findings support a model in which the polyglycine repeat insertion in Hoxc13 acted as an evolutionary gain-of-function mutation that amplified the Hoxc13-mediated gene regulatory network during the transition from non-mammalian to mammalian vertebrates. Rather than requiring wholesale changes in gene sequence or the emergence of entirely new genes, this scenario illustrates how a modest insertion of flexible, low-complexity sequence within a single transcription factor can reconfigure its binding profile, redirect its pathway targets, and reshape its protein interactions. Such low-complexity insertions are increasingly appreciated as a rich source of evolutionary novelty, capable of tuning protein behavior with relatively small mutational steps.

The research, conducted by Guanghao Yang, Chaoyong Huang, and colleagues at Inner Mongolia Minzu University, also carries practical implications beyond evolutionary biology. Cashmere goats, the species from which the two protein variants were derived, are prized for their fine undercoat fibers, and a deeper understanding of the Hoxc13-centered regulatory network could inform breeding strategies aimed at improving fiber quality and yield. More broadly, the study demonstrates the power of combining DAP-seq with pathway enrichment, motif analysis, and molecular docking to dissect the functional consequences of protein sequence evolution. While the polyglycine repeat is certainly not the sole driver of mammalian hair, the evidence presented here makes a compelling case that this tiny stretch of glycine helped tip the balance toward the furry integument that now defines the class Mammalia, from the wool of sheep to the whiskers of cats to the hair on our own heads.

Subject of Research: The role of a Hoxc13 polyglycine repeat insertion in mammalian hair and integument evolution

Article Title: Hoxc13 polyglycine repeat gain-of-function contributes to mammalian integument evolution by altering targeted genes and interactions

Article References: Yang, G., Huang, C., Yu, M., Li, W., Wu, D., Li, C., Hu, S., & Wu, J. (2026). Hoxc13 polyglycine repeat gain-of-function contributes to mammalian integument evolution by altering targeted genes and interactions. BMC Genomics. https://doi.org/10.1186/s12864-026-13314-5

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13314-5

Keywords: Hoxc13, polyglycine repeat, hair follicle, integument evolution, transcription factor, DAP-seq, Wnt signaling, gene regulation, mammalian evolution, cashmere goat, molecular docking, BMC Genomics