PYCR1 Levels Track with Premature and Chronological Skin Aging

Reviewer #1 (Public Review):

The article by Reversade and colleagues reports new mutations in the PYCR1 in a progeroid disease associated with premature skin aging. Using human cell culture and a newly generated mouse model of PYCR1deficiency they identify a role for this factor in maintaining dermal homeostasis and ECM production. I have some minor concerns about the role of PYCR1 in fibroblast survival vs function and the quantification of western blots.

Reviewer #2 (Public Review):

Summary:
Sotiropoulou et al. present an interesting study of an incredibly rare premature aging disease (De Barsy syndrome), examining both the underlying mechanisms at play behind the condition as well as how that biology may have a larger role in understanding features of normal aging, and in particular, human skin aging. The authors link one of the underlying genetic defects in De Barsy syndrome (PYCR1 mutations) to its phenotypic manifestations and then extrapolate those findings to present more preliminary data to suggest that a loss of PYCR1 may be a biomarker of normal human skin aging.

Strengths:
- The study is important as De Barsy syndrome is challenging to study given its rarity, thus making it an understudied condition. Here the authors combine both human patient samples and murine models to offer a nice contribution to further understanding the pathophysiology of this disease.

- The authors are able to link some of the observed features in De Barsy syndrome preliminarily to more common aging models and processes (senescence, human skin dermal aging). They nicely show that the loss of Pycr1 in mice can provoke thinning of the dermis of mice while not affecting the epidermis. Furthermore, they present compelling data to suggest that Pycr2 may be compensating in mice (while not in humans) and this may contribute to the differences in lifespan observed between the mice and humans.

- Should these results be further verified, this could suggest that further study of Pycr1 and Pycr2 biology may offer new insights into aging and senescence in other tissues.

Weaknesses:
- Some of the data appears preliminary and seems like it needs further analysis as described further below in my suggestions for the authors:

1. While the authors report that there is no difference in the lifespan of the Pycr1-KO mice, can they report whether there was overall weight loss or any size differences between the mice? This is helpful particularly when comparing the dermal thickness as well as considering how the global loss of Pycr1 may affect overall systemic health.

2. In Figure S2E, the comparison "pairs" seems somewhat arbitrarily chosen and it seems from the quantifications of these pairs that depending upon which young sample you compare to which old sample, you may end up with differing results. I think the more appropriate way to make this quantitative comparison would be to average the young samples and average the old samples and then compare them and perform statistics. This seems critical to really assess whether PYCR1 loss would be a consistent marker of human skin dermal aging. Additionally, it would be helpful to also look at Pycr2 expression in the normal young versus old dermis to see if the reported difference in Pycr1 is really something specific for Pycr1 and not something more general.

3. Are the labels mixed up in Figures 1J and 1K or am I reading it incorrectly? From what I can see the graph is showing that the dermal thickness and collagen intensity is higher in the Pycr1-/- mice. Similarly, the authors state that there is "significantly less collagen fiber staining", although in Figure S1G neither the quantification of collagen I or collagen III are shown to be significant. These discrepancies need to be discussed or corrected.

4. Can the authors speculate further on why Pycr2 is also diminished in human patients (while it clearly remains present in the mice).

5. Can the authors comment on whether other canonical senescence features are seen in De Barsy syndrome (p16 positivity, senescence associated secretory phenotype, etc.)? Along these lines, there is an abundance of publicly available RNA-seq datasets from various forms of senescent cells. It would be interesting to examine these and see whether there is any loss of expression of PYCR1 or PYCR2 in these data, or is the loss of PYCR1 only seen at the protein level?