Bone Growth
Runaway Oxygen May Explain Kabuki's Short Stature
Growth plate cells lacking KMT2D can't handle oxygen properly, triggering a chain reaction that cuts bone growth short — but a targeted antioxidant rescued normal development in the lab, pointing toward a potential treatment for Kabuki syndrome.
Kids with Kabuki are often short. Do we know why?
Bones grow longer through a structure called the growth plate, where cartilage cells go through stages — resting, multiplying, and then enlarging — before being replaced by bone. In Kabuki mice, the cartilage cells rush through these stages too fast, enlarging prematurely and then dying off early.
What's making them rush?
The cells' mitochondria — their energy generators — aren't working properly without KMT2D. This leads to a buildup of damaging molecules called reactive oxygen species, or ROS, which rise sharply as the cells mature and push them into premature aging.
Wait — why does this only show up after birth?
Before birth, the growth plate has very little oxygen, so the faulty mitochondria aren't being stressed much. After birth, oxygen supply increases, and the broken mitochondria can't handle it — ROS spike, and the cartilage cells senesce too early.
Could anything help?
In the lab, a mitochondria-targeted antioxidant called MitoQ slowed the runaway differentiation and fully rescued the premature cell aging. Lowering oxygen levels had a similar effect, supporting the idea that managing ROS could be a real therapeutic target for bone growth in Kabuki.
Short stature is one of the most common features of Kabuki syndrome — but what actually goes wrong inside a growing bone? A new study offers a surprisingly specific answer, and it has less to do with the bone itself than with how the cells that build it react to oxygen.
The finding centers on the growth plate, the strip of cartilage near the ends of long bones where all lengthening happens during childhood. In children with Kabuki syndrome type 1, caused by changes in the gene KMT2D, these growth plates appear to malfunction — but mainly after birth. This study set out to understand why.
What the researchers found is striking: cartilage cells lacking functional KMT2D have faulty energy-producing machinery inside them, making them dangerously sensitive to oxygen. When oxygen levels rise — as they naturally do after a baby is born — these cells accumulate damaging molecules called reactive oxygen species, or ROS, which push the cells to mature too fast, burn out early, and ultimately cut bone growth short.
How bones grow — a quick primer
Long bones don't grow by stretching. They grow from the growth plate, a thin band of cartilage cells (called chondrocytes) organized into three distinct zones. Cells in the "resting zone" act as a reservoir. They feed into a "proliferative zone" where cells multiply rapidly and stack into columns, laying down the scaffolding of new cartilage. Finally, cells enter the "hypertrophic zone," where they swell dramatically, change the composition of the surrounding matrix, and eventually die off — making way for blood vessels and bone-forming cells to move in and convert cartilage to actual bone.
The whole system depends on precise timing. If cells rush through hypertrophy too quickly — or stall — bone growth suffers.
What earlier work had already shown
This research team had previously found that a Kabuki mouse model had shortened leg bones with visible differences in the growth plate's structure. In lab-grown cartilage cells, knocking out KMT2D caused the cells to leap prematurely from the proliferating stage into hypertrophy. But why they did so remained unclear.
One important clue: the growth deficiency in Kabuki syndrome doesn't typically appear before birth. Babies with KS1 generally grow fairly normally in the womb; the shortfall shows up postnatally. What changes at birth? Among other things — oxygen. The growth plate is naturally low in oxygen, and the proliferating cells prefer it that way. But the hypertrophic zone, closer to blood vessels, sees more oxygen, and cells there shift to a more oxygen-dependent metabolism that generates more ROS as a byproduct.
Testing the hypothesis in the lab
To investigate, the team used a cartilage cell line in which the KMT2D gene had been completely knocked out. When grown under standard lab oxygen levels (20%, which is much higher than what growth plate cells normally see), these knockout cells secreted roughly three times more cartilage matrix than normal cells by day 7 — a hallmark of precocious maturation.
Single-cell gene-activity profiling of more than ten distinct cell subpopulations confirmed the picture: the knockout cells were racing ahead. A subpopulation resembling pre-hypertrophic chondrocytes expanded earlier in the knockout cells than in normal ones. Markers of hypertrophy appeared sooner, and a computational method for ordering cells along a developmental timeline showed the knockout population consistently further along.
Interestingly, the knockout cells also divided less often. This mirrors what happens in a real growth plate, where cells exiting the proliferative zone stop dividing as they enter hypertrophy. The cells weren't just differentiating faster — they were also losing their ability to multiply, compounding the growth problem.
Broken power plants and too much oxygen
The next question was mechanistic: what inside these cells was driving the acceleration? Gene-activity analysis flagged two unexpected pathways as being ramped up — "hypoxia" and "glycolysis" — even though the cells were sitting in ample oxygen. The key low-oxygen sensor, a molecule called HIF-1α, was elevated in the knockout cells throughout the experiment. And the cells were pumping out up to three times more lactate, a classic sign that they were relying on sugar-fermentation rather than oxygen-based energy production.
Was HIF-1α the culprit driving premature maturation? The team tested this directly by silencing it — and found it made no difference to the pace of differentiation. Likewise, lowering oxygen to activate the hypoxia response in normal cells didn't make them differentiate faster. So the hypoxia signal was a red herring, at least as a direct driver.
Instead, the researchers suspected the real problem lay upstream: in the mitochondria, the cell's energy-producing organelles. Previous work on cells from Kabuki patients had already revealed defects in the mitochondrial electron transport chain — the molecular assembly line that uses oxygen to generate energy. The ramped-up glycolysis, the team reasoned, was a workaround: if your mitochondria can't use oxygen properly, you fall back on less efficient, oxygen-free energy production.
They confirmed this with direct measurements. Knockout cells showed significantly reduced baseline energy output from their mitochondria, reduced maximum capacity, and reduced reserve capacity. When forced to rely entirely on mitochondrial energy production (by swapping glucose for a sugar called galactose that cells can't ferment), normal cells ramped up their mitochondrial output by about 13. 5% — but the knockout cells couldn't increase theirs at all. This pointed to a genuine defect in the respiratory machinery itself, not simply fewer mitochondria.
The ROS tipping point
Faulty mitochondria don't just produce less energy — they leak. Specifically, they leak reactive oxygen species. The knockout cells produced about 1. 5 times more mitochondrial superoxide (a type of ROS) than normal cells.
Here's where the story gets nuanced. The cells weren't defenseless: they had actually turned up their antioxidant alarm system. A master antioxidant regulator called NRF2 was more active in the knockout cells, and early on, levels of hydrogen peroxide (a downstream form of ROS) were actually lower than in normal cells. The antioxidant response was working — for a while.
But by day 7 of differentiation, the dam broke. Hydrogen peroxide levels in the knockout cells surged to approximately nine times those in normal cells. This spike coincided precisely with the burst of excessive matrix production and hypertrophic marker expression. By day 14, single-cell profiling showed expanding clusters of cells expressing DNA-damage and stress genes — hallmarks of cells heading toward senescence, or permanent shutdown. By day 20, a direct test for senescence showed knockout chondrocytes at nearly twice the senescence levels of normal cells. And by day 30, calcium deposition — the final act of bone formation — was doubled in the knockout cells, confirming premature ossification.
The picture that emerged was one of a tipping point: the mitochondrial defect generates extra ROS on top of the ROS that cartilage cells normally produce during differentiation. Once the total exceeds a critical threshold, the ROS signal flips from a helpful maturation cue to a destructive force that shoves cells into premature aging.
Can you fix it?
This is where the study gets genuinely exciting for the Kabuki community. The researchers tested two interventions — and both worked.
First, they treated knockout cells with MitoQ, an antioxidant designed to accumulate specifically inside mitochondria. MitoQ reduced the excess matrix secretion in a dose-dependent manner, and at the higher dose, the knockout cells differentiated at roughly the same rate as untreated normal cells. Early maturation markers were unaffected, but the hypertrophic marker was suppressed — suggesting that MitoQ didn't prevent differentiation from starting, but stopped the harmful acceleration into hypertrophy. Most importantly, MitoQ fully rescued the premature senescence of the knockout chondrocytes.
Second, simply lowering the oxygen concentration from 20% to 5% markedly reduced ROS accumulation in the knockout cells and slowed their differentiation, while having little effect on normal cells. This supports the core hypothesis: total ROS levels, which depend on oxygen availability, set the pace of cartilage cell maturation.
One cautionary note: the highest dose of MitoQ actually harmed normal cells, increasing senescence and blocking their differentiation entirely. This highlights the delicate balance of ROS biology — too much quenching can be as bad as too little.
What about real growth plates?
The team also examined actual growth plates from embryonic mice carrying one normal and one disrupted copy of Kmt2d — a closer model of the genetics in Kabuki patients. At embryonic day 17. 5, they found no significant differences in the overall size of the growth plate or its zones. However, a telling detail emerged: there was a significant increase in the overlap between the oxygen-poor region and the hypertrophic region. In other words, chondrocytes in these Kabuki growth plates were entering hypertrophy in areas where oxygen was still relatively low — starting to mature at a lower oxygen threshold than normal.
Why weren't the zone sizes different yet? The researchers note that at this embryonic stage, chondrocytes haven't fully switched to oxygen-dependent energy production — that metabolic shift happens mainly after birth. This aligns neatly with the clinical observation that growth problems in Kabuki tend to appear postnatally. The team had previously found clear growth plate differences in 6-week-old and 4. 5-month-old Kabuki mice, well after this postnatal metabolic transition.
A broader mechanism?
The implications may extend beyond bone. Cells from Kabuki patients — including skin cells and neurons — also show electron transport chain defects and increased ROS. The authors suggest that redox-sensitive differentiation defects could be a unifying mechanism behind multiple features of the syndrome, not just short stature.
Antioxidant treatment has already shown promise in mouse models of other growth and mitochondrial disorders. The current study identifies mitochondrial ROS as a specific, potentially targetable contributor to growth plate dysfunction in Kabuki syndrome, and demonstrates that limiting excess ROS can restore a more normal pace of cartilage cell maturation and prevent premature burnout.
There remains an open question: do the elevated ROS levels cause the mitochondrial defect in the first place, or does a subtle defect in the respiratory chain come first, with ROS then making it worse in a vicious cycle? Either way, breaking that cycle — whether with targeted antioxidants like MitoQ or by other means — appears to help.
For Kabuki families watching their children's growth curves, the takeaway is cautiously hopeful: we may be closing in on a specific, treatable mechanism behind the short stature so common in this syndrome.
generated by Claude