The sperm looked perfect but ran on empty.
In the quiet architecture of human reproduction, a single enzyme called AK9 has long carried an unrecognized burden — its absence leaving sperm structurally perfect yet energetically stranded, unable to complete the journey toward conception. Researchers at Xiamen University have now named this silence, identifying AK9 gene mutations as a rare but distinct genetic cause of male infertility affecting roughly 3% of studied cases. The discovery does not merely explain suffering; it points toward resolution, as ICSI treatment has already guided three affected men to fatherhood, demonstrating that understanding a limitation is often the first step to transcending it.
- For years, men with AK9 mutations received no answers — their sperm appeared flawless under the microscope while remaining functionally paralyzed, a cruel invisibility that left clinicians without a diagnosis and couples without a path forward.
- The mutation disrupts a hidden third energy system inside sperm cells, starving the flagella of the ATP they need to propel forward — a breakdown so specific it leaves DNA, structure, and fertilization potential entirely untouched.
- Whole-exome sequencing of 165 infertile men uncovered five carrying bilateral AK9 mutations, and cross-referencing global genetic databases confirmed the gene is tightly conserved across human populations, suggesting this subtype exists quietly worldwide.
- ICSI — which injects a single sperm directly into an egg, eliminating the need to swim — proved a definitive workaround: all three men who underwent the procedure achieved successful pregnancies and healthy births.
- The condition is permanent and cannot be corrected medically, but its narrow scope means the defect is circumventable rather than insurmountable, offering affected couples a clear and viable route to biological parenthood.
A man arrives at a fertility clinic with sperm that cannot swim — structurally flawless, genetically intact, yet unable to complete their journey. For years, no explanation existed. Now, researchers have identified the culprit: mutations in a gene called AK9, which governs a hidden energy system inside sperm cells that keeps their whip-like tails in motion.
Asthenozoospermia — severely reduced sperm motility — is the leading genetic cause of male infertility, touching nearly half of all infertility cases and, by extension, roughly 15% of couples trying to conceive. While several gene families were known to impair sperm movement, the full genetic landscape remained incomplete. AK9 fills one of those gaps.
The enzyme's role is deceptively precise: it recycles energy molecules that fuel the sperm's flagellum. Mammalian sperm are unusual in that they appear to draw on a third energy pathway beyond the two most cells rely on — and AK9 seems central to that system. When it fails, sperm run out of fuel before they can reach their destination.
A team at Xiamen University sequenced the genomes of 165 men with unexplained infertility and identified five carrying mutations in both copies of the AK9 gene. Mass spectrometry confirmed the biological consequence: depleted energy molecules and reduced ATP production. The sperm looked perfect. They simply couldn't move.
The pivot from diagnosis to hope came through ICSI — a procedure that extracts a single sperm and injects it directly into an egg, making motility irrelevant. Three of the five men underwent the treatment. All three resulted in healthy births. The genetic defect was narrow enough that bypassing it was sufficient — the sperm, once placed at its destination, could do everything else it needed to do.
AK9 mutations are lifelong and cannot be treated pharmacologically. But their specificity is also their limitation: they impair movement, not fertility itself. For carriers, ICSI does not repair the broken machinery — it simply renders it beside the point.
A man walks into a fertility clinic carrying the weight of a genetic mystery. His sperm swim poorly—so poorly that conception seems impossible. For years, doctors could only shrug. Now, researchers have finally identified why: a mutation in a single gene called AK9, which controls the energy that powers sperm movement. And more importantly, they have found a way to work around it.
Asthenozoospermia, the medical term for severely reduced sperm motility, is the leading cause of male infertility. It affects roughly half of all infertility cases, and infertility itself touches about 15 percent of couples trying to conceive. The condition is straightforward in its cruelty: sperm that cannot swim cannot reach an egg. Yet for decades, the genetic roots remained obscure. Researchers knew that mutations in several gene families could impair sperm movement, but the full picture stayed murky.
Enter adenylate kinase 9, or AK9. This enzyme sits inside sperm cells and does something deceptively simple: it shuffles phosphate groups around, converting one form of energy molecule into another. Sperm flagella—the whip-like tails that propel them forward—are energy-hungry structures. They need ATP, the cell's primary fuel, and they need it constantly. Most cells generate ATP through two main pathways. But mammalian sperm are unusual. They can keep moving even when one pathway is blocked, suggesting a third energy system at work. AK9 appears to be part of that hidden system, maintaining the balance of energy molecules the flagellum demands.
Researchers at Xiamen University studied 165 Chinese men with idiopathic asthenozoospermia—infertility with no obvious cause—and compared them to 200 men with normal fertility. Using whole-exome sequencing, they found five men carrying mutations in both copies of the AK9 gene. Two of these men had homozygous mutations, meaning they inherited the same defective version from each parent. The mutations scrambled the AK9 protein's three-dimensional structure so severely that it could no longer function. When the team checked global genetic databases, these mutations were either absent or vanishingly rare, suggesting the gene is tightly conserved across human populations.
The science revealed something striking: the sperm of AK9-mutated men looked normal under the microscope. Their DNA was intact. Their shape was perfect. Only their energy metabolism was broken. Mass spectrometry showed that AK9-deficient sperm had significantly lower levels of AMP and ADP—the energy molecules AK9 normally recycles—and crucially, they showed reduced ATP production. The men's sperm could not swim because they were running on empty.
But here is where the story pivots from diagnosis to hope. The researchers tested intracytoplasmic sperm injection, or ICSI, a procedure in which a single sperm is extracted and injected directly into an egg, bypassing the need for the sperm to swim at all. Three of the five men with AK9 mutations participated in the fertility treatment. All three resulted in successful pregnancies and the birth of healthy babies. The sperm's inability to move did not affect its ability to fertilize once placed inside the egg. The genetic defect was narrow and specific: it crippled mobility but left fertility intact.
This distinction matters profoundly. AK9 mutations are lifelong. They cannot be cured with medication or lifestyle change. But they can be worked around. For couples where the male partner carries these mutations, ICSI offers a clear path to biological parenthood. The treatment does not fix the underlying genetics—the man's sperm will always struggle to swim on their own. But it renders that struggle irrelevant. The researchers have essentially found a way to bypass the broken machinery entirely, letting the sperm do what it was always capable of doing once it reaches its destination.
Citas Notables
AK9 affects only sperm motility and energy modalities but does not alter the fertilization ability of the spermatozoa.— Study findings (Sha et al., 2023)
La Conversación del Hearth Otra perspectiva de la historia
So these men's sperm look completely normal under a microscope, but they can't move. How does that even happen?
It's all about energy. The sperm's tail needs constant fuel to beat, and AK9 is like a recycling system for that fuel. Without it, the cell runs out of usable energy even though structurally everything looks fine.
And the mutation is inherited from both parents?
Yes, autosomal recessive. You need two broken copies to have the condition. If you inherit it from both parents, you get asthenozoospermia. One broken copy and you're a carrier with no symptoms.
Why didn't anyone find this gene before?
Because there's no good way to study it in isolation. You'd need selective inhibitors to block just AK9 and watch what happens, but those didn't exist. It took whole-exome sequencing and careful comparison of infertile men to finally spot the pattern.
And ICSI just... works? Even though the sperm can't swim?
Completely. The sperm's DNA and fertilization machinery are untouched. It just can't get there on its own. Once you place it inside the egg, it does its job perfectly. Three couples, three healthy babies.
So this is a cure, not a treatment?
For these specific men, yes. The mutation doesn't change, but ICSI solves the infertility problem permanently. It's a cure for the condition, even if the genetic mutation stays with them.