Anyone who lives with more than one member of Felis catus knows that our beloved felines love to smell each other’s anal…
The last thing the field of gene therapy needs is another setback. Two studies, not yet peer-reviewed, point to adeno-associated virus (AAV) as a suspect behind the unusual hepatitis that emerged in children in April 2022. AAV has been critical to the development of gene therapy, as carriers of human genes in the single strand of DNA that is the viral genome.
AAV has been considered relatively harmless, as viruses go. It was discovered in 1965 as a tiny tag-along that will only replicate in human cells if adenovirus is also there at some point – hence the “adeno-associated.” AAV infection can also accompany certain herpes infections. Several subtypes of AAV have since been identified; AAV2 and AAV9 are gene therapy favorites. And they’re common. Eighty percent of us harbor AAV2.
AAV is a Superstar of Gene Therapy
Optimizing AAV as a gene therapy vector, tailoring payloads and altering trajectories to specific organs and cell types, like Amazon marking and delivering packages, is a huge field. Dozens of biotech and pharma companies provide the vectors.
AAV delivers the first two FDA-approved gene therapy products: Luxturna for the retinal blindness that I wrote my gene therapy book, The Forever Fix, about, as well as Zolgensma, to treat spinal muscular atrophy. These first gene therapies came on the market in 2017 and 2019, respectively.
Gene therapy trials, like so much else, came to a screeching halt as COVID descended. But the pandemic isn’t entirely responsible for the lag in approvals for gene therapies. Many factors have and are contributing to the slow pace:
• Clinical trial participants may succumb to the genetic disease, not the gene therapy, but are considered deaths.
• The first few participants in a trial may not respond as well as hoped, or at all, because optimal age range, best stage of disease to treat, and dosages are still being figured out. That’s why clinical trials are phased: to refine and redo.
• Even children who respond to a gene therapy may develop unanticipated symptoms, because no one had ever survived that long with the disease.
• Selection of participants may overlook a shared susceptibility that makes the treatment risky – a reason that may lie behind the possible link between AAV and the new pediatric hepatitis. The sick kids share a gene variant in an immune system gene.
AAV came onto the gene therapy scene following a few tragedies.
In 1999, 18-year-old Jesse Gelsinger died days after receiving gene therapy for an inborn error of metabolism (ornithine transcarbamylase deficiency). Adenovirus carrying genes into his liver entered immune system cells as well as the targeted hepatocytes, like a harpoon taking out a dingy in addition to a whale. The fact that Jesse had a fever the night before the procedure may hold a clue to the current situation of the possibly AAV-linked cases of pediatric hepatitis – perhaps his immune system overreacted.
Shortly after Jesse received gene therapy, so did five boys for an inherited immune deficiency, SCID-X1. A retrovirus delivered the genes but veered off course, piercing an oncogene, which manifest as leukemia 3 years later. One boy died from the cancer. The vector has since been retooled.
Much more recently, four boys died in a gene therapy trial for myotubular myopathy (MTM), which I wrote about here. That trial is using AAV as the vector. Initially investigators attributed the first three deaths to higher doses, but the fourth boy had a lower dose. All four boys, it turned out, had an unrecognized, unscreened for, liver condition.
Ironically, lentivirus is emerging as perhaps the safest gene therapy vector – that’s the sanitized name for HIV.
Zeroing in on the Cause of the New Hepatitis
The first cases of an unusual hepatitis in children and adolescents up to age 16 were reported in Scotland in April 2022, and called “non-A-E hepatitis.” As of now 35 countries have reported cases.
In a preprint, researchers from the MRC-University of Glasgow Centre for Virus Research report on 9 early cases of the hepatitis and 58 control subjects.
They identified AAV2 in blood serum and the livers of all 9 sick kids but in none of 13 healthy kids the same ages, in none of another 12 children with adenovirus infection and normal liver function, and also in none of 33 children who had been admitted to the hospital with hepatitis that had other, known causes.
Most telling is that 8 of the 9 children share an immune system connection that may be important: a variant of a gene that is part of the HLA complex. That’s the collection of genes that determines patterns of certain proteins on cell surfaces, particularly white blood cells, marking them as “self.” HLA type underlies tissue typing to match transplant donors to recipients.
Might the children with the new type of hepatitis share an underlying, rare variation of HLA type and a resulting immune response that overreacts to AAV?
Eight of the nine sick kids sharing the HLA type equals 89%, compared to the variant’s prevalence of 15.6% among blood donors in Scotland. Numbers elevate an association to a correlation, and so HLA type may predispose to the hepatitis.
As data accrue, identifying a plausible mechanism may elevate the correlation to a possible cause. For now, the researchers hypothesize that new infection with AAV2, or reactivation of a past infection, triggers a T cell response against the capsid proteins that protect the tiny DNA genome of AAV2. And that causes the inflammation of the liver that is hepatitis – something noted in a gene therapy trial using AAV2 in 2006.
The researchers call for larger case-control studies to tease out the roles of AAV2 and the two other types of virus it accompanies (adenovirus and herpes); testing stored clinical samples from kids who’ve had hepatitis that didn’t fit into categories A through E; and measuring specific antibody responses to distinguish newly acquired AAV2 from reactivation of infection.
The Bigger Picture: Immunity on Hold during the Pandemic?
These days it seems that whenever something new and odd that affects health appears, we look for COVID or it’s aftermath, long COVID. But SARS-CoV-2 was quickly ruled out as a cause of the new hepatitis. Perhaps we need now to look at the bigger picture, at the effect of the pandemic going forward in terms of susceptibility to other infectious diseases.
It may be time to return to the classic hygiene hypothesis. While we all hid from SARS-CoV-2, behind our doors and masks, the constant cataloging of the surfaces of potential pathogens that our immune systems continually carry out stopped.
The hygiene hypothesis is an old idea that has been demonstrated in many parts of the world: that “excessive cleanliness” reduces the “microbial stimulation needed to enable normal development of the immune system,” according to one definition. It’s why kids who grow up on farms are less likely to develop certain allergies and asthma than kids who grow up in cities, where immune system stimuli are far less diverse. Children in urban environments are also more likely to have greater exposure to antibiotics than rural kids, also perhaps dampening immunity.
What will happen when other respiratory viruses sweep by in the fall? Adenovirus? Rhinovirus? Respiratory syncytial virus? Coronaviruses? Mycoplasma? Will our immune systems be more vulnerable? I’m concerned about my friends who have barely emerged from their homes over the past two-plus years.
I suspect research is already underway to follow various groups of people to discover any links between environmental priming of the immune system, versus not, due to measures to keep COVID at bay. I fear that repercussions of the pandemic will undulate for years, perhaps in ways we can’t yet imagine.