The differences between gene therapies for hemophilia A and hemophilia B

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Hemophilia is a genetic condition
Both hemophilia A and B are caused by mutations in the gene for blood clotting factor. Hemophilia A is caused by a mutation in the gene that creates factor VIII (FVIII) and hemophilia B is caused by a mutation in the gene that creates factor IX (FIX). Both the F8 and F9 genes are located on the X chromosome at different points.

Low factor levels lead to the inability of blood to clot, resulting in numerous physical and lifestyle burdens, including unexpected breakthrough bleeds and other chronic health problems. Over half the people with hemophilia A or B have factor levels less than 1% of normal.

Gene therapies for hemophilia A and B target different genes
Gene therapy is a long-term treatment option for people with hemophilia that offers extended bleed protection, which could eliminate the need for prophylaxis. Gene therapy uses an innovative approach that redefines treatment by either introducing a functioning gene into the body, or turning off or changing the gene that is causing the condition. Current gene therapies approved for hemophilia introduce a new, fully functioning gene into the body. The mutations causing hemophilia A and B have been characterized in thousands of people, and it is clear from the large number of mutations that the molecular basis of the condition is extremely diverse.

There are differences between gene therapies for hemophilia A and B
All gene therapy for hemophilia targets the liver. However, since there are differences in how the body produces FVIII and FIX, there are also fundamental differences in how gene therapy works in the liver.

For people with hemophilia B, gene therapy targets liver cells, known as hepatocytes, where factor IX proteins are naturally made. By delivering a functional F9 gene straight to the liver, it enables a person to start creating their own factor IX proteins that are missing or not working and causing the disorder.

In hemophilia A, a functional F8 gene is delivered to the liver, allowing it to start creating the missing or nonworking factor VIII proteins that cause the disorder. However, the way gene therapy for hemophilia A works is slightly different, since FVIII is produced by different liver cells and tissues than those that produce FIX. The F8 gene is larger and structurally complex, which creates additional challenges.

How gene therapy works
Working genes are usually delivered into the cells of the body by inserting them into an inactive viral shell, known as the vector.

Vectors being used in research are commonly made from adeno-associated viruses (AAVs). The AAV, naturally existing in the world at large, is deactivated, eliminating its ability to cause any illness while it performs its new task to deliver a therapy. In AAV-based gene therapy or gene transfer, a working gene is inserted into an AAV vector. An AAV vector protects and delivers the new gene to its destination through a one-time infusion. Current gene therapies for hemophilia A and B use different AAV vectors to deliver that new gene.

The size and simplicity of the F9 gene made it a promising target for gene therapy
Hemophilia B has long been a promising target for gene therapy because it is caused by a single gene mutation, which is both small in size and structurally simpler in comparison to hemophilia A.

In 2022, HEMGENIX®, etranacogene dezaparvovec-drlb, was approved by the FDA as the first and only gene therapy for hemophilia B. A one-time dose of HEMGENIX has been shown to offer elevated factor IX levels for years, with 37% average factor IX activity sustained at 2 years. HEMGENIX also offers greater bleed protection than prophylaxis. In a clinical trial, annualized bleed rate (ABR) for all bleeds decreased by 54% from an average of 4.1 for patients on prophylaxis during the lead-in period to 1.9 in months 7–18 after treatment. And 94% of, or 51 out of 54, people remained entirely free of continuous routine factor IX prophylaxis.

Hemophilia A has been a more challenging target for gene therapy

Due to constraints with AAV vectors, hemophilia A proved to be a challenging target for gene therapy. However, that changed recently, when the first gene therapy for hemophilia A was approved by the FDA. Administered as a single dose, gene therapy for hemophilia A has been shown to increase blood levels of factor VIII and reduce the risk of uncontrolled bleeding vs prophylaxis.

With gene therapies being approved for both hemophilia A and B, the future treatment landscape has irrevocably changed for anyone managing the condition.



HEMGENIX®, etranacogene dezaparvovec-drlb, is a one-time gene therapy for the treatment of adults with hemophilia B who:

  • Currently use Factor IX prophylaxis therapy, or
  • Have current or historical life-threatening bleeding, or
  • Have repeated, serious spontaneous bleeding episodes.

HEMGENIX is administered as a single intravenous infusion and can be administered only once.

What medical testing can I expect to be given before and after administration of HEMGENIX?

To determine your eligibility to receive HEMGENIX, you will be tested for Factor IX inhibitors. If this test result is positive, a retest will be performed 2 weeks later. If both tests are positive for Factor IX inhibitors, your doctor will not administer HEMGENIX to you. If, after administration of HEMGENIX, increased Factor IX activity is not achieved, or bleeding is not controlled, a post-dose test for Factor IX inhibitors will be performed.

HEMGENIX may lead to elevations of liver enzymes in the blood; therefore, ultrasound and other testing will be performed to check on liver health before HEMGENIX can be administered. Following administration of HEMGENIX, your doctor will monitor your liver enzyme levels weekly for at least 3 months. If you have preexisting risk factors for liver cancer, regular liver health testing will continue for 5 years post-administration. Treatment for elevated liver enzymes could include corticosteroids.

What were the most common side effects of HEMGENIX in clinical trials?

In clinical trials for HEMGENIX, the most common side effects reported in more than 5% of patients were liver enzyme elevations, headache, elevated levels of a certain blood enzyme, flu-like symptoms, infusion-related reactions, fatigue, nausea, and feeling unwell. These are not the only side effects possible. Tell your healthcare provider about any side effect you may experience.

What should I watch for during infusion with HEMGENIX?

Your doctor will monitor you for infusion-related reactions during administration of HEMGENIX, as well as for at least 3 hours after the infusion is complete. Symptoms may include chest tightness, headaches, abdominal pain, lightheadedness, flu-like symptoms, shivering, flushing, rash, and elevated blood pressure. If an infusion-related reaction occurs, the doctor may slow or stop the HEMGENIX infusion, resuming at a lower infusion rate once symptoms resolve.

What should I avoid after receiving HEMGENIX?

Small amounts of HEMGENIX may be present in your blood, semen, and other excreted/secreted materials, and it is not known how long this continues. You should not donate blood, organs, tissues, or cells for transplantation after receiving HEMGENIX.

Please see full prescribing information for HEMGENIX.

You are encouraged to report negative side effects of prescription drugs to the FDA. Visit , or call 1-800-FDA-1088.

You can also report side effects to CSL Behring’s Pharmacovigilance Department at 1-866-915-6958.

HEMGENIX is manufactured by uniQure Inc. and distributed by CSL Behring LLC.

HEMGENIX® is a registered trademark of CSL Behring LLC.

©2023 CSL Behring LLC 1020 First Avenue, PO Box 61501, King of Prussia, PA 19406-0901 USA   USA-HGX-0466-NOV23

Know Thyself—and Thy Medical History

“Know thyself” is a philosophical maxim, inscribed upon the Temple of Apollo in Delphi. It’s also part of a quote from Socrates, “To know thyself is the beginning of wisdom.” True wisdom is knowing… but also recognizing what you do not know, and what you need to learn.

November is National Family Health History Month, according to the US Surgeon General. This is an important national public health campaign to encourage all Americans to share their family medical concerns with family members, and to learn more about their own family health history.

I just turned 66, and am keenly aware that some of my family members, and many friends, are undergoing health challenges now. Kidney disease, cancer (a big one), heart problems… all of these can have a genetic component.

Hemophilia did not seem to run in our family—ours is the first known case in several generations. But diabetes is rampant. From an early age I’ve been careful about monitoring this, keeping my weight down, and staying active.

Knowing about your family health history of a disease can motivate you to take steps to lower your chances of getting the disease. Good places to start? Stop smoking—now. Exercise regularly (hike, bike, walk, swim) and change your eating habits. Get rid of poisonous soft drinks, limit alcochol. Family history is considered one of the most important risk factors for health problems such as heart disease, stroke, diabetes, cancer and even certain psychiatric disorders.

Knowing whether hemophilia “ran” in your family was important in family planning. For us, it was a surprise, but now we know the chances of hemophilia being passed on—an important thing for each child to know, for themselves and for their future partners. There are several ways to test for hemophilia pre-birth, and post birth.

Umbilical cord blood is now used to treat more than 80 diseases and disorders, including some that are transmitted to newborns. If you know your family’s health history, you can decide whether or not to store your newborn’s cord blood at birth. The stem cells from this cord blood could possibly be used to treat future diseases in your family.

There are free tools on-line to help start recording your family history. But start with your primary care physician, who will know the right questions to ask. Know thyself, thy body, thy health!

The Supreme Court Decision and Genetic Testing

The recent decision by the Supreme Court regarding abortion rights had me thinking about genetic testing and hemophilia. I recall that when I decided to have another child, following my first who had hemophilia, doctors were pushing for genetic testing. Why? I asked. We had already decided that if a sonogram showed a boy, we would do another C-section. Otherwise, let nature take its course. The doctor kept offering genetic services, but… when I suggested that he wanted the test in case a parent might want an abortion if genetic tests showed hemophilia? The topic not everyone wants to discuss directly? And that would not happen in my case regardless? The conversation ended, happily on both sides.

I recall a case many years ago of a British couple who sued their doctor, because he told them that their unborn child would not have hemophilia. And he indeed did. The implication was that despite having a beautiful baby boy, they were arguing that they would have aborted the fetus, had they known he had hemophilia. So they wanted monetary compensation for bringing their baby with hemophilia to term.

Genetic testing, abortion… topics peppered with landmines of emotion, legality, and belief systems.

But the bottom line is: Do you wonder or know if you or your daughter might be a carrier for a genetic mutation that causes hemophilia? This topic also comes up in Facebook groups from time to time. Supreme Court aside, it’s a good question to consider.

There’s a chance that a female is a carrier if she is the mother, grandmother, or sister of a biological son, grandson, or brother with hemophilia; or the aunt, cousin, or niece of a male with hemophilia related through her mother.  If you or your daughter fits any of these scenarios, you might want to consider genetic testing to determine carrier status. There’s no need in the case of a female born to a male with hemophilia; she is an obligate carrier and will have the gene for hemophilia on one of her X chromosomes.

Genetic testing identifies changes in the normal structures of proteins, genes, and chromosomes. An integral part of genetic testing is the counseling that goes with it, to help patients understand and adapt to medical, psychological, and familial implications of genetics contributing to disease.

Many large health insurance plans cover genetic testing when it’s recommended by a doctor. Often, genetic testing for hemophilia is a covered benefit because learning hemophilia carrier status can impact medical care.  Many insurance companies consider genetic testing medically necessary if a person is at risk for inheriting a disease or disorder.

But insurance coverage for genetic testing is inconsistent among health insurance companies, and even within a single company’s plans.

There are pros and cons to genetic testing. It’s essential to consider the risks of not being tested, given chances of being a carrier, as well as the risks of being tested.

Some pros? Knowing carrier status before a medical procedure can help prevent bleeding complications. It’s probably good to test factor levels of females with a family history of hemophilia at as early an age as possible.  According to NHF’s Medical and Scientific Advisory Council (MASAC), at least 50% of females who are carriers for hemophilia also have factor levels below 50%, putting them at risk for excessive bleeding during delivery of a baby as well as during a surgery, accident, or menstruation.

It’s also important that your daughter understands the risks of being tested. For privacy reasons, some people choose to pay out-of-pocket for genetic testing so that the testing and results do not appear in their medical record. Ask a genetic counselor about the actual cost of testing before consenting. In 2017, eligible potential carriers could seek carrier testing at no cost through NHF’s “My Life Our Future” program at qualified hemophilia treatment centers.

It can be empowering to know if you’re a carrier of a genetic mutation that causes hemophilia. A woman who knows her risk of passing a bleeding disorder on to her child can better advocate for herself. And for her rights and beliefs—whatever they may be— on a national scale.

Why Harry Doesn’t Have Hemophilia

The randy royal Harry has been usurping the news lately, after his infamous interview with Megan on Oprah. But the royal who truly deserves some attention is Prince Phillip, Queen Elizabeth II’s spouse, who died last week at age 99. Hemophilia is known as the “royal disease,” primarily due to Phillip’s and Elizabeth’s joint family tree: Phillip’s great-great-grandmother was Queen Victoria, a known carrier of factor IX deficiency, also called hemophilia B. Let’s look at his family tree, to answer the question, why doesn’t Harry (or William, or Charles, or Phillip) have hemophilia?

Prince Phillip

Prince Phillip was born a prince (unlike Diana, Camilla, Kate and Megan, who married into the family). His mother, Princess Alice, was a great-granddaughter of Queen Victoria. Phillip was descended from the third child of Queen Victoria, also called Alice, who like her mother, was a carrier for factor IX deficiency, or hemophilia B. In fact Queen Victoria had nine children, of whom two were carriers (Alice and Beatrice) and one had hemophilia (Leopold). These are very good odds!

Alice married Louis IV, the Grand Duke of Hesse. Alice introduced hemophilia into the House of Hesse and this German lineage. There were 7 Hesse children, and like their grandmum, one had hemophilia (Frederick) and two were carriers (Victoria, Phillip’s grandmother, and Alix). Alix married the Tsar of Russia, Nicholas II, and gave birth to Alexis, who had hemophilia. So, Prince Phillip has a long and illustrious side to his family tree regarding hemophilia! It’s been proposed that Nicholas II was so distracted by his son’s suffering due to hemophilia, that eventually he lost his grip on the monarchy at a time when the Bolshevik Revolution was poised to strike. And it did.

Queen Elizabeth? Not so much drama with hemophilia. Why? She is not a carrier of hemophilia. She is a direct descendant of King Edward VII, a son of Queen Victoria who did not have hemophilia. Now, Queen Elizabeth and Prince Phillip are related as third cousins. But since Phillip did not inherit hemophilia from his mother (not a carrier) or grandmother (also not a carrier but with a flip of the genetic coin might have been), and since Queen Elizabeth had no hemophilia in her direct line, neither Charles nor his sons, William and Harry, have hemophilia.

Harry’s got other problems, but hemophilia isn’t one of them, thankfully!

The Royal Disease

Parents Sue Over Birth of Child With Hemophilia

The newswires are alive with this story: An Australian couple are suing their doctor because their baby was born with hemophilia. The mother knew she was a carrier. She knew in-vitro fertilization could not guarantee a daughter… and the result is a gorgeous baby named Jess who apparently has caused such emotional damage that the parents want financial retribution. Now age three, Jess is at the heart of what could be the first of a kind lawsuit.

“We love our little boy, but we are very sorry he has to go through so much in his life,” the couple told The Sunday Telegraph (Australia).”We tried everything to avoid this situation, and now our boy has to go through all the pain and treatment in order to survive. We now face the fact that Jess will require treatment for the rest of his life.”

Part of me wants to shriek “get real, grow up, toughen up, face the real world.” Suffer? After reading about what inhibitor patients endure, it’s unlikely Jess will suffer much. After visiting so many developing countries where NO treatment exists, Jess is living a dream come true. Good family, great health care system, plentiful medicine, no threat of viruses… from where I stand they are very lucky people… yet still they are seeking retribution.

“Paul and Fiona (who do not want to reveal their surname) are claiming damages for the shock and nervous anxiety caused by the unexpected nature of the birth.” Get in line… thousands of families have dealt with it–with no family history, no warning whatsoever. Their claim seems to negate all the emotional suffering the rest of us go through–and eventually get over.

I was speaking to another mom today about this case, which has everyone buzzing. She thinks the parents are just not informed about hemophilia today (the mother’s brother had hemophilia and was crippled). But I wondered, are they not aware or too aware? Are the parents profiting from something we all learned to accept, and they under took this risk willingly? They claim no one told them Fiona was carrying a boy, despite ultrasounds (and she didn’t ask?); had she known she would have aborted. Are they truly in agony, three years later? Are they just not aware of how good life can be with hemophilia in developed countries? Or are they looking to make a killing from the physician’s insurance, which also drives up health care costs for everyone?

But wait. In speaking with this mom tonight, I realized we are only skimming the surface. Snap judgments based on media reports are dangerous: what is the media not telling us? And despite what they print and even what the parents tell them, perhaps there is a bigger, more important plan here. Perhaps the situation wasn’t handled well medically. After all, they were assured at some point that what was selected and implanted was a female embryo. They must have paid good money for this procedure. Throughout her pregnancy, if she is correct, no one told the mother she was carrying a boy (hard to imagine no one thought to ask!! and where were the ultrasound pics?). And if the baby was a boy and the doctor knew, could the doctor have put the baby at risk of a head bleed by having a natural birth (the article did not say if the baby was born naturally or by C-Section)? If all this is true, maybe there was true medical negligence and what is being sought is not so much emotional damage (obviously they are not so damaged they cannot be good parents to Jess) but a punitive measure against the medical facility that failed them in so many ways. In this manner, the lawyer has to follow whatever strategy works to get that justice.

Tough case to judge. I hope the parents at heart are grateful for Jess. Knowing how fiercely almost all parents of children with hemophilia I have met love and protect their children, it is painful to read about the burdens they say this child is causing them –even if it is only said for the benefit of the courts to seek justice for all. It will be interesting to hear more of the facts come out and hear from the hemophilia families in Australia comment on this case. Thoughts, anyone?

Book I Just Read
The Alchemist by Paulo Coelho. I generally like Coelho who has a lovely, simple style of writing but usually manages to capture fundamental themes on life and the human character. Here, Santiago, a shepherd, leaves his life behind to follow his destiny after a dream about searching for a treasure at the Egyptian pyramids. He meets assorted people along the way who help him rethink his journey and ultimately his destiny. This book is a quick read, enjoyable, easy to understand. I don’t think it is Coelhos’s best by a long shot. It’s a bit too simple, though I enjoyed it because it was about traveling, far way places and above all following your destiny. I thought the ending was a disappointment–too much like a neat Hollywood ending. The book has met with mixed reviews. If you are spiritual, seeking or just like light tales with thought-provoking questions, you may enjoy this. Two out of four stars.

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