Interview with Tenna Toft Sylfest, XLH Alliance

OIFE has for many years had a project called “A stronger BOND between us”. The goal behind the project is to further develop an informal network between different stakeholders working on OI and other rare bone conditions (RBDs). The project has several subprojects and activities. We are formal members of the European Rare Bone Forum (ERBF) and we have an informal long standing collaboration with the umbrella for XLH – the International XLH Alliance. Because some of the other rare bone conditions are very rare or very little known, OIFE can play a role in creating awareness and spreading information about these conditions, which very often have a lot of commonalities with OI.

Who are you and what is your relation to XLH?

My name is Tenna Toft Sylvest, and I’m a 43-year-old woman living in Copenhagen with my husband and our twin daughters. I live with a rare bone condition called XLH (X-linked hypophosphataemia), which I have passed on to one of my daughters, who was diagnosed shortly after birth.

I co-chair the International XLH Alliance and lead the Danish XLH patient group. Additionally, I am an active member of the ERN BOND ePAG advocacy group.

In a few words – tell us about XLH and how it’s affecting lives!

XLH is a lifelong progressive condition characterized by low phosphate levels in the blood, leading to bone deformities, pain, dental complications, and reduced quality of life.

The conventional and standard treatment for XLH has been oral phosphate supplements and active vitamin D. However, since 2018, burosumab, a fully humanized antibody targeting FG23, has been approved for use in the treatment of XLH in some countries in Europe, the USA, and many other countries.

The clinical effects of XLH may include rickets in children, osteomalacia, bone pain, and leg deformities as well as premature fusion of cranial sutures and disproportionate short stature. Additionally, patients often experience dental issues making them more susceptible to tooth abscesses and periodontitis. In adults, XLH can lead to pseudofractures caused by osteomalacia, along with osteoarthritis, enthesopathies, spinal stenosis, hearing loss, depression, and an overall reduction in quality of life. You can read more in a new article called “Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia”.

How rare is XLH and are there different types? 

XLH is a genetic condition (X-linked). There is ongoing debate about how rare XLH is, but recent data suggest it may be rarer than previously thought, with estimates ranging from 1 in 65,000 to 1 in 90,000. Unlike OI, XLH doesn’t have distinct types, XLH’ers are affected to varying degrees, from mild to severe. XLH is a progressive condition.

Which patient organizations are you involved in?

I am actively involved at the National, European, and international levels. My journey in patient advocacy began around 2017, about a year and a half after I gave birth to my twins. Through a paediatrician, I discovered that what I had always referred to as D-vitamin resistant rickets was often known as XLH.

This opened a whole new world of knowledge and connected me with other patient group leaders across Europe and the US. My first international meeting took place in 2017, where a group of us patient representatives decided to establish the International Alliance for XLH.

Like OIFE, the XLH Alliance aims to bring together national patient group leaders to support each other, exchange knowledge and ideas, and collaborate on international projects. We strongly believe that this collaboration amplifies our voice. And we’ve already achieved many great things together, such as the annual awareness campaign for International XLH Day (October 23rd) and the International XLH Symposium. Similar to OIFE, our organization is patient-led, not by medical communities or other external interests. Since 2017, we’ve grown to 28 member countries worldwide.

On a national level, I helped establish XLH Denmark in 2018. The Danish group organises annual family days and maintains strong relationships with healthcare professionals. We’re also an active part of Rare Disease Denmark, working together with other rare disease patient groups, including the OI community in Denmark.

Finally, I’m involved with ERN BOND (European Reference Network) as an ePAG, working alongside other patient representatives, including two fantastic colleagues from the OI community in Norway and Germany.

Do you see any similarities or differences between XLH and OI?

Yes, I see many similarities. Although OI and XLH as a condition is very different, we have many similarities – where I think bone pain and deformities, dental issues, hearing loss are the most obvious ones. In many countries we also see the same experts and there is a lot of valuable in learning from each other when it comes to management and care.

Anything that’s very/completely different?

One of the most noticeable differences is that we don’t typically break our bones, and for children with XLH, bone fractures are less common. Instead, our bones are softer and may bow or become deformed. However, as adults with XLH, we’re more prone to pseudo-fractures.

The drug burosumab has been approved for both children in adults with XLH – but only in some countries and for some age groups. What are the reasons for this?

I can primarily speak to the situation in Europe and Denmark. Many countries have their own Health Technology Assessment (HTA) bodies, which must approve a new treatment before it can be authorized for the market. While EMA approval is required, in many places it is only the first step in allowing patients access to a potential new treatment.

In 2017/2018, the EMA recommended Burosumab as a new therapy. In some European countries, this led to automatic approval, enabling patients to be considered for the treatment. However, in many countries, the pharmaceutical company must apply for market authorization and gain approval from a national body.

In Denmark, the pharmaceutical company has applied for authorization for children, and as a result, children with XLH who meet certain criteria have begun receiving burosumab. In England, Burosumab was initially approved as a standard treatment for all children, but after a new HTA review, it is now the standard treatment for both children and adults.

Do you have any advice for the OIFE’s members, who might face similar challenges in the near future?

It’s a good idea to familiarize yourself with the HTA process in your country to understand how your patient group can effectively voice its concerns. Each country adopts unique approaches to patient involvement in the HTA process, ranging from no involvement at all to limited representation in a few meetings, or even actively contributing data and providing decision-makers with detailed reports about your condition, personal experiences, and quality of life.

Personally, I was struck by how lengthy the process can be and surprised that it requires the company to initiate it. I’ve also observed significant variations in how the process and results unfolds across Europe.

If you should suggest one topic OI and XLH could work together on – what would it be?

I think the pain topic is an extremely relevant topic for both groups and a great idea to dedicate a conference to, as you did in Stockholm. I have many ideas on what we could work with in the future whether it be women’s health, geriatrics, dental issues. My main focus will be to continue on the excellent collaboration between our groups – both between the International XLH Alliance and OIFE but for me personally also nationally in Denmark. If you want to get in touch with me you can contact me on email Tenna@xlhalliance.org or visit our website www.Xlhalliance.org

What do you do when you don’t do XLH work?

I work full time as a Senior IT Consultant. When I’m not working or volunteering, I’m usually in family mode, which brings its own joys. I used to love music, attending concerts, and dining out. These days, the kids’ favourite tunes have taken over my playlists, but I still enjoy catching up with friends over dinner and trips to the theatre.

This article has first been published in OIFE Magazine 1-2025