Cure RTD

I is for Intrafamilial Variability

Same family. Same two RTD gene mutations. Different journeys.

Among the more than 500 individuals in the Cure RTD Patient Registry, there are over 150 affected sibling. This is not surprising because Riboflavin Transporter Deficiency (RTD) is usually inherited in an autosomal recessive pattern.

When both parents carry an altered copy of the same RTD-associated gene, each pregnancy has a 25% chance of producing a child who inherits both altered copies and develops RTD.

What often surprises families and doctors is how differently RTD can affect siblings.

What is intrafamilial variability?

Intrafamilial variability describes differences in the age of onset, symptoms, severity, and progression of a condition within the same family.

In RTD, these differences can often be dramatic.

Two siblings may carry the exact same disease-causing variants (mutations) in the SLC52A2 or SLC52A3 genes, yet follow very different clinical paths.

Without riboflavin treatment, one child may become severely unwell during infancy and develop rapid neurological decline and even pass away. Their sibling may remain apparently healthy for many years before developing milder, and sometimes different and slower-progressing symptoms.

In some RTD families, one sibling may require extensive medical support while another with the same variants is much less affected.

How can the same mutations cause different outcomes?

There is no single proven explanation. The RTD-causing variants are important, but they are only part of the story.

Modifier genes

Siblings may share the same RTD-causing variants, but they do not share their entire genetic makeup. Other genes can influence many things important in RTD, including riboflavin metabolism, mitochondrial function, oxidative stress, and the body’s ability to compensate for reduced transporter activity. Depending on their effects, these modifier genes may make the overall presentation of RTD either milder or more severe.

Illness and metabolic stress

In more than half of people with RTD, the first symptoms appear shortly after a metabolic stressor. This may include an infection, prolonged fever, surgery, or another physical stress that increases the body’s energy demands. In someone already close to a metabolic threshold, a major stressor may trigger or accelerate symptoms. This could help explain why siblings exposed to different stresses throughout their lives can follow very different clinical paths.

Epigenetic and biological differences

Even when siblings share the same RTD variants, those genes may not behave in exactly the same way. Differences in growth, hormones, mitochondrial function, and other biological processes can affect when symptoms appear, how quickly they progress, and which areas of the body are most vulnerable.

Nutrition

In some young adults, RTD symptoms first appeared after a major diet change that sharply reduced riboflavin intake. This suggests that, before treatment begins, diet may also play a factor in determining when symptoms appear and how the disease progresses, which can be different between siblings.

Researchers are still trying to understand why these differences can be so large. Many of these explanations remain hypotheses rather than proven causes.

Why does this matter?

Doctors should never assume that an apparently healthy sibling is unaffected simply because they do not resemble the first family member diagnosed with RTD.

Once one person in a family is diagnosed with RTD, all biological siblings should be evaluated genetically as soon as possible. If an undiagnosed sibling shows symptoms consistent with RTD, riboflavin treatment should begin while genetic testing is still underway. Delaying treatment could allow avoidable and potentially permanent damage to occur.

The same RTD variants do not guarantee the same symptoms, severity, or age of onset.

In RTD, the first diagnosis should open the door to testing the entire family. Early identification may change the course of a sibling’s life.

H is for Heterozygous

Can one altered gene copy cause Riboflavin Transporter Deficiency?

RTD is a autosomal recessive genetic disorder. The basic idea is simple.

We inherit two copies of most genes, one from our mother and one from our father. In a recessive disorder, both copies need to be altered for the person to develop the disease.

• RTD Type 2 is linked to the SLC52A2 gene.

• RTD Type 3 is linked to the SLC52A3 gene.

Among the more than 500 patients in the Cure RTD Patient Registry, every person with RTD Type 2 has two identified SLC52A2 variants (mutations), one inherited from each parent.

RTD Type 3 is more complicated.

In approximately 20% of people with RTD Type 3, genetic testing finds only one SLC52A3 variant. This is called being heterozygous.

Think of a gene as an instruction manual. Someone who is heterozygous has one altered copy and one copy that appears normal.

Traditionally, this person would be considered a healthy “carrier” and develop no RTD symptoms. However, the growing number of RTD Type 3 cases with only one identified variant shows that the story is not always that simple.

These patients often develop symptoms later in life (normally 15 to 40+ years old) and can vary widely in severity. Many have a milder form of RTD than children who present early with two altered gene copies. In most reported cases, symptoms improve or stabilize with standard riboflavin treatment.

How can one altered copy cause symptoms?

The honest answer is that we do not fully know, but there are several possibilities.

It is possible that a second variant may be hiding. Genetic tests are very good at reading the main protein-coding regions of a gene, but they do not detect everything. A second variant may be deep inside the gene or in a region that controls how much RFVT3 protein is produced.

It is also possible that one altered copy may sometimes be enough to cause RTD in some people. Certain SLC52A3 variants may act in a dominant way, meaning one altered copy can increase the risk of disease. This does not mean everyone with the variant will develop symptoms.

This leads to two important genetic terms:

Reduced penetrance means some people with the variant develop symptoms while others remain healthy.

Variable expressivity means people with the same variant can have very different symptoms and levels of severity, even within the same family.

Other genes may influence the outcome. Additional genetic differences affecting areas such as riboflavin metabolism, energy metabolism or cell/nerve health may help determine whether symptoms develop.

Illness, infection or other biological stress may trigger symptoms in someone who had previously been coping with reduced transporter function. Many RTD patients developed or worsened after an infection, and more recently, major dietary changes have also been reported as possible triggers in a few RTD adults. The exact connection remains unclear.

The important message

A person with RTD-like symptoms should not be dismissed simply because testing found only one SLC52A3 variant.

At the same time, one variant does not automatically prove RTD. It must be considered alongside the person’s symptoms, family history, genetic findings and response to treatment.

RTD genetics are teaching us that biology does not always follow simple textbook rules. Sometimes one altered gene copy is only part of the answer. Sometimes it may be enough to matter.

H is for Heterozygous: one altered copy, many unanswered questions and a diagnosis that should never be overlooked.

H is for Homodimer

We talk a lot about RFVT2, the riboflavin transporter made by the SLC52A2 gene. Its job is to help move riboflavin (vitamin B2), into cells. When RFVT2 does not work properly, it causes Riboflavin Transporter Deficiency Type 2.

New Cure RTD funded research adds an important layer to this story.

RFVT2 does not appear to work as a single protein. It can pair with another RFVT2 protein to form what scientists call a homodimer.

That word sounds complicated, but it simply means two identical copies of the same protein working together. Think of it like a buddy system, a zipper, or two matching halves that help stabilize each other.

A recent study showed that this pairing may be an important part of how RFVT2 folds, stabilizes, reaches the cell membrane, and functions.

That matters because RTD-causing mutations can damage this pairing.

When RFVT2 cannot form a proper pair, several things may go wrong at once.

First, the protein may become unstable. A damaged copy may not fold correctly, and the cell may recognize it as defective. It can then get trapped in the endoplasmic reticulum (ER), which is the cell’s protein-folding factory.

When too much misfolded protein builds up in the ER, the cell turns on an alarm system called ER stress or the unfolded protein response. At first, this response is protective. The cell tries to refold the protein or clear it away. But if the stress continues, it can become harmful and push vulnerable neurons and other cells toward dysfunction or death.

That is what makes this research so important. RTD patient's motor neurons had large increases ER stress. Even more interesting, this happened despite normal cellular flavin levels (Riboflavin, FMN and FAD). That means RTD may not be only a “not enough riboflavin” problem. In some cases, the damaged RFVT2 protein itself may be causing stress inside the cell.

Second, pairing may help RFVT2 reach and remain stable on the cell surface. Many membrane proteins have quality-control checkpoints. If they fold and assemble correctly, they are sent to the cell membrane where they can do their job. If they do not, they may be held back inside the cell or degraded. A mutation that weakens dimer formation could mean fewer working transporters on the membrane.

Third, RFVT2 has loops that extend inside and outside the membrane. These loops may help the transporter fold, pair, move, and communicate with other proteins or cell-signaling systems. If mutations distort the structure, those loop interactions may also be affected. That could help explain why RTD cells show disrupted calcium signaling and struggling mitochondria.

Researchers have tested several RTD mutations. When mutant RFVT2 copies paired with the same mutant copy, dimer formation dropped by about 50% to 80% compared with normal RFVT2. But most patients carry two different mutations, one from each parent. When two different damaged copies were tested together, dimer formation dropped by more than 90%.

That could be a big clue.

It means two mutations may not simply add up. They may interact in a way that makes RFVT2 much harder to fold, pair, or stabilize. This could partially help explain why RTD can look so different from one person to another, along with many other genetic factors.

Some mutations may mostly affect riboflavin binding. Some may destabilize the protein. Some may trap it inside the cell. Some may damage dimer formation. Some may do several of these things at once.

This does not make riboflavin less important. High-dose riboflavin remains the core treatment and can be lifesaving, especially when started early. But this research helps explain why riboflavin alone may not fully solve the disease for every patient. It also raises an interesting possibility: riboflavin may be helping in more than one way. Beyond increasing riboflavin availability, it may also help some damaged RFVT2 proteins fold more correctly, become more stable, and form homodimers more effectively. That idea still needs more research, but it could help explain why some patients improve with high-dose riboflavin even when the biology is more complex than a simple vitamin shortage.

RTD may involve two connected problems: not enough riboflavin getting where it needs to go, and a damaged transporter protein that may cause additional stress and cell injury.

That opens the door to new research questions. Can we help RFVT2 fold better? Can we stabilize the transporter? Can we reduce ER stress? Can we protect mitochondria while riboflavin does its job? Could future treatments include chaperone-like compounds, ER stress modulators, mitochondrial support, or gene-based approaches?

The homodimer story gives us another piece of the RTD puzzle. In simple terms, RFVT2 works best with a partner. When mutations prevent that partnership, the effects can ripple through the entire cell.

And every new piece like this gives researchers another possible target.

G is for Gene Therapy

RTD is caused by changes in the SLC52A2 or SLC52A3 genes. These genes provide the instructions for making riboflavin transporter proteins, which move riboflavin (vitamin B2), into the cells that need it.

In RTD, those instructions do not work properly, leaving vulnerable nerve cells struggling to survive and function.

High-dose riboflavin remains the foundation of RTD treatment and has changed many lives. But riboflavin does not correct the underlying genetic problem. That is why Cure RTD has spent years supporting gene therapy research.

So how would gene therapy work?

A simple way to think about it is this: the damaged gene is like a recipe with errors in it. The type of gene therapy we are using does not erase or rewrite that recipe. Instead, it aims to deliver a clean, working copy into the cells that need it most.

To do this, scientists use a delivery vehicle called AAV, short for adeno-associated virus. The virus is modified so it cannot behave like a normal virus. It is emptied out and repurposed as a microscopic delivery truck carrying a healthy copy of the SLC52A2 gene. The goal is for affected cells to begin producing working riboflavin transporter protein.

A Cure RTD-funded study at OPBG in Italy has already tested AAV9-SLC52A2 gene therapy in motor neurons made from RTD patient cells. Researchers showed that gene therapy could dramatically improve these cells. Nerve length was restored to levels similar to healthy motor neurons, and treated cells were better able to survive and maintain a healthy nerve cell network over time.

That matters because it shows gene therapy is not just an idea on paper. It can rescue a real RTD disease feature in living patient-derived nerve cells.

This work builds on years of Cure RTD-funded progress.

In 2019, Cure RTD partnered with Jackson Laboratories and Dr. Steven Gray’s lab at the University of Texas Southwestern to develop an RTD mouse model and test whether gene therapy could rescue these mice.

In June 2024, newborn RTD mice received a single injection of an rAAV9 vector carrying a healthy human SLC52A2 gene. After 26 weeks, treated RTD mice were thriving and appeared similar to healthy controls, while all untreated or riboflavin-only treated RTD mice died.

In June 2026, the next phase of preclinical testing began. RTD mice are now being treated with a new gene therapy called rAAV9fire. In this experiment, mice are treated at the equivalent of young childhood, with new measures of brain health and metabolic correction being studied.

This work is designed to build the critical data needed to understand safety, durability, and effectiveness before moving toward human clinical trials hopefully in the next few years.

There is still a long road ahead. Mouse studies and cell studies are not the same as treating children or adults with RTD. Gene therapy must be tested carefully and thoroughly before it can become a human treatment.

But this is real progress.

We now have RTD patient cells showing rescue after gene therapy.

We have RTD mouse models that can test treatment. We have animal data showing remarkable survival. And we now have another major mouse study underway.

Research takes time.

Gene therapy takes time.

But we will not stop until we cure RTD.

Happy Father’s Day to all of the incredible dads in our RTD community! We hope you have a wonderful day filled with all the people and things you love! 👏

F is for FAD

When most people hear “vitamin B2,” they think of riboflavin.

But riboflavin is only the starting point.

Inside the body, riboflavin is converted into two larger molecules called FMN and FAD. These are the active working forms of vitamin B2.

FAD, short for flavin adenine dinucleotide, is especially important. It helps power mitochondrial energy production, antioxidant defense, and many other essential cell functions. In simple terms, FAD helps cells turn food into usable energy, and cells cannot survive without it.

That is why FAD matters so much in riboflavin transporter deficiency (RTD).

RTD is caused by mutations in the SLC52A2/A3 genes that move riboflavin into cells. And that is the key point:

Riboflavin gets transported into cells - Not FAD.

FAD and FMN are larger, more charged molecules. They cannot simply pass through riboflavin transporters and enter the cell directly. The cell needs riboflavin first, then it builds FMN and FAD on the inside.

Once FMN and FAD are made inside the cell, they are trapped there. That makes sense because cells need to hold onto them. So many enzymes depend on them to function.

Even the food we eat follows this same riboflavin rule.

Most food we eat does not contain riboflavin, despite humans needing it to survive. Instead, food contains riboflavin in the form of FMN and FAD, the same energy molecules used by the plants and animals.

But our bodies cannot absorb them that way. First, the gut breaks them down into riboflavin. Then riboflavin is absorbed, transported into cells, and rebuilt into FMN and FAD where it can work.

It is a strange little loop: break it down, move it in, build it back up.

As discussed in the recent “E is for Ectoenzymes” post, most flavins in the bloodstream also do not travel as free riboflavin. Instead, about 80% circulate as FAD. This blood FAD may also be an important supply source for riboflavin to cells.

In RTD, the problem may not be just how much riboflavin is present. It may also be about whether the body can move flavins, convert them, transport them to the right places, and rebuild them in the tissues that need them most.

RTD has taught us that riboflavin is not just a vitamin. It is part of a much larger transport and conversion system.

And FAD may be one of the most important pieces of that puzzle.

Today, we remember Brooklynn, four years after her passing from complications related to RTD.

Brooklynn faced more in her life than any child should, but she did so with remarkable strength, joy, and spirit. She left a lasting mark on everyone who knew her, and her story continues to be part of the work we do in the RTD community.

We are also deeply grateful to her parents, Vanessa and Paul, for the love, advocacy, and commitment they have shown over the years. Through their awareness efforts and past fundraisers for Cure RTD, they have helped carry Brooklynn’s legacy forward in a way that continues to make a difference.

E is for Ectoenzymes

Riboflavin (vitamin B2), is essential fuel for our cellular engines. But most flavins in the bloodstream do not travel as free riboflavin. Instead, about 90% circulate as larger molecules called FAD and FMN.

These forms are too bulky and too charged to slip through the cell membrane, and riboflavin transporters, known as RFVTs, cannot bring FAD or FMN into the cell.

So how does the cell access riboflavin?

RFVTs can transport free riboflavin outside the cells that is in blood or brain fluid. But FAD and FMN may also play an important role in supplying riboflavin to cells, as long as they are converted first.

That is where ectoenzymes come in.

Ectoenzymes sit on the outside surface of cells and act like molecular scissors. Between 2022 and 2024, researchers first discovered that these enzymes help convert FAD and FMN into free riboflavin, allowing cells to take it up. Until recently, the purpose of the large amounts of FAD and FMN in the blood and brain was not understood.

First, NT5E, ENPP1, and ENPP3 help convert FAD into FMN. Then ALPL removes the final phosphate group from FMN, turning it into free riboflavin.

This provides additonal free riboflavin for the RFVT transporters to pull into the cell.

This may matter in Riboflavin Transporter Deficiency (RTD). In RTD, the RFVT transporters are already impaired, which means cells depend even more on having enough free riboflavin available at the cell surface.

One fascinating question is whether the distribution or regulation of these ectoenzymes might play a role in why certain tissues are affected more than others in RTD. Cells have very different mixes of ALPL, NT5E, ENPP1, and ENPP3.

This is one of many important questions Cure RTD-funded research is working to answer. Understanding this process could open a new window into RTD biology and may help point toward future treatments.

For RTD, these tiny enzymes on the outside of cells may turn out to be another important piece of a much larger puzzle.