Thank you so much, it's been a pleasure having this invitation to speak to you today on a topic that's very important to me and that is the topic of rare bone disease. And I'll start off by asking a question, a couple of questions, one of which is, any of you seen patients who have hypophosphatasia? Raise your hand so I can see. So there's a couple of you. What about XLH, X-linked hypophosphatemia? So a few more of you. If you're in the pediatric realm, you're probably seeing more of this. I wanna emphasize to you that these disorders aren't in isolation, that they in fact may have important implications for mineralization disorders that affect all of us as we get older. And mineralization of soft tissues, such as in the arteries, we do a calcium score on the coronary arteries, for example, are really defects of mineralization. So understanding the process of mineralization and what can go wrong with it, I think is very important for our understanding of what happens with aging and a variety of other health problems. So I am a speaker for Alexian, Amgen, and Kiowa-Kirin. Briefly, I am past president of the American Association of Clinical Endocrinologists and the American College of Endocrinology, as well as the International Society for Clinical Densitometry and the co-chair of the Osteoporosis Guidelines for ACE. Probably the most interesting thing I do is that for 18 years, I've been the main endocrine and bone densitometry consultant for NASA, the Johnson Space Center. So I do all the astronaut stuff. If you want to know about long-duration space flight, I would recommend not doing it. It has a lot of issues and that's beyond the scope of what we're going to talk about today. I'm also on the writing committee and faculty for the ISCD certification course. And I just retired three months ago from being chair of endocrinology at the Houston Methodist Hospital. So I am having a lot of fun doing other things than just medicine all the time. Hopefully in your futures, you have the same experience. All right, so first of all, our objectives. First of all, to understand the chemistry underlying mineralization disorders. I will show some slides that have a lot of chemistry in them. I will allude to them very briefly because it's not really the major focus that I'm going to want to labor today. Importantly, to differentiate hypophosphatemia and hypophosphatasia as forms of osteomalacia. This is a problem for a lot of trainees trying to figure out low bone density, whether or not it's osteoporosis or a mineralization problem. And unfortunately, this is not always the case where you can distinguish this easily. And so it's oftentimes problematic because you're putting patients on osteoporosis medications that actually have mineralization disorders and that can actually make the problem worse. We'll explore the diagnosis of hypophosphatasia and XLH a bit and the management in brief. And we'll talk about some other rare and novel metabolic bone disorders that you may actually have in your practice if you're not recognizing them yet. You may recognize them after I give this talk. So I'll want to emphasize those of you who are in clinical practice to go back to your clinics and re-evaluate whether or not you may have these patients in your practice. And that's why, unfortunately, these rare diseases stay rare because people think they'll never see them. And if they think they'll never see them, they tend not to look for them. So I would want to make the effort to go ahead and look for them. And we'll talk about why this matters for everyone, that these mineralization disorders actually affect soft tissues in a way that can cause significant pathology. And as we get older, the calcifications in our arteries and other parts of our body can result in significant morbidity and mortality. So these are not just rare diseases. They, in fact, have implications for very common disorders. So there are seven key points that I want to make and I will repeat these at the end. But I want to emphasize the fact that osteomalacia and osteoporosis are not the same thing, that when you have a low bone mineral density, that mineralization defects may look like osteoporosis. And if you're just jumping to the conclusion that that's what it is, you tend to skip steps and put patients on medications that are not appropriate for them. So I emphasize to all my trainees to note that osteomalacia has to be evaluated first before you make a diagnosis of osteoporosis. I would also emphasize that these are considered rare diseases, that's what I specialize in, but they're rare mainly because people don't look for them. They may be uncommon, they may be more common than we think, but in fact, we have lots of patients that have these disorders that are out there that simply are not recognized. The mean time to diagnosis, if you ever make a diagnosis on some of these patients, is seven years or more. So I think that it's important to understand this in the adult patients. In pediatrics, it's much more common to make a diagnosis early on because they're presenting with significant bone problems in their early years, and most pediatric evaluations will include this type of a metabolic bone evaluation. Even though I'm a bone specialist, I love endocrinology of bone. I will emphasize that these disorders are systemic disorders. They involve every organ system in the body because these mineralization disorders can affect soft tissues. So even though we call this a bone disease, this is not a bone disease, this is a systemic disease, although we tend to put it into the category of bone disease. And I will emphasize two things to look at in your patients. Number one, check the phosphorus level. In the United States, phosphorus is not on our chemistry panel anymore. It was when I was a fellow. That was a long time ago. But the economics of laboratory testing apparently left it off the panels, at least for the past 20 years or so. So you actually have to think about, at least in the US, of ordering a phosphorus as part of your initial evaluation, or otherwise you will never make these diagnoses in a timely fashion. So I think it's very important to routinely check a phosphorus level in anyone presenting with symptoms that suggest muscle disease or bone disease. So I would emphasize that importantly. XLH is the one thing, of course, I'm gonna emphasize in this talk, but there are other hypophosphatemic disorders that I will allude to that also may be significant. I'll also emphasize that alkaline phosphatase, which is our key test for hypophosphatasia, is a low alkaline phosphatase. And I see a lot of clinicians who are so relieved that the alkaline phosphatase is not elevated, right? They're looking for things like Paget's disease and liver disease and bad things. And when it's low, it tends to be ignored. And most laboratories don't flag this as being a significant abnormality in many cases. So it's very important to note that a low alkaline phosphatase shouldn't be ignored. And if the low alkaline phosphatase is something that is chronic, that has been present all the person's life or for long periods of time, that in fact hypophosphatasia may be a diagnostic consideration. I'll also talk about some therapy considerations that depend on clinical circumstances. So not all these diseases need to be treated. Once you identify them, it may explain the symptoms and that may relieve the patient of seeing multiple specialists and try to figure out what's wrong with them. And this may be enough. Some patients do require therapeutics and that is something that you need to consider on an individual basis. So let's talk about mineralization chemistry. So all of that have had chemistry long ago. I was a chemistry major and a computer science major in college, but chemistry is something that we oftentimes will come up in our daily practices, but we don't emphasize as much. So I will talk about a little bit of mineralization chemistry and why it's important. It is tightly regulated in biologic systems, although we don't generally emphasize this in our educational programs. In life forms such as the invertebrates in the ocean, we'll allude to this later, they started off without any skeletal structures at all. The life forms in the ocean use calcium carbonate on an organic matrix in order to provide protection against other organisms that were trying to eat them. Invertebrates, of course, hydroxyapatite, which is a calcium phosphate composite, not only holds us upright, but also is a source of mineral for our metabolic processes that are critical. So we can walk around with these minerals that are in our bone that can be mobilized if we need them in the proper time. Physiologic mineralization, the bone osteoblasts are important in mineralizing our skeleton, but other areas such as the chondrocytes in the growth plates in which there are hypertrophic zones result in the bone lengthening, what we call bone modeling, that is important for a growing skeleton. And in the teeth we have a different set of cells, odontoblasts, that also produce hard materials in order to allow our teeth to form properly. And the mineralization characteristics in these structures is different and it's regulated differently. I will emphasize that pathologic mineralization is the reason why we should care about these disorders. For instance, soft tissue calcifications and articular cartilage, we deal with arthritis patients, for example, patients with chondrocalcinosis. The cardiovascular system, the aging of the arteries is something that we're all gonna face at some point if we get old enough. And these calcifications that form in the blood vessels have significant pathology associated with them. And when we look at a calcium score, for example, in cardiac evaluations, these pathologic mineralizations are very important. So understanding the mineralization process has implications for everybody. In addition, in patients with kidney disease, these patients have problems with phosphorus already and tend to calcify tissues abnormally. I will also emphasize that in patients with osteoporosis, which is the more common problem that we see with mineralization problems, and I won't talk about that much here because you're all very familiar with it, that there's an imbalance in resorption formation. And this certainly is something that is undergoing intensive further study, but it won't be the topic of today's discussion. So let me start out by saying, 520 million years ago and below, invertebrates were the rule. So the oceans developed these organisms in which calcium and other minerals were available in the environment. Sea water contains everything they needed. As organisms started to eat each other, they needed protection. And so the use of calcium carbonate on an organic matrix was oftentimes the way these organisms protected themselves. And we see this today, of course, in its shells, corals, and other mollusks. This is Abu Simbel, this is me standing next to Ramses. I'll point out that I have relatively normal mineralization, so I'm standing there showing what a normal vertebrate mineralization should look like. Ramses is hypermineralized, so you see him as a statue there. And I will allude to disorders in which hypermineralization is a huge problem. Mineralization of soft tissues can turn us into stone, just like we see here in this particular statue. So the mineralization process is complicated. In fact, it's much more complicated than our understanding has extended to. And there's active research continuing to go on to what seems to be a very simple process. Calcium phosphate solubility product, when it's exceeded, causes deposition at various sites. When it's occurring at proper sites, such as in collagen, we get bone, which is something that clearly is a normal structure that we need to maintain. The extracellular fluid, though, does have a lot of calcium and phosphorus in it, and so we're always on the verge of calcifying our soft tissues. So there is an important countercurrent mechanism in order to protect us against calcifying our skin. Sometimes when that goes awry, we have calcinosis in our skin and in our blood vessels, and this produces pathology. So there is a complicated system in which to protect us against these abnormal calcifications. Alkaline phosphatase is one of the key enzymes that controls mineralization, and we'll talk about that in some more detail. And the key other factor is inorganic pyrophosphate. That is a major inhibitor of calcification. And I'll point out that, unfortunately, there is no test that's commercially available for inorganic pyrophosphate, but it is important. So those of you who are interested in innovation, if you can come up with an easy commercial test, there is research tests for pyrophosphate, but I think this is really key in order to determine some of these mineralization defects that are otherwise cryptic at present. So those of you who have an innovative streak in yourself, I think developing a common, easy test to do for inorganic pyrophosphate may help open the door to understanding many of these disorders more clearly. As a inhibitor of calcification, this is why our tissues don't mineralize. It's why they shouldn't be mineralizing. The cells are complicated in that they sequester calcium, generally in the mitochondria. The mitochondria have high calcium concentrations, whereas the cytoplasm tends to have more phosphorus. This enables calcium to be an important mediator of cellular processes because the calcium concentrations in the cytoplasm are low. But if cells undergo normal apoptosis, they're engulfed by macrophages, and so this whole system tends to balance itself. But when you have inflammatory processes, the cells rupture, releasing their contents. This extra calcium and phosphorus exclude out into the extracellular fluid, where these abnormal calcifications can occur, which is why inflammatory processes in the body wind up being repaired by calcification because you're letting all of these minerals into the extracellular fluid that then can mineralize and cause damage. Bisphosphonates work because they use a carbon instead of an oxygen to connect the phosphate groups. They lock onto bone the same way that pyrophosphate does, but they inhibit mineralization because they're toxic to the osteoclasts. This results in mineralization that's abnormal because normally there's asynchronous resorption and formation. This results in the minerals in the bone being put into the bone asymmetrically. So you have areas of decreased and increased mineralization. This actually contributes to bone strength. If you have hypermineralization of bone, where it's all very homogeneous, you're much more likely to have these types of atypical femur fractures that we worry about with long-term antiresorptive therapies. I'll point out just briefly that osteopontin is also a regulator of mineralization. In dental disease, which we see in some of these disorders, especially in XLH, in which dental caries and dental abnormalities are quite common, osteopontin is degraded by FEX, P-H-E-X, which is the problem in XLH, and it's why the dental disease is so prominent in these patients, because the degradation of osteopontin isn't going on, and so you get abnormal mineralization in the teeth. So the first point that I was gonna allude to, of course, is that hypophosphatasia and hypophosphatemia are not the same thing. They're terrible names because all of our trainees think that the phosphate in the names is the key problem. Hypophosphatasia is a disorder of alkaline phosphatase, whereas hypophosphatemia is a disorder of phosphorus. So the name is bad. We all know what it means, but oftentimes our trainees are confused, and so I always tell them that on your board exams, there may be a question that says hypophosphatasia involves, and the answers may be low phosphorus, high phosphorus, low alkphos, high alkphos, and something else, and I tell them if they're in my program, they had better get that question right. So that's clearly something that they all know. They're both forms of osteomalacia. So the low phosphorus level impairs production of hydroxyapatite mineralization by the osteoid in patients with low phosphorus levels. In hypophosphatasia, it's the low alkaline phosphatase levels that inhibits production of phosphorus from inorganic pyrophosphate, and inorganic pyrophosphate, that impairs mineralization. But keep in mind that as systemic disorders, there are other targets for alkaline phosphatase, such as vitamin B6. This is why pyridoxal 5-phosphate, which is vitamin B6, circulates in the blood, but cannot get into the nervous system in that form. It has to be split off by alkaline phosphatase to pyridoxal, which then enters the nervous system and reconstitutes as pyridoxal 5-phosphate, where it does good things for us, like make neurotransmitters, like dopamine and serotonin, all sorts of those good things that keep us in balance. In patients who have significant enough hypophosphatasia, this doesn't happen, and the vitamin B6 can't get into the nervous system. This can produce infantile seizures, which can be fatal in the infantile version of the disorder, and in the more mild versions, may produce neurocognitive dysfunction. So a lot of patients running around with brain fog with this disorder may be related to the vitamin B6 problem. The bone mineralization problem, of course, is different than in osteoporosis, and so anti-resorptive therapies in these disorders is not appropriate, since it may further inhibit the ability of the bone to function normally. I'm just gonna briefly tell you that this is the balance, the mineral balance, and I'm going off to the side here, because I can't actually see my slide, but inorganic pyrophosphate is transported extracellularly, and ATP in the extracellular compartment is one of the big sources of inorganic pyrophosphate. Inorganic pyrophosphate is what keeps our soft tissues from mineralizing, but it also can inhibit the mineralization and hydroxyapatite in the bone, so this balance has to be struck in a proper way. So there's a balance between alkaline phosphatase, ectonucleotide pyrophosphate, phosphodiesterase type one, and I'm only gonna say that one time, because it's a long thing to say, and phosphocholine. So this balance has to be maintained in order to keep calcium and phosphorus and the inhibitor of inorganic pyrophosphate in balance, and this is what that looks like. But it's the extracellular ATP that provides a lot of the inorganic pyrophosphate that winds up making sure our soft tissues remain balanced. In hypophosphatase, the alkaline phosphatase isn't functioning well, it's abnormal, and it's abnormal to different degrees. So it may be mildly abnormal, or it may be highly significantly abnormal. This results in the inability to cleave off the phosphate group from inorganic pyrophosphate, and therefore the hydroxyapatite is abnormal because of that, because your inability of getting phosphorus into the structure of the apatite crystals. In hypophosphatemia, the problem is a little different in that the phosphorus level's actually low, and this causes problems with your hydroxyapatite formation, and therefore the chemistry of this disorder, although quite similar in some respects, does have significant differences that makes targeting this for therapy a little bit different. So let's talk about hypophosphatasia first. This is a chronic decrease of alkaline phosphatase. So this is really easy to find, because alkaline phosphatase is part of your typical chemistry panel. So if you're looking at it, you see, gee, the alkaline phosphatase is low, and you look back and see, gee, it's always been low, this may be the disorder that you're dealing with. The patient may not have significant symptoms, so the symptom range, and we'll talk about that, can be severe and fatal, all the way to extremely mild and of very little clinical significance. So the variability of this disorder makes it very difficult to sometimes diagnose. The chronic decrease of alkaline phosphatase causes an increase of pyrophosphate inhibition and mineralization, and abnormal crystal size and distribution, so the bone is mechanically more brittle. So these patients will have various fractures. Metatarsal fractures are gonna be more common. So you may find patients with recurrent metatarsal fractures but they may also have femur fractures and fractures at other sites, although they're less common. Hypophosphatasia has a genetic evaluation that is complicated. It's complicated because there are probably over 400 different mutations, some of which are pathologic and some of which have no significance. There's probably some 200 abnormalities that actually have clinical significance. I'll remind you that there are four types of alkaline phosphatase. There's the tissue nonspecific ALKFOS, which is the problem that we're dealing with. There's also ALKFOS in the intestines. Now the intestinal alkaline phosphatase is actually in the lumen of the intestine and it's not part of the cellular metabolism. The placenta has alkaline phosphatase as well. And so in patients in whom pregnancy is occurring, alkaline phosphatase is part of that normal sequence of mineralization for the fetus. The genes for these alkaline phosphatases are in different places, but the one we worry about, the tissue nonspecific, is on chromosome one. It has 11 coating exons, two liter exons, and has binding sites for calcium, zinc, magnesium as well. So disorders, for example, of calcium deficiency, zinc, or magnesium deficiencies can also produce alkaline phosphatase abnormalities, so correcting those minerals are important in order to properly make a diagnosis. And there are promoters in these regions as well, 125 vitamin D and retinoic acid, for example. Hypophosphatasia is ubiquitous. The gene for tissue nonspecific alkphos is present in liver, bone, and kidney. As you know, the post-translational modifications make it possible to use electrophoresis to screen for bone-specific alkphos versus the other forms of alkphos. The substrates, as I pointed out, the main one that we worry about is inorganic pyrophosphate, but vitamin B6 is also one, and that can lead to neurocognitive dysfunction in patients or in seizures in children. Another substrate, phosphoethanolamine, the PEA test, which you can do in the urine, we don't really understand what that does. So those of you who are young investigators who wanna figure this out, you are certainly, this is an area ripe for research. Alkaline phosphatase is an ectoenzyme. It's linked to the membrane by a foot, glycosylphosphatidyl inositol foot, so it's locked onto the membrane. It hydrolyzes the esters, and the disorders of alkaline phosphatase may be dominant or recessive, and as I pointed out, there's many of them. There's probably well over 400 mutations. Severe mutations, though, about one in 100,000 to one in 300,000, but there are certain populations in which these genes are concentrated as abnormalities. For example, the Mennonite population in Manitoba, about one in 25 are carriers, and their newborns with severe forms of disease is about one in 2,500. So there are these clusters of patients in whom these genetic abnormalities may be much more prominent. So knowing the origins of your patient sometimes can make a difference as far as how much concern you have for the disorder. There are a lot of mild forms. It doesn't require therapy. Many of these patients, just recognizing they have a problem, they explain their muscle symptoms or their bone symptoms, and they may not need further treatments. Just explaining it to them may lead to them not seeking out further specialization and further testing that can be expensive and nonproductive. The forms of the disease range from fatal, very deadly, all the way up to very mild without much clinical symptoms. There's various forms. The perinatal form is often severe with death, and the death is resulting from the poor mineralization of the skeleton, so they have respiratory insufficiency. So when they're born, they're often ventilator-dependent, and treating them with enzyme therapies are lifesaving in these settings. So the advent of enzyme replacement therapy has dramatically decreased the mortality rates in the severe infantile and perinatal versions of the disorder. Seizures can also occur. They may have infantile seizures that can be fatal, and this is, again, because pyridoxal phosphate, vitamin B6, can't get into the nervous system, and if it's severe enough, seizure disorders result from that. The childhood disorders that you may see in the pediatric realm vary as well. They can have deformities, they can have fractures, they can have muscle symptoms. These children may be behind their peers in activities. For example, in gym classes or sports activities, they may be last, and they may be running around the track and may not be able to keep up with their peers. So these muscle symptoms may be more prominent than the bone symptoms in many of these patients. So keep in mind that if you have a patient that's having muscle symptoms, and in adults, we worry about fibromyalgia. Rheumatologists see fibro patients all the time. Few of them may have this disorder, but when you find this disorder, it clearly makes a difference because they don't have fibromyalgia. They may, in fact, have hypophosphatasia. The adult versions of the disorder are relatively more mild. Fractures is what I tend to see in my practice, so they come to me with a metatarsal fracture history, and I will find this disorder in them because of that evaluation. But keep in mind that muscle symptoms may be more prominent. These patients may have significant muscle weakness, pain, and be confused with other muscular disorders, and fibromyalgia in the rheumatology world tends to be the main one that looks like this. Neurocognitive symptoms may also occur. So patients may be depressed. They may have unusual headaches. These patients may be subtle in their presentations, and so the disorder may be difficult to diagnose when those are the major diagnostic presentations. The dental manifestations, the odonto-hypophosphatasia disorders is probably just a spectrum of the same disorder, but the dentists may be seeing these disorders because of the predominant dental symptoms. So it's a very difficult diagnosis to make in some patients because it's multisystemic. Again, the perinatal form is the more severe version. The adult forms may present with osteomalacia and fractures, which is why we see them. They may also develop chondrocalcinosis because the high levels of pyrophosphate that are circulating may then deposit in the inflammation in the joints and produce crystal disease in the joints. Premature tooth loss is often present. So before the age of five, they may have tooth loss involving the entire tooth, including the root. This is a key factor in making the diagnosis if it's complicated to make a diagnosis otherwise. So this spectrum of disorder makes it very difficult to point out that this may be the reason for the patient's symptoms. Recently, Alia Khan and her colleagues came up with this, a diagnostic criteria for HPP in adults. So low ALFAS is the starting point. So that's the beginning point for this. The ALPL gene variants, elevation of a natural substrate such as PEA or PLP or atypical femur fractures would be considered the major criteria. Minor criteria would be muscle pain, nephrocalcinosis, tooth loss, as I talked about before age five, poorly healing fractures and chondrocalcinosis. And they propose that the diagnosis requires two major criteria or one major and two minor criteria. This is a useful tool, but some patients may certainly fall through the cracks. The evaluation for hypophosphatasia is really easy. And I want to emphasize this. A low alkaline phosphatase chronically is the key test. So if your alkaline phosphatase is low, don't ignore it. You're worried about high, right? You don't want to see high, but low is not good either. If it's chronically low, this is probably the problem. The substrates also have diagnostic values. So vitamin B6 levels may be elevated. You want to make sure you're testing that off of B6 supplementation. PEA is easy to measure, but sometimes hard to order, but it's not specific for HPP. Unfortunately, inorganic pyrophosphate, which may be a valuable test, is only available in the research setting. It's not available commercially yet. Genetic testing, I would recommend it in every one of these patients. And it's not necessary oftentimes, but I think it's important in order to better characterize the disorder. Since some insurance companies in the U.S. do require genetic testing, genetic testing may be negative because the regulatory sequences and the interaction between other genes may actually be regulating the activity of alkaline phosphatase. And so having a negative genetic test does not rule it out. There also may be some variants that lack an anchor. So the alkaline phosphatase may circulate in the blood. We actually reported this, and I'll talk about that article we published later in this talk, in which the alkaphosphate may be normal in the blood, but in fact, it's not targeting the bone compartment because the anchor isn't present. So it looks like it's normal, but it may not be locking on to the osteoblasts. And so the patients may have bone disease, but normal alkaphosphate levels. This has only been reported in a couple of cases, and we were the second report. Other family members may have it. So this is genetic. So if you have a single patient that has this, you may want to look at siblings, parents, and children, because other family members may have the same disorder with different complaints. So they may have complaints that are different than your index case, but you may be able to make a diagnosis and explain symptoms that are otherwise cryptic. Low alkaline phosphatase has a differential. So hypophosphatase would be the major one for a persistently low disease. Meslene joint disease in South Africa is very rare. It's only in a certain tribal community. And cladiocranial dysplasia is also a rare diagnosis that can cause persistently low. Acutely low cardiovascular bypass surgery, myeloma, transfusions, and other things. Laboratory issues such as EDTA, citrate or oxalate in the test tube and hemolysis. And it may be temporarily low in a wide range of disorders, including the antiresorptive therapies that we use for osteoporosis. Patients with nutritional deficiencies of zinc, calcium, magnesium, and other things, because those are co-factors for alkaline phosphatase. And chemotherapy patients. So these are transiently low. So if the patient's chronically low on alkaline phosphatase, hypophosphatase would be almost certainly the diagnosis. There is treatment for it. This was cool. In the old days, they actually took serum from Paget's patients that have high levels of alkaline phosphatase and tried to infuse it into the infantile, the childhood versions to try to fix the disorder. And it did not work. It was completely ineffective. And the reason is already what I alluded to, that the alkaline phosphatase has to be in the bone compartment. If it's not locked into the bone compartment, it's circulating, but it's not effective because it's not targeted to that bone compartment. So I think that it's important to understand that a normal alkphos level doesn't entirely rule out the disorder either, which further complicates, of course, in these very rare patients that have this variant. Gene therapy may be possible in the future, but because there's so many genetic variants, this is not likely to be useful. Enzyme replacement therapy does exist now. And I think it has made a huge difference in the infantile pediatric versions in which symptoms may be significant and life-threatening, if not fatal. Enzyme replacement therapy, asphatase alpha, was first used in the life-threatening versions of HPP. It was FDA approved in the U.S. in 2015. And it is a hybrid molecule. It's a alkaline phosphatase dimer with an IgG1 fragment and a deca-aspartate in order to lock it onto the hydroxyapatite. Because if you just give it alkaline phosphatase, it doesn't go to the bone compartment. So by using this deca-aspartate group, which is highly charged, it locks itself onto the hydroxyapatite crystals and the osteoblasts, and so is effectively targeting the bone compartment. This is given as sub-Q dosing, two milligrams per kilogram three times a week, or one milligram per kilogram six times a week. Injection site reactions are the most common side effect. Occasional other things can occur, like hypersensitivity reactions and the potential of ectopic calcifications, although that is characteristic of the disorder anyway. There's a phase three study in progress with a new version of this, Lexian's version ALXN 1850, which may have fewer side effects and may be dosed less frequently. So we'll turn now to phosphorus. So phosphorus is really cool. Phosphorus was the first compound found to have luminescence, chemoluminescence. The discovery in 1669 was done by an alchemist, Hennig Brandt, who tried to create the philosopher's stone. So those of you who try to make gold from lead, I know there's a lot of gold in UAE, but you don't make it from lead. But this was always the goal of the alchemist, and so he took lots of urine, 1,500 gallons of urine, and boiled it with sand and charcoal, and this resulted in sodium phosphate, because there's a lot of phosphorus in urine. And this heating product gave carbon monoxide and phosphorus and this white vapor glowed. So everybody was really astounded by this. About 120 grams of phosphorus resulted from this, and the recipe was kept a secret, although later independent discoveries by Robert Boyle and Johann Knuckel wound up showing us how to actually make this. The glowing is resulting from the oxidation of the white phosphorus on the surface, so it's not related to radioactivity or any bioluminescence. It's related to oxidation of the phosphorus on the surface of the phosphorus molecule. So it actually oxidizes, and that oxidation produces luminescence, chemoluminescence. So hypophosphatemia, which we'll talk about now, is a phosphorus level below the age-appropriate range, which is typically less than 2.5 milligrams per deciliter. Periproteins can cause spurious disease in phosphate assay interference, so just keep in mind that phosphorus levels may be abnormal because of assay interference in some cases, such as in periproteinemia. The differential hypophosphatemia is extensive, and I'm not going to belabor all of these other potential issues. I'm gonna focus on the disorders associated with increased urinary excretion related to FGF23 abnormalities, which are XLH and TIO, tumor-induced osteomalacia, is something that you probably do see, especially if you see cancer patients. A mineralization disorder that we'll talk about, and another pediatric case that we'll talk about. But XLH, TIO is gonna be the main focus. Fanconi syndrome can cause a loss in the urine as well because of tubular dysfunction, and a variety of medications may also cause increased urinary excretion. One of the cool ones that you might not be aware of is ferric carboxymaltose. So patients with chronic anemias get iron infusions, and the particular version of ferric carboxymaltose actually increases FGF23 levels, so it kind of mimics what TIO and XLH look like. The mechanism isn't clearly understood, but the use of ferric carboxymaltose can sometimes result in profound hypophosphatemia. So patients who get their iron infusions who suddenly have significant muscle symptoms, and may have rhabdo, in fact, in rare cases, worry about that. Iron, iron product, other types of iron products don't tend to do that. But ferric carboxymaltose is one that you should be aware of. The evaluation is really simple if you get a phosphorus level. So please include a phosphorus level on all of your metabolic bone evaluations. Again, in the US, it's not part of our chem panel. I've been trying for years to try to get it put back. I'm only one person. Some of the societies have tried to also do that, but the laboratories clearly don't have an economic incentive to do this. But trying to diagnose metabolic bone disease without a phosphorus is extremely hazardous, because it results in inappropriate treatments with bone drugs that aren't needed because the patients have osteomalacia. So just in your own practices, make sure you're getting phosphorus levels. Calcium levels need to be looked at as well, clearly. And you're gonna wanna calculate this urinary phosphate excretion. So in patients who have XLH or TIO, they're leaking phosphorus out of their kidneys because of FGF23. And so a value for urine phosphate more than 100 milligrams for 24 hours, which is high, is definitely suggestive. And a urinary excretion more than 5% is considered elevated. So do a urinary phosphate excretion in order to determine the resorption of phosphorus in your patients. And that equation is easy to find on the up-to-date and other resources. XLH produces chronic hypophosphatemia. The genetics involves a gene called FEX. There are more than 200 variants of FEX and results in overproduction of FGF23, which is our favorite molecule in our osteocyte that regulates bone biology in a very profound way. And you're all quite familiar with it. How FEX actually influences FGF23 is not known. So this is another area of interest for those of you who are young investigators to try to figure out how FEX actually affects FGF23. FEX is an endopeptidase. As I pointed out before, it also degrades osteopontin in the teeth. So when FEX abnormalities occur, osteopontin levels go up, and dental caries and dental disease is much more rampant in XLH patients because of this. X-linked, it's X-linked dominant, but 20% may be spontaneous. So you may have a family history, but 20% of the disorders is spontaneous. Since it's X-linked, that means that if only there's an affected father, all the daughters will be affected, but not the sons. And if the mother is affected, then each child, regardless of sex, would have a 50% chance of inheritance. And you want to do genetic counseling in these patients so that you can anticipate problems. The prevalence is about one in 20,000. And what you're probably more familiar with in the adult endocrinology world are these perineoplastic syndromes involving tumor-induced osteomalacia in which these mostly benign mesenchymal tumors produce excess amounts of FGF23. This may be a more acute presentation, so you're not gonna find the patients have had a long history of this in TIO. But keep in mind that they may have a history that may take seven years to make this diagnosis in some instances. And even though most of these tumors are benign and small, some of them are malignant. There are a few of them that, in fact, are associated with perineoplastic syndromes associated with malignancy. So it's not always the case in which it's benign. Hypophosphatemia is resulting from excess FGF23 or tumor production, and this results in low phosphorus levels because of decreased 125 vitamin D and decreased renal phosphate transporter function. I won't go through this cartoon again, only to point out that because of the hypophosphatemia, your bone is not capable of mineralization. Because of time constraints, I'm gonna move a little quickly. XLH results in osteomalacia fractures and pseudofractures. TIO is something you need to be aware of because it is more recent onset. It can occur at any age. These tumors are generally small and benign, but may be malignant in rare cases. And medical management, either conventional management with calcitriol and calcium or borosimab. Low phosphorus is the key test. FGF23 laboratory testing is readily available now in most instances, so you want to be able to check that. And you're looking for a decrease in your tubular max of phosphorus. Again, management is either conventional with calcitriol and potassium phosphorus, or a blocking antibody, borosimab, that can be used for XLH and TIO therapy. It's approved for use in the US. In a nutshell, HPP and XLH result in low ALKFOS levels in hypophosphatasia, low phosphorus levels in hypophosphatemia, and since I already went through this in our discussion earlier, I'm not gonna belabor it here. I'm going to very quickly go through a disorder called pyrophosphate deficiency. So this causes generalized arterial calcification. This is fatal in many instances. This is a pyrophosphate deficiency, so your tissues are calcifying abnormally, and I'm not gonna spend time on this except to point out that it exists. Enzyme replacement therapy is now available for this disorder as well, and there are human trials going on. I'm gonna skip through this, except to say that in order to differentiate these disorders, alkaline phosphatase is a key test you're gonna want to look at, and the phosphorus, and that'll help distinguish these various disorders. So again, to repeat, osteomalacia is not osteoporosis. These disorders are rare, but you need to look at them. They're multisystemic and familial, and you want to make sure you're looking at these tests carefully. We reported on two patients recently. This is from our center, hypophosphatasia with normal ALKFOS because it's not targeting the bone compartment, and so I think having a normal ALKFOS doesn't rule out the disorder, and we also have a patient with Ehlers-Danlos syndrome and HPP. And just very briefly, secondary hypertrophic osteoarthropathy is also a disorder that's poorly understood. This is called Bamberger-Marie disease, and in dogs, my wife's a veterinarian, so she sees this sometimes, in dogs with hypertrophic osteoarthropathy, but we went to Ashfall Fossil Bed in Nebraska, and about 12 million years ago, the Yellowstone volcano blew up, and the ash wound up killing off a huge number of animals in the area of Nebraska, and these fossils contain hypertrophic disease in the bones resulting from the inhalation of ash, and so this is really cool because this is a disorder that we know about in modern days, but probably was present 12 million years ago relating to this pulmonary finding in kids, so just in these fossils. So just understanding mineralization problems may help us target disorders of other tissues as well. There are important implications for cardiovascular disease and aging. Genetic panels are always improving. I would keep looking for these disorders, and lastly, I want to thank you for the invitation, and my cat did help me with the presentation, but wasn't all that useful, but thank you. Thank you.

hypophosphatasia

X-linked hypophosphatemia

bone diseases

mineralization

enzyme replacement therapy

FGF23

phosphorus levels

cardiovascular disease