Sickle Cell Disease (SCD): Practice Essentials, Background, Genetics

Is Long Life Without Disease Possible? A Rare Genetic Syndrome May Point The Way

Ecuadorean doctor Jaime Guevara Aguirre, examines patients Maritza Valarezo (L) and her sister ... [+] Lugartda, both with Laron syndrome, during a consultation in Quito, on January 11, 2014. Physician and diabetes expert Jaime Guevara Aguirre, studies Laron syndrome, or Laron-type dwarfism, a disorder caused by a variant of the growth hormone receptor. The genetic mutation causes short stature and also prevents cancer and diabetes. AFP PHOTO / RODRIGO BUENDIA

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We hear the words "genetic mutation" and tense up: if it's a mutation, it has to be a bad thing, right? Not always. In some cases, a mutation can offer protective or beneficial effects. These types of gene variants are a hotspot of research since they carry the promise of new treatments — if we understand how the mutation works, we may be able to mimic its protective effects artificially.

Laron syndrome, known technically as growth hormone receptor deficiency (GHRD), falls into this category. Individuals with the syndrome are much less likely to suffer from age-related diseases like cancer and diabetes. A new study suggests they may also be more resilient against heart attacks and other cardiovascular issues.

What is Laron Syndrome?

Laron syndrome is an extremely rare disorder —there are less than 500 confirmed cases in the world— caused by mutations to the growth hormone receptor gene (GHR). As with any gene, the strand of DNA that comprises the growth hormone receptor is made up of thousands of "base pairs" — the chemicals that form the foundation of DNA. Just a single change to any of these base pairs may be enough to cause a difference in the function of the protein that the gene encodes.

These changes to the growth hormone receptor gene interfere with the production of important proteins involved in childhood growth. As a result, people with the syndrome are almost never taller than four and a half feet (1.5 meters). They are also prone to developing obesity and tend to have higher levels of low-density lipoprotein (LDL), or "bad cholesterol."

Despite this, people with Laron syndrome are long-lived compared to unaffected relatives. Back in 2011, a group of researchers suggested that this may be due to almost nonexistent levels of cancer and type two diabetes, lucky protective byproducts of the genetic mutation that causes the syndrome. Individuals with Laron syndrome tend to produce less of a growth hormone called insulin-like growth factor 1 (IGF-1), which although crucial for growth during childhood, has also been linked to the kind of haywire cellular proliferation that leads to cancerous tumors.

The same longevity boost has been observed in mice with Laron syndrome, who, compared to their peers, tend to live 40% longer. They also develop fewer tumors and exhibit the same smaller stature.

Another potential explanation for the increased lifespan of those with Laron syndrome is the fact that their cells are significantly more likely to self-destruct after suffering damage than those of unaffected individuals. This prevents the cells from accruing mutations or DNA damage over repeated generations, which is considered one of the hallmarks —and potential causes— of aging.

The same group of researchers, led by Dr. Jaime Guevara-Aguirre at the San Francisco de Quito University and Dr. Valter D. Longo at the University of Southern California, followed up these initial findings with a second study in 2017. This time, their research indicated that the protective effects of the syndrome were not restricted to the body alone: cognitive performance also remained high with age, and there were barely any cases of dementia. All in all, the brain function of older adults with Laron syndrome was closer to that of younger adults in the general population.

What About Heart Health?

But one big question mark remained. Many researchers speculated that since those with the syndrome were more likely to develop obesity, they would also be more likely to develop heart issues. These issues could possibly outweigh the protective factors. To answer this question, Dr. Guevara-Aguirre and colleagues returned to the Ecuadorian families they had worked with in the past. They recruited 24 individuals with the syndrome and compared them to their unaffected relatives.

The results suggest that those with the growth-factor deficiency are no likelier to suffer from heart issues than their unaffected counterparts. If anything, the syndrome seems to be slightly protective against cardiovascular diseases. Affected individuals had lower blood pressure and glucose levels. They also had fewer issues with atherosclerosis, which is when plaque builds up in the arteries and begins to restrict blood flow. If left untreated, which is not uncommon since it is hard to notice, the plaque buildup can lead to heart attacks and strokes.

Same But Different: Not All Individuals With Laron Syndrome Share Protective Benefits

Something worth noting is that not all individuals with Laron syndrome enjoy the same protective effects. Zvi Laron, professor emeritus at Tel Aviv University, was the first to recognize and define the syndrome in 1966 — that is why it carries his last name. But in the population that he studied, made up of consanguineous Jewish families from Yemen, a portion of the patients did develop an insulin intolerance and diabetes. Also, only a handful of individuals displayed the usual resilience against cancer seen in the Ecuadorian families with Laron syndrome.

How should we make sense of these discrepancies? Laron syndrome is caused by mutations to the growth hormone receptor gene, but these mutations can take many different forms. More or less of the gene may be affected, and in different areas. So even though all of the individuals in question suffer from the same syndrome, it may be brought on by subtly different mutations. Indeed, to date we know of 17 different genetic mutations that cause the disease. Based on the research of Zvi Laron, it seems that the protective benefits against cancer are only present when both parents share the same mutation in the same place and pass it on to their child, known as homozygosity.

The Ecuadorian community, whose roots can be traced back to Sephardic Jews who fled Spain during the Inquisition, all share the same mutation. It may just be that this particular mutation is the one that happens to grant longevity benefits. In which case, focusing on this community may prove most useful when it comes to the development of treatments that mimic the life-lengthening and disease-busting qualities of the syndrome.

Implications

Although Laron syndrome comes with clear challenges, it also seems to provide certain benefits. Those with the disorder, at least the Ecuadorian contingent, live longer and struggle with fewer age-related diseases than their unaffected counterparts. Most of these benefits can be traced back to lowered levels of insulin-like growth factor 1.

By studying the syndrome and learning more about its effects at the molecular level, we may discover ways of transferring the protective effects to the general population. The point here is not necessarily to grant longevity so much as to increase "healthspan," the number of years of your life you spend in good health. Indeed, Dr. Valter D. Longo —the senior author of the study— has experimented with different fasting-like diets that can reduce the levels of insulin-like growth factor 1 circulating in the body and along with it, various risk factors for disease. There are also molecules that block the protein, slowing down cancerous growths in the process. By fine-tuning these approaches, the protective effects of the syndrome might soon be available to everyone.

Tracking Down The Genetic Causes Of Lupus To Personalize Treatment

Lupus is a lifelong, often painful and occasionally lethal autoimmune disease. Few treatments exist today beyond powerful steroids to knock down a patient's immune system -- a therapy that has its own serious risks.

The good news is that new and promising treatments are in clinical trials. But the term lupus belies the fact that the disease has a variety of causes, which means that treatments will have to be highly personalized to guarantee that each patient is given the drug that targets the specific genetic mutation responsible for their variety of lupus.

Researchers are just now beginning to link specific genetic mutations to subsets of lupus patients, allowing physicians to target therapies to those who will benefit most. In the latest advance, researchers at the University of California, Berkeley, report in a new paper the discovery of two sets of patients with genetic mutations that are nearly identical to mutations that the researchers had earlier pinpointed in mouse and cell lines as linked to autoimmune disease.

These two genetic links are among several dozen mutations that the UC Berkeley team recently discovered and linked to lupus, all in one gene that regulates a prime suspect in a subset of lupus patients -- proteins called toll-like receptors (TLR), which enable immune cells to recognize foreign DNA and RNA.

According to study leader Gregory Barton, UC Berkeley professor of molecular and cell biology, identifying these mutations could help doctors deliver a personalized treatment to patients with oversensitive TLRs and, in particular, oversensitive TLR7 receptors.

"We basically have a map now," said Barton, who is also an investigator in the Howard Hughes Medical Institute. "It's not like everybody that has lupus has a mutation in the gene that causes overactivation of TLRs and TLR7. But there are drugs coming online that very specifically inhibit TLR7. As we sequence more and more people, it will become easier to identify those patients and put them on those drugs. That's a lot better than the current course of therapy for lupus, which is brutal."

"This is exciting because the drug will be orally available and is in clinical trials now," said Victoria Rael, a UC Berkeley graduate student who, with fellow graduate student Julian Yano, is a co-first author of the paper.

The results of the genetic screens and details of the patients' mutations were published today (May 23) in the Journal of Experimental Medicine.

A problem recognizing 'self'

Autoimmune diseases, which range from rheumatoid arthritis and Crohn's disease to scleroderma and numerous thyroid conditions, stem from attacks by the immune system on the body's own cells that destroy normal, healthy tissue.

Many studies have linked at least two types of autoimmune disease, lupus and psoriasis, to TLRs, which are part of the innate immune system that initially detects foreign invaders, such as viruses and bacteria, and stimulates a first line of attack. Normally, TLRs are delicately tuned to react only to foreign DNA and RNA, but if that tuning is off, they can react to a body's own nucleic acids and proteins associated with nucleic acids, which look much like those of pathogens.

What makes this autoimmune reaction so deadly is that the TLRs also activate the body's second-line defense, the more powerful adaptive immune response, mobilizing T and B cells, macrophages, and other cells. These cells then mount a sustained attack that destroys the body's healthy tissue and causes chronic inflammation.

The most common form, systemic lupus erythematosus (SLE), for example, is characterized initially by skin rashes -- in particular, a butterfly-shaped rash on the face -- but later by damage to joints, muscles, organs and skin, causing pain and fatigue. It's most commonly seen in females, often starting during the teen years. Lupus, in general, is two to three times more prevalent among women from many ethnic and racial minority groups than among white women.

"We think the way the system works is that if nucleic acids find these receptors, most likely they're going to be from a virus," Barton said. "But in some people, the receptor is more responsive, so now levels of self-nucleic acids that otherwise wouldn't stimulate the receptor in a normal person activate the receptor. We think that one of the ways that these mutations are working is that they're making levels of self-nucleic acids that normally wouldn't be stimulatory, stimulatory."

Barton and his lab colleagues have been investigating the role of TLRs that are misregulated in lupus, and in particular, one of the main proteins that regulates them: UNC93B1, or UNC for short. Several years ago, a team of postdoctoral fellows and graduate students in his lab screened in cell culture more than 100 genetic mutations in the UNC gene to see which ones overstimulated TLRs and would be good targets for further study. While they published some details in earlier papers, they didn't publish the complete list because there seemed to be little point -- almost no data was available on the genome sequences of lupus patients to compare with the mutations that overstimulated TLRs.

But that has changed in recent years, thanks to a plunge in the cost of genome sequencing. That's how the mother of a young girl with severe autoimmune disease found Barton. Her daughter's DNA had been sequenced and showed a mutation in a region of UNC that Barton's team had noted in an earlier paper.

Lupus in the family

Rael and undergraduate Madeleine Weiss used the same cell culture screening technique to test the novel mutation from the young girl and found that it had an overstimulating effect, similar to the effect of other mutations in that area of the UNC gene. Surprisingly, the patient had the genetic mutation on only one of the two UNC alleles, meaning that she had one normal UNC gene, yet she still suffered severe autoimmune symptoms.

Barton and his team also connected with a family of five afflicted with lupus. All had mutations on one UNC allele in another area of the UNC protein that Barton's team had previously identified. That mutation, when screened in cell lines, also produced overactive TLRs.

"We were skeptical that just one copy of a gene would be sufficient to cause a disease," Rael said. "It wasn't until we put the patients' mutations into cell lines and saw that they led to very convincing TLR hyper-responsiveness that we realized they had the possibility of being sufficient to be disease-causing."

Rael and Yano then repeated the screening work previously performed in the lab and confirmed that 32 distinct mutations in the UNC gene -- about one-third of the mutations tested -- increased the sensitivity of TLR7 to nucleic acids at least twofold. About another five mutants increased TLR7 sensitivity but to a lesser degree. Before these screens, only two mutations in the UNC protein had been linked to increased sensitivity of TLR7 in mice, though three additional human mutations were reported within the last two months.

Barton is hopeful that by publishing the complete list of TLR hypersensitivity mutations, doctors can identify other lupus patients who could benefit from the anti-TLR drugs now in clinical trials. One drug, M5049, or Enpatoran, appears to work by latching onto two human receptors, TLR7 and TLR8, and preventing them from binding nucleic acids.

Rael, Yano and other members of Barton's lab are investigating further how these unique UNC mutations affect the way a patient manifests the disease. They have recreated these patients' mutations in mice so that they can model human lupus.

"With mouse models, you can start thinking about how, even though the mutations are in the same protein, the different mechanisms of TLR regulation break down, which immune cells get activated as a result, and how this can lead to differences in the symptoms patients suffer from," Rael said.

The lab also is trying to understand how UNC tunes TLRs, which may be by regulating the number and arrangement of TLRs on immune cells. More TLRs may make a person more sensitive to the small number of self-nucleic acids circulating in the body.

"UNC93B1 is important for getting the receptors to the place where they can function, but it also is important for regulating them when they get there," Barton said. "The protein is a very baroque way of trying to make decisions about whether the nucleic acid that you just bound to a TLR is from a virus or from one of your own cells."

He hopes that physicians add this gene to the list of lupus-associated genes, "so if they see a mutation like these, even a heterozygous mutation, they will investigate further."

Other senior authors of the paper are Bo Liu of the Chinese Academy of Sciences in Shanghai and Olivia Majer of the Max Planck Institute for Infection Biology in Berlin, Germany. The co-authors also include physicians from UC San Francisco, Stanford University, and hospitals in Missouri, North Carolina and Washington.

The work was funded in part by the Lupus Research Institute, the Lupus Research Alliance and the National Institutes of Health (R01AI072429).

Genetic Mutations Leading To Truncated Proteins In Schaaf-Yang Syndrome

image: 

From left to right, Mónica Centeno, Aina Prat, Juan Diego Gutiérrez, Susanna Balcells and Raquel Rabionet, members of the UB, the BUB, the IRSJD and the CIBERER.

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Credit: UNIVERSITY OF BARCELONA

Mutations in the MAGEL2 gene, which cause Schaaf-Yan syndrome (SYS) —an ultra-rare disease that affects neuronal and cognitive development— generate truncated, non-functional proteins that tend to accumulate in the cell nucleus. Moreover, this progressive accumulation of abnormal proteins could cause a toxic effect in patients affected by the syndrome, who suffer congenital malformations, intellectual disability, alterations in facial features, sleep apnoea and joint contractures.

These advances in SYS research appear in a study published in the Journal of Medical Genetics. The study was led by a team from the Faculty of Biology and the Institute of Biomedicine of the University of Barcelona (IBUB), the Sant Joan de Déu Research Institute (IRSJD) and the Rare Diseases Networking Biomedical Research Centre (CIBERER). This team is also the author of the publication of the first clinical guide on Schaaf-Yang syndrome (Journal of Medical Genetics, 2022), aimed at healthcare professionals and families of children affected by this pathology.

A better understanding of the function, genetic variants and impact of nuclear retention of the MAGEL2 protein will open new ways to design patient-specific gene therapies to prevent the synthesis of the altered protein and address SYS, a disease without treatment.

Genetic mutations leading to truncated proteins

The MAGEL2 gene is located on chromosome 15, is expressed in the nervous system and produces the MAGEL2 protein, which is involved in the retrograde transport and recycling of proteins in the cell cytoplasm of neurons. To date, more than eighty mutations in the MAGEL2 gene have been documented in the scientific literature, some of which are found repeated among patients. Currently, it is estimated that there are about 250 people diagnosed with Schaaf-Yang syndrome worldwide.

The new study, conducted with human cells in vitro, shows how almost all truncated proteins associated with Schaaf-Yang syndrome lose part of their molecular structure due to genetic mutations. "Functional MAGEL2 proteins have a complete molecular structure that allows them to interact with other proteins and carry out their normal biological functions. They are usually found in specific locations within the cytoplasm of the cell, mainly in subcellular compartments related to the transport and recycling of proteins", explains Susanna Balcells, professor at the UB's Department of Genetics, Microbiology and Statistics, and coordinator of the study. "In contrast — she continues — truncated proteins are shorter versions of the MAGEL2 protein, as they have been affected by genetic mutations. Therefore, truncated proteins lack certain regions necessary to function correctly in the cell".

Due to genetic mutations, truncated proteins lose key structural domains, such as the MAGE homology domain, which is crucial for interactions with other proteins. "The absence of this domain could prevent these essential interactions for the correct functioning of MAGEL2, such as its role in retrograde transport and protein recycling", explains Roser Urreizti.

When toxic proteins accumulate in the cell nucleus

Truncated proteins tend to accumulate inside the cell nucleus, and this could further aggravate the symptomatology of people affected by Schaaf-Yang. Mónica Centeno, "it is likely that, in a real cellular context, some of the truncated proteins synthesised can also be found in specific locations within the cytoplasm, such as endosomes, for example. However, as their structure is altered, they might not be able to perform their normal functions correctly".

The severity of clinical manifestations in those affected by Schaaf-Yang syndrome may be related to the accumulation of altered proteins in the cell nucleus. "In other words, the mutations that cause more severe symptoms also cause the truncated MAGEL2 protein to accumulate more in the nucleus. This could be explained by the fact that a higher accumulation of truncated proteins in the nucleus could be interfering with important nuclear processes and affect the functioning of the cell under normal conditions to a greater degree," says Raquel Rabionet.

Searching for treatments to address Schaaf-Yang syndrome

Cellular mechanisms that drive protein degradation — for example, the ubiquitin-proteasome system — could help reduce the negative effects of truncated proteins and contribute to mitigating the progression of the pathology. However, if the rate of protein production exceeds the degradation capacity of the cell, aberrant proteins may escape degradation and continue to exert toxic effects. The expert Aina Prat says that "in this regard, we have found that normal and truncated MAGEL2 proteins have very similar half-lives. Therefore, truncated MAGEL2 would be stable in the cell, where it could be exerting toxic effects".

"If we knew how truncated MAGEL2 proteins alter cell function, we could develop strategies to promote the degradation of these toxic proteins, restore cell function or compensate for the metabolic and signalling dysfunctions caused by their accumulation", concludes the team, which will drive further research to contribute to the development of innovative treatments for people affected by SYS.

Journal

Journal of Medical Genetics

Method of Research

Experimental study

Subject of Research

Cells

Article Title

Subcellular localisation of truncated MAGEL2 proteins: insight into the molecular pathology of Schaaf-Yang syndrome

Article Publication Date

28-Mar-2024

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