Around 70% of all rare diseases arise during childhood, in part because many rare diseases are also genetic (and are therefore present from birth), for example, conditions such as Batten disease and spinal muscular atrophy.
One childhood disease where significant progress has been made is Hunter syndrome – which affects the body’s ability to break down sugars, leading to impairments, such as delayed growth and stiff joins, and a life expectancy of 10 to 20 years. A stem-cell based therapy is to be trialled in young children, based on research at the EARA member the University of Manchester, UK. Earlier studies in mice had shown that the therapy could successfully correct the condition and some of its effects on the brain, and the trial has the potential to dramatically improve treatments for children over their lifetime. It is hoped the new therapy will remove the need for weekly enzyme replacement therapy, as it is based on inserting a gene that will allow the body to create a working lysosome enzyme produced by the body’s blood cells.
Mice have also played a long-term role in the development of therapies for congenital athymia, where children are born without a thymus gland and are therefore susceptible to seemingly harmless infections. After three decades of research, a therapy was used to treat patients so that they lived well past the average life expectancy of two to three years, and was approved as a treatment by the US Food and Drug Administration in 2021.

Perhaps surprisingly, rare cancers make up a significant number (20%) of all cancers and include sarcomas (that begin in the bones and tissue), oesophageal (throat) cancer, hepatocarcinoma (liver) and certain types of leukaemia, which affect the blood and bone marrow.

Chronic myeloid leukaemia (CML) is one such rare cancer that develops slowly over a person’s lifetime, but thanks to medical advances can now be kept under control with the right treatments. Studies in mice were an instrumental part in finding the protein abnormality that causes CML. The precision medicine Gleevec was developed, by Oregon Health & Science University, USA, to just target this protein abnormality in cancer cells in mice (not targeting all cells, like most chemotherapy drugs), and was later used to reduce cancer growth in CML patients. Other species, including dogs, rabbits and monkeys, were also involved in the development of Gleevec (and its rapid approval by the US Food and Drug Administration), by shedding light on how the drug behaved in the body and potential toxicities. 
​There are several different types of rare cancer that start in childhood, including neuroblastoma that mainly affects children under five-years-old and starts in nerve cells called a neuroblast, often in the adrenal glands located on the top of each kidney. Several studies in mice have contributed to improving survival rates and developing treatments with less severe side effects, for example a team at the University of Gothenburg, Sweden, were able to find an effective drug that worked against one of the two gene mutations that cause neuroblastoma.  
Mice were also used in a study of the rare eye cancer retinoblastoma (which mainly affects young children) at the St Jude’s Research Hospital, USA, to confirm that the cancer’s cause was not a series of mutations, like most cancers, but the loss of a single gene – explaining why it develops so quickly.

Genetic diseases, such those that are more generally familiar – cystic fibrosis (affecting the lungs), haemophilia (blood) and muscular dystrophy – arise either because of a faulty gene or genes, DNA damage, or a combination of genetic mutations and environmental factors. Sometimes, the mutations can develop throughout a lifetime, while in other cases they are already inherited. An example of the latter is sickle cell anaemia (or sickle cell disease), which affects the red blood cells and blocks blood flow due to the presence of a faulty gene. It is a lifelong condition that can only be cured with a bone marrow transplant, but requires a genetically similar donor. Researchers have therefore investigated alternative approaches, including the use of CRISPR gene editing to modify the red blood cells of people with the condition – which was safety tested in mice before it could progress to human trials (CRISPR itself was developed using animal research).

As well as using genetically altered mice, other treatments for sickle cell disease hold great potential such as producing cells of interest from human stem cells, which have the ability to develop into any type of cell, and then transplanting these into animals to grow and develop. 

A variation on this approach has been used at the University of Alabama at Birmingham, USA, which also previously developed the first mice that could be used to study sickle cell anaemia. By making mouse skin cells develop into stem cells, the researchers successfully made these mice models produce healthy, functioning blood cells and the diseased animals showed no symptoms.

The blood condition haemophilia, also known as the ‘Royal Disease’ because it ran in the family of certain royal lines, is where the ability of the blood to clot is severely hampered, leading to excessive bleeding. It is now known to be caused by a change in one gene in the X chromosome, that is inherited and passed down through generations. Studies in dogs that naturally had a type called haemophilia B led to the development of a gene therapy that later went to human trials, from research at Auburn University, USA, while a factor from the blood of pigs has historically been used as a treatment for specific types of haemophilia (such as haemophilia A), to successfully stop bleeding episodes in severe cases.

Creutzfeldt-Jakob Disease (CJD) which affects behaviour, memory, co-ordination and vision, can see life-threatening complications develop within a year of people showing symptoms. The disease has been investigated experimentally in monkeys and mice (such as in a 2008 study by EARA member the Mario Negri Institute, Italy), by giving the animals the human prion protein – the abnormal version of this is thought to cause CJD, or the same prion gene defects. The condition can be either genetic or environmental. 

Many people with another genetic brain disease, Rett syndrome, which causes mental and physical disability, can live to adulthood, but certain complications such as pneumonia and problems with heart rhythms, can mean some die during childhood. Mice have been important for testing gene therapies for Rett syndrome, particularly by allowing researchers to study it in females, since it affects females more than males. 

For some neurological diseases, the fruit fly has proved to be a valuable species, playing a considerable role in not only our understanding of how these diseases work, but also in the discovery of rare diseases. It was thanks to investigating a specific gene mutation generated in fruit flies that researchers at Baylor College of Medicine, USA, first identified Mitchell syndrome(previously a mystery) to understand how it caused progressive loss of various body functions, including movement and hearing, in people with the mutation.

In turn, through investigating potential treatments for the syndrome, again using fruit flies, members of the same team were then also able to apply their insights to other neurological diseases such as multiple sclerosis (which is more common, but includes the common feature of brain inflammation).

Meanwhile, in a study led by EARA member Uppsala University, Sweden, researchers concluded that routinely used blood-thinning drugs could be a hugely beneficial option for people with cerebral cavernous malformations (abnormal groups of blood vessels in the brain that can lead to bleeding, resulting in seizures), by studying mice with mutations that reflected the human condition, as well as human tissues. 

As the most used species of animal in biomedical research, it is no surprise that mice have been used to study many different rare diseases. Some types of leukaemia, which affect the blood-forming tissues, such as the bone marrow, are rare, including acute myeloid leukaemia(accounting for 1% of all cancers) and prolymphocytic leukaemia (less than 1% of chronic leukaemias, with a life expectancy of less than two years for certain types). For these, humanised mice have been used to explore bone marrow transplants as a treatment, as well as develop immunotherapies that work by getting the body’s own immune system to kill cancer cells.

The fact that mice can be easily genetically manipulated to carry specific genes, or have their genes edited, has paved the way for treating various diseases, including familial hemophagocytic lymphohistiocytosis (FHL), a disease of the immune system commonly affecting young children.

Research led by EARA member the Max Delbrück Center (MDC), Germany, used CRISPR gene editing to repair the defective immune cells in mice as well as in blood samples taken from two babies with FHL, to dampen the immune response and return it to normal.

Meanwhile, scientists at ICVS, University of Minho, Portugal, studied Machado-Joseph disease (MJD) – also called spinocerebellar ataxia type 3 – that affects muscle control and in the severest cases limits life expectancy to around 35 years. The team genetically engineered mice to express a protein that is responsible for the rare genetic disorder, and by using these mice to test possible treatments, found that an existing drug for depression could reduce the severity and slow the progression of MJD. 

"The animal models provide essential information regarding nervous system function that is not possible to obtain in cell culture systems and allow us to evaluate the effect of treatments on the organism as whole, namely on the movement problems associated with the disease," said​ Dr Patrícia Maciel, ICVS Minho, Portugal.

Mice can also be engineered to study cystic fibrosis (CF), a condition that results in thick mucus building up in the lungs and digestive system. The mice used have the CTFR gene that is responsible for the condition and this allows researchers to investigate the faulty mechanisms that cause the mucus to thicken abnormally, as well as ways to prevent common features such as lung damage. 

In a study at Yale School of Medicine, USA, researchers were also able to correct some of the mutations in CTFR in multiple tissues in mice, using gene editing, and found that this treatment led to CTFR partially gaining back its normal function. This has the potential to treat not only CF, by addressing its numerous gene mutations, but other genetic conditions. 

Humanised mice can shed light on the gene defects that affect the health and function of the muscles, such as muscular dystrophy and spinal muscular atrophy (they can display similar symptoms to the human condition). Researchers at Harvard University, USA, explored a different approach by successfully transplanting human muscle stem cells into mice with muscular dystrophy, to improve their muscle function and help repair future muscle injuries. 

While the use of mice for studying rare diseases is generally the most widespread, zebrafish are increasingly being employed in these studies too, as they can be maintained in very large numbers in the lab, can mature faster than rodents, and, like mice, can be easily genetically manipulated. 

Zebrafish have therefore contributed to the study of various genetic diseases, including Batten disease, an incurable disease that results in several symptoms, such as vision loss, seizures and problems with cognition. By using tools such as CRISPR gene editing, researchers can initiate in zebrafish the same gene mutations as people with the condition (mostly children), thereby replicating key aspects and testing the effect of possible drugs.

EARA member the Luxembourg Centre for Systems Biomedicine (LCSB), at the University of Luxembourg, was able to identify three possible early markers of Batten disease, using zebrafish larvae which had advantages over using mice or other mammals, which do not develop the disease until adulthood. 

"Our new model may also facilitate drug screening to identify compounds that could slow or halt disease progression, a breakthrough that the 14,000 patients living with this still uncurable disease are eagerly waiting for," said Dr Carole Linster, LCSB, Luxembourg.

Meanwhile in work at Sidra Hospital, in Qatar, researchers are using genetically modified zebrafish to gain key insights into the biology and symptoms of a range of rare childhood diseases (including inherited blood disorders such as sickle cell anaemia), in turn paving the way for precision medicines. 

Another benefit of zebrafish larvae is their transparent bodies which makes them particularly suited to research into rare skeletal disorders, such as idiopathic scoliosis (abnormal spine curvature arising after birth). Their transparency means that genes linked to the development of such conditions can be easily seen under a microscope, as well as the bones and skeleton itself, to understand the fundamentals of bone formation and where it can go wrong. Adult fish can also develop similar bone complications to people (they have shown osteoporosis-like features, for example), but are also able to repair and renew their skeletal tissue, which can offer insights into possible treatment strategies.

For cancer, the benefit of genetic manipulation has led to zebrafish that have specific gene changes linked to the disease. For example, in a study at the University of Birmingham and University of Warwick, both UK, zebrafish were edited to have a so-called GATA2 deficiency – a hallmark of a rare bone marrow disorder that is tied to the development of blood cancer – to shed light on how this deficiency increases the risk of the cancer. 

Although we may seem very different to fruit flies, they in fact share three-quarters of the genes that cause human diseases (and 60% of genes in general). Fruit flies are therefore very useful for genetic studies, particularly when they have the same genetic mutations as humans, or are engineered to express genes of interest to understand how it causes or increases the risk of the disease developing, or is responsible for the symptoms seen in people.

"Lots of neurological phenotypes in humans can be assessed in fruit flies, including seizure, motor skills, aging and neuroinflammation," said Dr Hyunglok Chung, Houston Methodist Hospital, Texas, USA.

The causes of the rare neurodevelopmental disorders, Harel-Yoon syndrome and Yoon-Bellen syndrome, were both identified as due to specific genetic mutations thanks to studying fruit flies, by researchers at Baylor College of Medicine, USA. Although this gene mutation had been linked to neurological symptoms in humans, it was only by determining the effect in fruit flies that researchers could confirm the cause – paving the way to targeted treatments. 

Meanwhile, in a fruit fly study at the University of Sydney, Australia, researchers were able to pinpoint how a mutation underpinned a key mechanism behind episodic ataxia, a condition that severely affects balance and co-ordination. 

Fruit flies reach adulthood in just two weeks, so using them in research opens doors to exploring the long-term effects of a disease and how it progresses over a lifetime, which is more difficult to achieve in larger animals and humans. For example, research at the Institute for Research in Biomedicine and SJD Paediatric Cancer Center Barcelona, both Spain, engineered fruit flies to express a variant of a cancer gene that causes a rare bone cancer called Ewing sarcoma, which has a survival rate of only just over 50% in teenagers, and less than 30% after the cancer has spread to other parts of the body. This resulted in the same tumour development as seen in humans, and was the first time the disease was modelled in an animal, after failed attempts to do so in mice. 

Like us, dogs are also naturally prone to rare diseases. Some of these canine conditions have many parallels with human ones, and so research in dogs has helped to inform and improve both human and veterinary medicine, and holds the potential to do so for even more diseases in the future. Cancer-driving gene mutations, the biological make-up of tumours, and the spread of cancer (metastasis) all share similarities between dogs and humans. 

A rare disease that dogs can get is Batten disease – called neuronal ceroid lipofuscinoses in the canine version. Here, studies in dogs by a US team led by the University of Pennsylvania, have led to new findings about how best to deliver gene therapy to slow the development of the disease in children and in dogs. Studies at the University of Missouri have also looked at how to preserve the function of the retina – a common hallmark of Batten disease is vision loss. 

It was from a study at EARA member the University of Helsinki, Finland, that researchers found that the canine version of hypophosphatasia (impaired mineralisation of the bones and teeth) closely resembles the disease in human babies, and also marked the first report of this condition naturally occurring in dogs. The findings then allowed the team to develop a gene test for the specific dog breed they investigated, to identify the defect before birth. 

"Although hypophosphatasia has been extensively studied in humans, the results of the canine study are significant, as they provide the first spontaneous animal model for the disease, which may also open new avenues for the development of novel therapies," said Prof Hannes Lohi, University of Helsinki, Finland.

Dogs can also suffer from the blood disorder haemophilia. Another condition that is shared with dogs includes muscular dystrophy (MD), because the gene mutation in human MD matches that in golden retrievers with MD. In an unexpected development of studying a version called Duchenne MD in this breed, researchers at the University of São Paulo, Brazil, discovered a new gene mutation that can protect against muscle degeneration and weakness from the disease, opening new avenues to treatment.

As a larger research animal, pigs are emerging as a suitable choice for investigating certain diseases, where their similar make-up to us (especially in terms of anatomy, genetics and body processes) make them more suitable to use than a rodent. 

While pigs do not naturally get cystic fibrosis (CF) they can be bred to develop very similar symptoms to the condition, such as lung disease which can naturally develop as a result – the lungs of pigs have been shown to be affected in a very similar way to those of humans. This means that interventions can be tested at an earlier stage than in a patient, in turn allowing researchers to come up with measures that could help to ease symptoms or detect the condition earlier on.

Pigs have also been genetically engineered to develop a disease that mimics CF in newborns, in a study at the University of Missouri, USA – although CF is present from birth, it is not always easy to diagnose it early in life, meaning that treatment does not start at the optimal time.

Meanwhile, the same similarities in body make-up to humans have also meant that pigs have been used to model Huntington’s disease, a rare disorder where the brain’s nerve cells are damaged, and that can affect movement, behaviour and communication, among other effects. 

In a study from Emory University, USA, and Jinan University, China, the genome of pigs was edited using CRISPR to insert the human gene that causes Huntington’s. This approach resulted in pigs that showed very similar symptoms to people with the condition compared to rodents, such as difficulties with walking and running with age. In addition, certain brain changes were similar, such as the loss of specific neurons and immune responses.