A new gene therapy created by US researchers appears to have cured eight infant boys born with a rare genetic disorder referred to as “Bubble Boy” disease. The new therapy was shown to safely correct the immune systems of eight infants affected by X-linked severe combined immunodeficiency (X-SCID).
The results of the clinical trial were published yesterday in The New England Journal of Medicine.
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What is X-SCID?
X-SCID is an autosomal recessive condition, meaning the gene mutation associated with the illness is located on the X chromosome. Since males only have one X chromosome, only one copy of the gene mutation is needed to cause the disease. Girls have two X chromosomes, so are much less likely to inherit the condition as both chromosomes would need to be affected.
In X-SCID, immune cells that fight off bacteria, viruses and fungi do not develop or function properly, leaving the infants susceptible to recurrent and persistent infections. Affected boys often suffer from chronic diarrhea, fungal infections such as thrush and skin rashes, and also tend to grow more slowly than unaffected children. Babies born with the condition are confined to completely sterile conditions and, if left untreated, usually die within the first two years of life.
Currently, infants with X-SCID are treated through bone marrow transplants, ideally from a tissue-matched sibling donor. However, in more than 80% of cases, newborns with the disease do not have a genetically matched donor and instead, need to rely on transplants from an unmatched donor. This only partially restores the immune system and causes the children to need lifelong treatment, with frequent and complex health problems, including persistent infections.
Previous gene therapy approaches have been developed to provide alternatives to bone marrow transplants, but the therapies sometimes need to be accompanied by chemotherapy and have been associated with the development of other illnesses including metabolic syndrome and blood disorders such as thalassemia and sickle cell anemia.
A new treatment for X-SCID
Now, eight infants have been treated with a new gene therapy, which was co-developed with National Institutes of Health (NIH) scientists, seem to have developed fully functional immune systems and are still growing normally up to two years after receiving the treatment.
The therapy, which was given to babies at St. Jude Children's Research Hospital, Memphis and UCSF Benioff Children's Hospital in San Francisco, appears to be safer and more effective than the gene therapies previously developed for the condition. The approach seems to have cured the infants, without causing any side effects or complications.
New-borns with X-SCID have a mutation in a gene called IL2RG, which encodes for a protein needed for the growth and maturation of lymphocytes and normal immune system function. Lymphocytes protect against invading pathogens by producing antibodies and, without them, the body cannot defend itself against infection.
Previous attempts have been made to correct the IL2RG mutation, but although these gene-therapies restored T cell function, the function of other important immune cells including B cells and Natural Killer (NK) cells was not fully recovered.
Furthermore, some children treated in these trials developed leukemia. Scientists suspect this could have been due to the gene carrier or “vector” used to replace the IL2RG mutation with a healthy version of the gene, also activated genes involved in the regulation of normal cell growth.
By contrast, in the current trial, not only did seven of eight infants develop normal numbers of functioning T cells, B cells and natural killer (NK) cells, within three to four months of receiving the therapies, but the vector used to correct the mutation was designed to avoid the possibility of leukemia or any other illness developing.
Correcting the faulty gene
For the phase 1/2 trial, Ewelina Mamcarz and colleagues inserted a normal copy of the IL2RG gene into the infants’ own blood-forming stem cells. All infants, who were aged between 2 and 14 months, were newly diagnosed with X-SCID and did not have a genetically-matched sibling donor.
After extracting the stem cells from the patients’ bone barrow, an engineered lentivirus that cannot cause other illnesses was used as the vector to carry the corrected IL2RG into cells. The stem cells were then infused back into the infants.
The babies were also given a low-dose of a chemotherapy drug to aid the establishment of the corrected stem cells in the bone marrow so that they could start producing new blood cells.
Mamcarz and team report that most of the babies were well enough to be discharged from the hospitals within just one month.
These patients are toddlers now, who are responding to vaccinations and have immune systems to make all immune cells they need for protection from infections as they explore the world and live normal lives."
Director of NIH's National Institute of Allergy and Infectious Diseases, Anthony Fauci, says the exciting new results suggest that gene therapy may be an effective treatment option for infants with this extremely serious condition, particularly those who lack an optimal donor for stem cell transplant:
This advance offers them the hope of developing a wholly functional immune system and the chance to live a full, healthy life."
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