Amyloidosis is a rare, advanced, and deadly disease caused by the depositing of damaged proteins in the body. With time, these depositions interfere with the heart as well as the nervous and digestive systems, resulting in severe organ damage. For example, amyloidosis polyneuropathy leads to nerve damage, and cardiomyopathy can cause heart failure. Symptoms include fatigue, swelling, shortness of breath, pain, diarrhea, weight loss, enlarged tongue, skin changes, irregular heartbeat, and difficulty swallowing. Unfortunately, as these symptoms are common, the disease is underdiagnosed.
In addition, the root cause of the disease is not currently treated. Instead, only life-supporting therapies are available, depending upon disease onset and condition. Therefore, CRISPR is being explored as a promising treatment option to eradicate the real cause of amyloidosis. This article summarizes current treatments and discusses potential CRISPR-based therapeutics for amyloidosis.
Amyloidosis disease and its current treatment strategy:
Amyloidosis is a rare disease caused by the interference of abnormal proteins (amyloids) in normal body function and produced abnormally within the body of the patient. The estimated incidence of amyloidosis is 3-12 cases per million per year, with approximately 50,000 cases in the United States and Europe. According to the Genetic and Rare Diseases Information Center, the approximate life span of a patient with ATTR amyloidosis is about 7 to 12 years.
The amyloidosis types depend on the types of protein produced abnormally. Treatment options depend on the type of amyloidosis, stage of disease, and majorly damaged organ of the patient and may include chemotherapy, stem cell transplant, organ transplant, or a combination of these treatments. The four main types of amyloidosis are:
- AL (primary) amyloidosis: Affects light chain antibody-protein, and is caused by cancerous plasma cells within the bone marrow. Options for treatment include chemotherapy to eradicate plasma cells, potentially combined with autologous stem cell transplantation.
- AA (secondary) amyloidosis: Affects serum amyloid A protein, which is produced in response to inflammation or infection. High levels of protein do not cause disease in the short term but lead to disease progression over the long term by accumulation. Therefore, treatment often depends on the cause of inflammation and may include immunosuppressants (for rheumatoid arthritis), antibiotics (for chronic tuberculosis), or renal transplantation (for renal disease).
- Familial ATTR amyloidosis: Involves mutant transthyretin (TTR) protein. This form is hereditary and can be treated by liver transplant, recently approved drug regimes, or supportive therapies in a personalized way, depending on the patient’s requirements. Genetic counseling is essential for familial ATTR amyloidosis patients and their family members.
- Wild-type (senile) ATTR amyloidosis: Involves non-mutated TTR protein. Similar to familial ATTR amyloidosis, life-supporting treatment is given to patients. Further treatment options to treat the underlying cause of the disease are under investigation.
Limitations of current options to treat amyloidosis:
Existing therapies mainly support the life of patients but do not cure amyloidosis. Liver transplantation or multi-organ transplantation could slow the progress of the disease but cannot completely halt it. The US FDA approved Onpattro (patisiran) and Tegsedi (inotersen) in 2018 to support polyneuropathy for adults with familial ATTR amyloidosis, and Vyndaqel (tafamidis meglumine) and Vyndamax (tafamidis) in 2019 for ATTR cardiomyopathy adult patients. However, tafamidis is only approved for stage I familial ATTR amyloidosis uses in Europe.
Patisiran (small interfering RNA, or siRNA) reduces the production of the TTR protein via gene silencing, and inotersen (antisense oligonucleotide) reduces levels of TTR protein by mRNA degradation, whereas tafamidis (small molecule) stabilizes the quaternary structure of the TTR protein to prevent the formation of amyloids.
However, the price burden of these drugs is too high for many patients to afford. Tafamidis is the most expensive cardiovascular drug approved in the United States. Also, the treatment benefits are limited to a narrow sub-group population within amyloidosis patients. For example, patisiran and inotersen treat polyneuropathy (nerve disease) caused by familial ATTR amyloidosis, whereas tafamidis treats familial and wild-type ATTR amyloidosis cardiomyopathy patients. In addition, these drugs are not approved for pediatric use.
These drugs also include a number of side effects, ranging from vitamin A deficiency to internal bleeding; for these reasons, the use of inotersen should be personalized, presenting its own set of challenges. Fortunately, tafamidis is an oral medication with fewer side effects reported, but its use is restricted during pregnancy or breastfeeding, or in cases of liver disease or interference with other medicines.
The strategy in treating amyloidosis with CRISPR:
Recent research progress of CRISPR-Cas9 tools aims to reduce the amount of faulty TTR protein, which is a root cause of protein deposition in the body. The knockout strategy involves breaking the TTR gene instead of repairing it. CRISPR introduces a break in the double-stranded DNA such that the host repair mechanism cannot find the correct template match, therefore introducing a mutation upon DNA repair that can lead to the cell no longer producing TTR protein. All CRISPR techniques target specific cells or tissues so the genetic changes do not pass to future generations through germline cells.
Earlier successful attempts in the cases of eye, muscle, and liver using the CRISPR-Cas-9 system were reported in animal models. To date, various viral and non-viral vectors have been designed to deliver CRISPR elements into the retinal cells. For instance, the EDIT-101 vector was designed to deliver CRISPR components in Leber congenital amaurosis type 10. The vector successfully met the therapeutic stage to support restoration from vision loss in the animal model.
The first CRISPR-based treatment to directly inject into the body was approved for a clinical trial (NCT03872479) to treat hereditary blindness disorder (Leber’s congenital amaurosis 10). Further, Intellia Therapeutics already developed lipid nanoparticles to deliver SpCas9 mRNA and modified gRNA into a mouse liver. The experiments successfully reduced the TTR protein in the mouse model.
Ongoing trials related to CRISPR:
The results of the clinical phase 1 trial funded by Intellia Therapeutics and Regeneron Pharmaceuticals (NCT04601051) are very encouraging. The initial results were published in June 2021 in The New England Journal of Medicine. The designed single-dose NTLA-2001 (CRISPR components) were injected in six patients with hereditary ATTR amyloidosis with polyneuropathy.
The study was designed for lower (0.1 mg per kilogram) and higher (0.3 mg per kilogram) doses to evaluate the safety and effects of the treatment. After 28 days, TTR protein reduction among patients in the lower dose group was 52%, and the higher dose group was 87%. This is the first trial to deliver a genetic component using a lipid nanoparticle. The team are planning to extend the trial for another phase.
Intellia Therapeutics received US FDA Orphan Drug Designation on October 21, 2021. This prestigious designation is provided for new rare disease treatment options. The benefits to the sponsor party include tax credits for clinical trials, user fee relief, and seven years of market exclusivity after approval.
Another TTR protein reduction using the CRISPR SpCas9 system was studied in a humanized mouse model. The study compared the single and dual approaches of loading the CRISPR SpCas9 system in adeno-associated virus 8 (AAV). The single AAV-mediated CRISPR system was more efficient than the dual approach.
Challenges and opportunities in developing CRISPR-based treatments:
One of the greatest challenges for the CRISPR technique is the method of delivery. Viral vectors have been primarily used to deliver gene-editing products so far. However, the CRISPR-Cas-9 therapy developed by Intellia Therapeutics and Regeneron Pharmaceuticals used lipid nanoparticles, as lipids tend to deposit in the liver naturally in animals and TTR is produced primarily in the liver.
The most concerning risk of any gene-editing system is off-target effects. The system could cut DNA in the wrong location, which could trigger other issues such as cancer. The chances of mutation error are higher when using viral vectors, since along with the gene of interest, these vectors also carry other DNA sequences that help to deliver the target gene. Different viral vectors have their own sequence length and risk factors, whereas the lipid nanoparticle gene delivery system includes a designed lipid envelope around the target gene. Therefore, the chances of mutation error risk decrease greatly.
The efficiency of treatment with genome editing therapies is a common concern. The efficiency is measured by the percentage of edited cells. The TTR protein reduction to cure the disease in non-human primates is achieved by editing only 35%-40% of liver cells. Therefore, the patients receiving CRISPR-based treatments during clinical trials should be followed up to get a clear idea about the efficiency of the treatment and to understand what percentage leads to results.
Unlike other drug trials, this treatment could take a longer time to show effects in patients. Also, this trial was designed for patients having amyloidosis with polyneuropathy, but most patients have more than one symptom and organ involved. However, the NTLA-2001 gene-editing therapy can target other conditions by specifically designed guide RNA in the CRISPR set.
Future opportunities might include modifying the DNA instead of knocking it out. Presently, the efficiency of integration is very low in animal cell culture models. However, techniques are being developed for promising approaches. The research team from Thermo Fisher Scientific, the Swiss Federal Institute of Technology, the University of Nebraska Medical Center, and the University of Washington have developed some potential strategies for CRISPR-mediated gene knocking systems. Genetic modification could open several positive possibilities, such as replacing faulty DNA with any enzyme system that is needed elsewhere in the body.
Conclusion:
The results of CRISPR treatment are supportive enough to move forward toward clinical studies. However, movements toward the ability to inactivate, repair, or replace genetic material are more challenging.
Intellia President and Chief Executive Officer John Leonard, MD, said in a press release that the company will continue to work with the ATTR amyloidosis community and the FDA to bring better treatment options to the market. Intellia Therapeutics’ NTLA-2001 had already received European Union Orphan Drug Designation on March 30, 2021. Another Orphan Drug Designation from the FDA has encouraged the company to move forward, including submitting additional regulatory applications in other countries. In addition, Intellia is active in developing uses of the CRISPR technology in other areas, including cancer and autoimmune diseases.
Beyond the benefits that successful development of the CRISPR approach will bring to patients of amyloidosis and their families, lessons learned in overcoming the challenges and embracing the opportunities of this approach will prove invaluable in extending the applications of CRISPR to other conditions.
