Gene Therapy for Hereditary Amyloidosis

Introduction

Hereditary amyloidosis is an inherited disorder characterized by the accumulation of abnormal proteins in diverse body tissues. A specific subtype, known as ATTR (amyloid transthyretin) amyloidosis, results from mutations in the TTR gene (transthyretin gene), leading to instability in the protein transthyretin and subsequent deposition. In addition to TTR-related cases, mutations in other protein genes contribute to non-TTR amyloidosis. Gene therapy endeavors to rectify genetic aberrations by employing diverse methods to modify, deactivate, or substitute faulty genes. Although still in its nascent development and clinical evaluation stages, whole gene therapy harbors considerable potential as a groundbreaking treatment paradigm for addressing this complex ailment.

What Is Hereditary Amyloidosis?

Hereditary amyloidosis is a severe and uncommon affliction characterized by the infiltration of abnormal amyloid proteins into various tissues throughout the body, including vital organs such as the nervous system, heart, and kidneys. These pathological proteins wreak havoc on tissue integrity and functionality. The inheritance of hereditary amyloidosis is familial, with genetic factors playing a crucial role. The onset of this condition is precipitated by mutations in the gene responsible for encoding the transthyretin (TTR) protein, which facilitates the transportation of retinol and thyroxine throughout the body. Operating under an autosomal dominant inheritance pattern, at least one mutated gene copy is requisite for disease manifestation. The resultant unstable TTR protein undergoes amyloid deposition, further exacerbating the pathology.

Two distinct forms characterize hereditary amyloidosis:

• ATTR Amyloidosis: Mutations in the TTR gene precipitate the instability and subsequent deposition of transthyretin, leading to the onset of ATTR. Synthesized predominantly within the liver, transthyretin encompasses approximately 136 genetic variations within the spectrum of ATTR. Notably, the deposition of this amyloid protein primarily targets nerves, cardiac tissues, and the gastrointestinal tract.

• Non-TTR Amyloidosis: Among the classifications of hereditary amyloidosis, the Non-TTR variant emerges as notably scarcer than its ATTR counterpart. Mutations in genes encoding proteins like fibrinogen Aa, apolipoprotein AI, apolipoprotein AII, gelsolin, lysozyme, and cystatin C characterize this subtype. Around 60 genetic variations contribute to the complexity of non-TTR hereditary amyloidosis.

How Is Gene Therapy Utilized in the Treatment of Hereditary Amyloidosis?

Gene therapy represents a groundbreaking intervention strategy that aims to modify the genetic makeup of individuals, thereby modulating the biological properties of living cells for therapeutic purposes.

This innovative therapeutic approach unfolds through various mechanisms, including:

• Introducing a healthy gene to supplant the defective one.

• Disabling the flawed gene will prevent its detrimental consequences.

• Introducing a tailored or fresh gene into the body to alleviate the ailment.

Gene therapy encompasses different strategies, such as plasmid DNA (deoxyribonucleic acid, which carries the genetic material), where engineered circular DNA molecules are employed to transport therapeutic genes into cells. Viral vectors offer another avenue involving the modification of viruses to eliminate their pathogenicity, repurposing them as carriers of therapeutic genes in human cells. Additionally, bacterial vectors entail modifying bacteria to act as carriers, aiding in delivering therapeutic genes into human tissues. Furthermore, patient-derived cellular gene therapy products involve the extraction of the patient's cells, genetic modification, and subsequent reintroduction into the patient's body.

Alternative strategies in gene therapy for addressing hereditary amyloidosis encompass:

1. Human Gene Editing Technology: This approach disrupts or repairs deleterious genes. Methods such as CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats CRISPR-associated protein 9) enable the direct rectification of genetic mutations within the individual's DNA. Gene editing targeting TTR involves the application of CRISPR-Cas9 technology, a powerful tool for precise DNA alterations, comprising two components:

• The Cas9 protein functions akin to molecular scissors, exhibiting the capability to cleave DNA at designated sites with precision.

• Guide RNA (gRNA) is a compact RNA fragment crafted to align with the precise DNA sequence earmarked for editing, effectively steering the system to the targeted locus within the DNA.

Combined, they constitute a mechanism capable of accurately modifying genes by creating double-stranded DNA breaks at specific sites. NTLA-2001, a therapeutic gene-editing medication, utilizes a single guide RNA to direct the Cas9 protein to the disease-causing gene (TTR). Additionally, it incorporates lipid nanoparticles, which effectively transport the Cas9 protein's directives into the patient's cells. In this research, a notable decrease in the concentration of the detrimental TTR protein in the bloodstream was achieved, accompanied by only minor adverse reactions. Clinical trials demonstrated a remarkable reduction of up to 87 percent in TTR levels, with negligible side effects reported.

2. Gene Silencing: Gene silencing constitutes a technique aimed at diminishing or preventing the expression of a targeted gene, utilizing small interfering RNA (siRNA) or antisense oligonucleotides (ASO). It is pertinent to mention that in 2019, the FDA (Food and Drug Administration) sanctioned two medications:

• Inotersen: This antisense oligonucleotide is delivered through weekly subcutaneous injections and reduces the blood levels of a protein known as transthyretin. The medication functions by binding to the mRNA (messenger RNA), which is responsible for TTR synthesis. This binding leads to the degradation of the mRNA, thereby decreasing the production of the TTR protein in the body.

• Patisiran: This medication employs siRNA technology to reduce the body's protein levels, which is called transthyretin (TTR). Patisiran, the active component, targets a specific sequence within the mRNA that encodes TTR. Administered via intravenous infusion, it disrupts the mRNA, lowering TTR production.

How Does Gene Therapy Offer Benefits?

The benefits of gene therapy include:

• Gene therapy offers a precision approach, directly pinpointing and tackling the root cause of ailments.

• Gene therapy promises to usher in lasting or even permanent solutions to diseases by rectifying genetic mutations at their core.

• Gene therapy extends a lifeline in the realm of rare genetic disorders, aiming to correct faulty genes and provide newfound hope for affected individuals.

• Gene therapy delves into the genetic origins of ailments and strives to enhance the quality of life by directly addressing their root causes.

• Gene therapy exhibits a customizable nature, adapting its strategies to the unique genetic mutations of each case.

What Are the Drawbacks Associated With Gene Therapy?

The drawbacks linked to gene therapy encompass the following:

• The immune system might identify and attack the viral vectors employed for gene delivery as foreign invaders, leading to negative side effects.

• Viral vectors might mistakenly target incorrect cells, causing unintentional alterations in the genome.

• The costs associated with gene therapies tend to be notably high.

• Gene therapy carries the risk of introducing errors into the genetic makeup.

• The enduring consequences of gene therapy remain uncertain due to its novelty as a medical approach.

• The viral vectors employed in gene therapy may pose the risk of inducing infections.

Conclusion

The diverse spectrum of hereditary amyloidosis results in varied manifestations, prompting individualized treatment strategies tailored to the specific subtype. Timely intervention during the initial stages markedly improves treatment outcomes. Gene therapy, employing advanced methodologies such as inactivating defective genes, replacing malfunctioning ones, and introducing modified genes, presents a promising approach to tackling the genetic origins of hereditary amyloidosis. Nonetheless, given its nascent stage, continued research is essential to refine and optimize gene therapy's effectiveness.