Familial ATTR Amyloidosis - An Overview

What Is Familial ATTR Amyloidosis?

Familial ATTR amyloidosis is driven by an inherited mutation in the gene responsible for coding the transthyretin (TTR) protein. This mutation triggers the production of an anomalous amyloid protein, which deposits in various tissues and organs, impairing their function. Despite its genetic origin, symptoms may not manifest until adulthood, and some individuals may experience only minor symptoms or remain entirely asymptomatic. Among the diverse genetic variations of familial ATTR amyloidosis, which encompass around 136 variations, notable examples include the ATTRV30M and V-122-I mutations, among others.

What Is the Pattern of Inheritance for Familial ATTR Amyloidosis?

Familial ATTR amyloidosis follows an autosomal dominant inheritance pattern, meaning that inheriting just one copy of the mutated gene from one parent is adequate to trigger the disorder. A child with a parent carrying the mutated gene has approximately a 50 percent chance of inheriting it.

What Is the Genetics Involved in Familial ATTR Amyloidosis?

The genetic disorder involves the TTR gene located on chromosome 18, which instructs the production of the transthyretin protein. Transthyretin facilitates the transportation of retinol and thyroxine throughout the body. Thyroxine is transported by forming a tetramer with transthyretin proteins, whereas retinol requires a tetramer complex with retinol-binding protein. The primary production site of this protein is the liver, with smaller amounts synthesized in the retina and choroid plexus. When mutations affect this gene, they induce the misfolding of the transthyretin protein, leading to its aggregation and deposition in diverse tissues across the body.

What Symptoms Are Associated With Familial ATTR Amyloidosis?

Symptoms of familial amyloidosis are diverse and contingent upon genetic variations. Predominant deposition sites typically include nerves, the heart, and the gastrointestinal tract. However, other organs like kidneys, carpal ligaments, and eyes can also be affected, shaping the symptomatology based on the organs impacted by amyloid deposits.

1. Cardiac System: Amyloid deposition in the heart tissue induces cardiomyopathy, characterized by the stiffening of heart muscles. This can present with symptoms like dizziness, fatigue, sleep disturbances, shortness of breath, palpitations, mild valve regurgitations, irregular heart rhythm, chest discomfort, and lower limb swelling. In advanced stages, congestive heart failure may develop, heightening the likelihood of stroke.

2. Nervous System: This condition can initiate nerve damage, leading to peripheral neuropathy characterized by impaired nerves outside the brain and spinal cord. This can evoke tingling or burning sensations in various body parts. Furthermore, sensorimotor impairment may emerge, involving a loss of motor and sensory functions. Additionally, autonomic neuropathy may occur, affecting nerves responsible for regulating bladder control, body temperature, blood pressure, and breathing.

3. Digestive System: In the digestive system, the accumulation of amyloid deposits can trigger bladder dysfunction, constipation, diarrhea, and nausea, disrupting the normal functioning of intestinal muscles controlled by affected nerves. Furthermore, individuals may experience loss of appetite, weight loss, and persistent feelings of fullness due to these deposits.

4. Central Nervous System: The presence of amyloid deposits in the brain may result in symptoms such as seizures (uncontrolled electrical activity in the brain causing abnormal movements or behavior), ataxia (poor muscle control), headaches, progressive dementia (impaired ability to remember which progresses over time), spastic paresis (characterized by muscle weakness along with increased muscle tone), and episodes resembling strokes.

5. Genitourinary System: Amyloid deposition can manifest in symptoms such as recurrent urinary tract infections, irregularities in sweating patterns, sexual dysfunction, and orthostatic hypotension (low blood pressure that occurs when standing up from sitting or lying down position).

6. Kidney: The influence of amyloid protein can disrupt the filtration mechanism, leading to nephrotic syndrome characterized by excessive protein excretion in the urine and swelling in the legs. Furthermore, in some instances, kidney failure may occur.

7. Ocular: The accumulation of amyloid in the ocular area can cause vitreous opacification (clouding of vitreous humor, which is a clear gel that fills the space between the lens and the retina in the eye), abnormal conjunctival vessels (the blood vessels of the conjunctiva, a thin, transparent membrane that covers the front of the eye, appear pathological), glaucoma (type of eye diseases that damages the optic nerve which can affect vision), and papillary abnormalities (irregularities or changes in the appearance or function of the pupil).

8. Others: The build-up of amyloid deposits in the wrist, fingers, or thumb may cause tingling or numbness sensations and could eventually lead to the onset of carpal tunnel syndrome.

What Methods Are Used to Diagnose Familial ATTR Amyloidosis?

The diagnosis of familial ATTR amyloidosis includes:

• Blood and urine tests are employed to detect the presence of amyloid proteins in the body.

• Tissue biopsy involves the microscopic analysis of a small tissue sample, allowing for the identification of amyloid deposits.

• Application of a Congo red stain to a tissue biopsy leads to a distinct alteration in light wavelength, resulting in the emergence of an apple-green coloration, indicative of amyloidosis.

• In addition, protein sequence analysis and DNA sequencing play a crucial role in identifying the variant. TTR gene sequencing is a genetic test dedicated to analyzing the DNA sequence of the TTR gene, uncovering any genetic variations or mutations.

What Are the Treatment Options for Managing Familial ATTR Amyloidosis?

The treatment options for managing familial ATTR amyloidosis include:

• Supportive Treatment: This encompasses a tailored approach to address symptoms such as neuropathy, kidney complications, gastrointestinal disturbances, and cardiac ailments, each managed according to its specific clinical manifestation.

• Liver Transplant: Among the diverse genetic variations of ATTR amyloidosis, the liver stands out as the principal producer, making the liver transplant a promising avenue for arresting amyloid deposits. For individuals grappling with cardiac concerns, the prospect of a combined heart and liver transplant holds potential advantages.

• Gene Silencing Therapy: This represents a cutting-edge technique designed to suppress or thwart the expression of specific genes. Patisiran and Inotersen, two approved drugs, operate by binding to messenger RNA (mRNA) and reducing the production of TTR.

• Gene Editing: This represents another cutting-edge approach to intervene in deleterious genes, whether by disrupting their function or facilitating repair. Leveraging CRISPR-Cas9 technology enables precise gene manipulation at designated locations, with the potential to effectively diminish TTR protein levels in the bloodstream.

• TTR Stabilizers: These are pharmaceutical agents designed to stabilize the naturally occurring, circulating form of TTR, thereby preventing its misfolding and subsequent deposition. Notably, drugs such as Tafamidis have been employed for this purpose.

• Genetic Counseling: This serves as a pivotal process, enlightening individuals about the risks and patterns of genetic disorders in forthcoming generations, while also elucidating available treatment avenues. In families where mutations are identified, genetic counseling assumes fundamental importance.

What Is the Expected Outlook for Individuals With Familial ATTR Amyloidosis?

Disease onset at an early age typically grants a survival window of 10 to 20 years, contrasting sharply with late-onset cases characterized by swift progression and frequent cardiac complications, leading to an average survival of approximately seven years. In cases with cardiac phenotypes, the prognosis is notably unfavorable, with a mere two to five years of life expectancy from the onset of symptoms.

Conclusion

The familial ATTR amyloidosis keeps getting worse over time without stopping. Given its genetic basis, early diagnosis is paramount for effective management, necessitating genetic screening to pinpoint mutation carriers. Systematic and consistent monitoring plays a vital role. Successfully navigating familial ATTR amyloidosis requires a comprehensive, multidisciplinary approach supported by continuous strides in medical research and therapy evolution.