Exploring Biomarkers in Hereditary Transthyretin Amyloidosis: From Clinical Severity Assessment to New Disease Mechanisms

Hereditary transthyretin amyloidosis is a severe and heterogeneous systemic condition due to mutations in the transthyretin (TTR) gene. This autosomal-dominant neurogenetic disorder is characterized by an adult-onset with variable penetrance and an extracellular deposition of amyloid in different organs with a prevalent involvement of the somatic and autonomic peripheral nervous system (PNS).

At the same time, the heart, the kidney, the gastro-intestinal system, and the eyes are frequently involved, leading to a lifethreatening multisystem disease with a huge clinical variability and course, and death within 10 years on average. The prevalence of the disease is highly variable between endemic and non-endemic countries and the global prevalence ranges from 5.526 to 38.468. However, the real numbers of ATTRv could even be higher, considering the missing diagnoses and pre-symptomatic carriers regularly followed in each centre. The last few years have been characterized by a significant change in disease management due to a deeper knowledge of the phenotype-genotype correlations and most importantly to the availability of disease-modifying therapies (DMTs). This raises the need for reliable disease biomarkers, in order to monitor the efficacy of the pharmacological treatment and the progression of the disease but also to identify the disease onset in the presymptomatic carriers of TTR gene pathogenic variants.

While enormous progress has been made in diagnostics and treatment in ATTRv, advances in the identification of biomarkers useful for assessing the severity of the disease and the efficacy of pharmacological approaches have not been parallel to this so far. Once molecular diagnosis has been established, ATTRv patients are evaluated in clinical practice and in clinical trials through clinical scales such as the Familial Amyloid Polyneuropathy (FAP) staging system, the Polyneuropathy Disability (PND) score, the Neuropathy Impairment Score (NIS) and its subset, the NIS-lower limbs (NIS-LL) score, and the Norfolk Quality of Life-Diabetic Neuropathy (QoL-DN) questionnaire. However, these purely clinical scales detect a change only when patients have already progressed clinically. It would be desirable to have outcome measures in order to effectively treat presymptomatic patients with a high risk of future clinical worsening.

Furthermore, investigating extraneurological involvement in ATTRv can help identifying disease severity biomarkers and shed light on new general and specific pathogenetic pathways besides those already known. In this context, neurofilament light chain (NfL), a neuronal cytoplasmic protein highly expressed in large calibre myelinated axons, has emerged as a good candidate as a disease biomarker in ATTRv, suggesting a key role in monitoring disease progression.

Providing information about somatic and autonomic small fiber, skin biopsy appears to be a minimally invasive exam able to disclose pathological changes several years before the onset of symptoms. Skeletal muscle can be secondarily involved in ATTRv amyloidosis, as already described for other inherited neuropathies. In this context, muscle MRI can represent a useful tool to evaluate the progression of the pathology.