Hereditary Transthyretin-Related Amyloidosis: Genetic Heterogeneity and Early Personalized Gene Therapy.

Point mutations of the transthyretin (TTR) gene are related with hereditary amyloidosis (hATTR). The number of people affected by this rare disease is only partially estimated. The real impact of somatic mosaicism and other genetic factors on expressivity, complexity, progression, and transmission of the disease should be better investigated. The relevance of this rare disease is increasing and many efforts have been made to improve the time to diagnosis and to estimate the real number of cases in endemic and non-endemic areas. In this context, somatic mosaicism should be better investigated to explain the complexity of the heterogeneity of the hATTR clinical features, to better estimate the number of new cases, and to focus on early and personalized gene therapy. Gene therapy can potentially improve the living conditions of affected individuals and is one of the central goals in research on amyloidosis related to the TTR gene, with the advantage of overcoming liver transplantation as the sole treatment for hATTR disease.

Somatic mosaicism with reversion to normality of a mutated transthyretin allele related to a familial amyloidotic polyneuropathy.

Familial amyloidotic polyneuropathy (FAP) is a progressive neuropathy, with onset in adulthood and high mortality. It is related to an altered transthyretin (TTR) plasma protein, mainly produced by the liver and responsible for amyloid deposit in the peripheral nervous system. SNPs in the TTR gene were associated with FAP, and the G>C substitution (NM_000371.3:c.325G>C) in the 109th codon (GAG vs CAG; NP_362.1:p.E109Q) was previously described in Sicily (Italy). Here, we report on a Sicilian family with several patients affected by FAP related to the E109Q mutation, which displayed a somatic mosaicism with the reversion to normality of the c.325G>C mutation. After exclusion of isodisomy and allele deletion, this event seems to be due to a rare, post-zygotic interallelic gene conversion with the wild-type allele serving as a donor. Further investigations will be necessary to better understand the molecular basis of this phenomenon, and could help determine if this can be induced in a targeted manner in the context of natural gene therapy to treat TTR-related FAP patients, as previously proposed for other diseases. Moreover, our results confirm the need to perform DNA-based diagnostic tests with at least a second tissue when a suspected germline mutation in a candidate gene is not identified in the first tissue.