CPD: Transthyretin Amyloidosis

AUTHORS: Dr Zara Togher, Professor Sinéad M Murphy

Dr Zara Togher is a Specialist Registrar in Neurology currently working in University Hospital Limerick. She has an interest in neuropathies and neuromuscular disorders having spent time in dedicated Neuropathy and Neuromuscular clinics in Tallaght University Hospital (TUH).

Prof. Sinead Murphy is a Consultant Neurologist at TUH and Clinical Associate Professor in Trinity College Dublin. She has particular subspecialty interest and expertise in inherited neuropathies and ataxias, runs Neuropathy and Neuromuscular clinics and co-leads the National Ataxia Clinic in TUH. She is lead of the Irish Consortium for Rare Neurological Disorders, with TUH recently being recognised as a site for the European Reference Network in Rare Neurological Diseases (ERN-RND). She chaired the Neurology sub-group of the HSE Amyloidosis Working Group which developed the Amyloidosis Model of Care and Guideline recently approved by the HSE.

60 Second Summary

Transthyretin amyloidosis (ATTR) is a multi-system fatal disease which occurs as a result of deposition of abnormal transthyretin-derived amyloid fibrils in a variety of body tissues. Both inherited (ATTRv) and acquired i.e. wild-type forms (ATTRwt) can occur. Due to the variety of systems affected there can be a wide variability in symptom manifestation, although cardiac and neurological symptoms generally predominate. Commonly symptoms are nonspecific and may be mistaken for symptoms of other more frequently occurring causes (e.g. orthostatic hypotension being attributed to medication rather than to an underlying autonomic neuropathy). This, along with a lack of awareness among healthcare professionals, can lead to significant delays in diagnosis. With emergence of disease-modifying treatment, prompt diagnosis is of increasing importance. Licensed treatment options now include transthyretin stabilisers and small interfering RNA therapeutics, while gene editing with CRISPR-Cas9 based therapy is showing promise in clinical trial.

This CPD piece will provide a general overview of ATTR including subtypes, clinical presentation, diagnosis and disease-modifying treatment strategies. Symptomatic management is outside the scope of this piece. The HSE Model of Care for Amyloidosis which aims to improve standards of care for patients with amyloidosis in Ireland is discussed; an important aspect of this is improved awareness and earlier diagnosis. We also discuss methods for earlier or potentially pre-symptomatic identification of patients.

Introduction

Transthyretin amyloidosis (ATTR) is a progressive and fatal disease caused by deposition of abnormal transthyretin-derived amyloid fibrils in the extracellular tissues of various organs. 1 Transthyretin protein (TTR) is predominantly produced by the liver, although small amounts are also produced in the eye by the retinal and ciliary pigment epithelia, and in the brain by the choroid plexus. 2,3 Normally TTR circulates in the form of a tetramer made up of four identical monomeric subunits in both the serum and cerebrospinal fluid (CSF). Its main function is as a transporter of both thyroxin and retinol-binding protein, hence the name. 2 In ATTR, dissociation of the tetrameric TTR occurs followed by misfolding, aggregation and formation of insoluble amyloid fibrils which are then deposited systemically. Accumulation of amyloid fibrils in tissues progressively disrupts their structure and function, leading to loss of function in various organs, particularly heart and peripheral nerves. There are two sub-types of ATTR: wild-type ATTR (ATTRwt) and hereditary or variant ATTR (ATTRv).

ATTRv

In ATTRv pathogenic genetic variants destabilise TTR, causing misfolding and aggregation. 3 The TTR gene is located on chromosome 18, and over 120 TTR mutations have been identified to date 3,4 Worldwide V30M (p.V50M) is the most frequent, accounting for approximately half of all cases. 5 However, with regard to Irish patients, in 1995 Reilly et al. described a cluster of cases concentrated in North West Donegal. This cluster was associated with the T60A (p.T80A) variant. Further population-based studies showed a carrier rate for this mutation of approximately 1.1% in this region. 6 This is the most common TTR mutation in the Irish population, although other variants also occur.

It is important to note that there can be significant differences in presentation depending on the particular genetic variant. For example, while the typical presentation of ATTRv is with a combination of cardiomyopathy and neuropathy, patients with ATTRL12P usually present with oculoleptomeningeal disease. 7 In addition, penetrance and age of onset varies between populations even with the same mutation. For example, patients with ATTRV30M of Portuguese origin have mean age of onset of 33 years with penetrance of 91% at 70 years, whereas patients of Swedish origin with the same variant have a later age of onset of 56 years and lower penetrance of 36% at 70 years. 8

ATTRwt

ATTRwt (previously called senile systemic amyloidosis) is generally considered a disease of older adults in which amyloid fibril deposition occurs due to agerelated structural changes of the TTR protein which cause it to misfold. Studies which examined autopsy specimens revealed increasing amounts of wt TTR deposition in cardiac tissues with age. 9,10 It is likely that ATTRwt is significantly underdiagnosed; it is estimated that up to 25% of patients with heart failure with preserved ejection fraction (HFpEF) have ATTRwt. In two autopsy studies specifically looking at supercentenarians (individuals who have reached the age of 110 years or more), the primary cause of death in 70% of patients was related to cardiac deposition of wt TTR. 11

Clinical Features

The clinical presentation of ATTR varies between patients depending on the subtype (ATTRwt vs ATTRv) and, in ATTRv, the specific mutation and the origin of the patient. However, the heart and peripheral nerves are the most common clinically involved organs in patients with ATTR. Cardiac symptoms classically consist of heart failure and arrhythmias, whereas neurological symptoms are due to involvement of peripheral nerves which can include large fibre, small fibre or autonomic involvement. 12 In both subtypes, amyloid deposits in connective tissues may cause carpal tunnel syndrome, spinal stenosis, rotator cuff tears and joint pain which often predate the neurological or cardiac symptoms. 13,14,15

ATTRv

Clinical presentation in ATTRv can show a wide heterogeneity depending on the specific TTR mutation (Image 1). Diagnostic delay is frequent, typically 4 years from onset of symptoms. 16 Average survival depends on the mutation, overall 10-15 years following onset of symptoms, but only 2.5-5.5 years following diagnosis for ATTRT60A. 17,18 With regard to ATTRT60A, the common Irish variant, a case series of 60 patients published by Stangou et al. in 2012 showed that cardiac involvement was present in nearly all cases by time of diagnosis, in contrast to Portuguese patients with ATTRV30M where cardiac involvement occurs much later. 19 A review of 15 patients attending the Irish cardiac amyloidosis referral centre in 2020 demonstrated that patients presented with a mixed phenotype of neuropathy and heart failure, with cardiac scanning using 99m Tc-DPD tracer (Tc-DPD) showing an advanced stage of cardiac disease by the time of diagnosis. 20

Mean age of symptom onset is 62 years. The first symptom in more than half of patients with ATTRT60A is of carpal tunnel syndrome (usually bilateral) which occurs approximately 7 years before other symptoms develop. 21 Sensory symptoms typically predominate initially with numbness, burning, paraesthesiae and occasionally lancinating pains. Motor weakness is a later feature. Although typically the neuropathy is length-dependent, some patients have early upper limb involvement out of proportion to lower limb involvement. 22 Autonomic dysfunction is common, with symptoms including alternating diarrhoea/constipation, orthostatic hypotension, nausea and vomiting, early satiety, urinary symptoms and erectile dysfunction. 23 Weight loss results from a combination of autonomic and gastrointestinal involvement. Cardiac features include those of heart failure: shortness of breath, oedema, exercise intolerance, orthopnoea, syncope, as well as palpitations, arrhythmias and electrical conduction abnormalities. 24 Table 1 lists diagnostic clues to ATTR.

ATTRwt

Historically ATTRwt has been regarded as a cardiomyopathy affecting elderly males. 9 However; it is a disease of aging, with the amount of amyloid deposition increasing in proportion to age. A Finnish study demonstrated that 25% of the autopsied population over 85 years had evidence of wt TTR deposition in myocardium, with no difference in prevalence between males and females although the severity of deposition was more in males. 25 ATTRwt is an important differential to consider when patients present with HFpEF, accounting for up to 25% of cases. 26 It also manifests as carpal tunnel syndrome which often predates symptoms of cardiomyopathy. 27 Neuropathy is usually a less prominent clinical feature in ATTRwt; however, some smaller studies 28,29 suggest a higher prevalence than previously thought and occasionally it can be the presenting feature. 30 Prognosis is poor as most patients have cardiac involvement at time of diagnosis, with mean survival 3.5 years.

Diagnosis

As previously discussed, there may be a wide variability in presenting symptoms many of which are non-specific. This can lead to misdiagnosis or often significant delays in diagnosis. Previous studies into ATTRv report rates of misdiagnosis of up to 32-74%. 31,32 The Amyloidosis Research Consortium reported that 47% of patients received at least one misdiagnosis before being diagnosed with amyloidosis. 33 In this survey, ATTR patients went undiagnosed for more than 4 years (27% ATTRwt and 15% ATTRv). 19% of patients also reported having to travel over 2 hours to get diagnosed. While this is data from the US, this would likely be mirrored in Irish patients, with the majority of patients until recently travelling to the UK for diagnosis.

An amyloid Model of Care has been drafted by the HSE Amyloidosis Working Group to improve the standard of care for patients in Ireland 34 . As part of this, a number of symptom ‘gates’ are highlighted where physicians are encouraged to think of a diagnosis of amyloidosis where it may not already be under consideration to aid in timely diagnosis. We will briefly mention here the symptoms of relevance to ATTRv and ATTRwt.

The first of the symptom gates is carpal tunnel syndrome, with a rationale that patients (in particular with ATTRwt) may present with carpal tunnel syndrome up to 10 years before developing symptomatic cardiomyopathy. Therefore the recommendation is for patients who undergo surgical decompression to have a biopsy of the flexor retinaculum taken at the time of surgery to exclude ATTR. Another symptom gate is that of aortic stenosis with low flow-low gradient. It is advised that these patients be referred for DPD scan to exclude the presence of ATTR. Other presentations that should prompt consideration of ATTR are unexplained spinal stenosis, or an unexplained combination of autonomic and peripheral neuropathy.

Several diagnostic algorithms have been proposed as part of the Model of Care, depending on the presenting symptom, a modified version of the neuropathy algorithm is shown below in Image 2. (page 34)

Disease-modifying treatment

In this section we will focus primarily on disease-modifying treatment for ATTRv. Symptomatic management is equally important, especially for quality of life, but is outside the scope of this article. Initially, disease-modifying therapy had relied predominantly on liver transplantation, the concept being that this would prevent further variant TTR being produced by the liver. This improved outcomes in some patients (especially those with early-onset ATTRV30M) but was not a viable option for patients with cardiac disease (and therefore was not usually considered in those with ATTRT60A who usually have cardiac disease at presentation). 19,35 Thankfully, there have been significant advances in the last number of years in ATTR therapies. Tafamidis is a TTR stabiliser which was approved for use in the treatment of adults with ATTR cardiomyopathy in the European Union in 2016. However, it is not approved for, nor has shown a significant improvement in ATTRv associated neuropathy therefore use is currently limited in Ireland to patients with ATTR (either ATTRwt or ATTRv) with cardiomyopathy. 36 Diflusinal is an oral non-steroidal antiinflammatory drug which has been repurposed for use in ATTR (off-license) as it stabilises the TTR tetramer in vitro. It has been shown to stabilise neuropathy progression and reduce cardiac dysfunction, however with potential side effects including gastrointestinal symptoms and renal impairment. 36

Newer treatments including gene silencing therapies and gene editing have recently emerged as effective treatment strategies for ATTRv. Gene silencing therapies for ATTRv include antisense oligonucleotides (Inotersen) and small interfering RNAs (siRNAs), namely patisiran. The APOLLO trial 37 demonstrated biochemical and clinical efficacy of intravenous patisiran in ATTRv, with significant reductions in circulating TTR and improvements in autonomic neuropathy, quality of life, peripheral neuropathy symptoms, nutritional status and cardiac function. An open label extension reported sustained benefits supporting the long-term effectiveness of this therapy. 38 Similar benefits were reported with subcutaneous Inotersen therapy in the NEURO-TTR trial, although it has additional monitoring requirements due to the occurrence of thrombocytopaenia and glomerulonephritis 39,40 . At present only patisiran is reimbursed in Ireland for patients with ATTRv associated neuropathy and is given by three-weekly intravenous infusion; the initial three doses are given in hospital and subsequent doses can be given at home if tolerated. With regards to gene editing, the first ATTRv patient cohort treated with a single dose of CRISPR- Cas9 based therapy showed a significant dose-dependent and long-lasting reduction of serum TTR protein levels. 41 Clinical outcomes are eagerly awaited.

Given that gene silencing and gene editing therapies only affect TTR production by the liver, as they do not cross the blood brain barrier, there is concern that with prolonged survival will come the development of a new phenotype of ATTRv due to continued unchecked production of mutant TTR in the eye and brain. 42 Indeed, symptomatic cerebral amyloid angiopathy has been seen in ATTR patients surviving after liver transplantation. 43 In contrast to the systemic circulation where other thyroxin carriers exist, TTR is required for thyroxin transport centrally as well as having some neuroprotective effects. Thus, novel approaches will be needed as prognosis improves with the recent advances in therapy.

Future of ATTR in Ireland

One avenue for improvement is the potential for identification of pre-symptomatic disease in both ATTRwt and ATTRv. As mentioned, carpal tunnel disease may present up to 10 years before development of systemic symptoms. Sperry et al published an observational study of 98 patients where the flexor retinaculum in men ≥50 years and women ≥60 years undergoing carpal tunnel release surgery was routinely sent for analysis with Congo red staining to determine the prevalence of amyloid. 44 This detected amyloid deposits in 10.2% of patients (in this study due to a combination of ATTRv, ATTRwt and light chain amyloidosis). In a similar Japanese study, 34% of patients had amyloid deposition in the tenosynovial tissue sample. 45 Similar studies looking at ligamentous tissue in patients undergoing surgery for lumbar spinal stenosis showed amyloid deposition in 88.4% 13 and 96% 46 of patients respectively. We suggest that routine analysis of surgical tissue would be an easy way to diagnose presymptomatic patients and therefore enable earlier treatment, improving outcomes.

Another consideration is of genetic testing of patients with family members with ATTRv. Although the dominant genotype in the Irish population (p.T80A) shows incomplete penetrance, screening allows mutation carriers to be followed in order to identify early symptomatic disease and offer early treatment. A recent consensus paper suggests that adult firstdegree relatives of ATTRv patients may be offered genetic counselling and screening and, if positive, should start having neurological and cardiac assessments within 10 years of anticipated onset of symptoms. 47 The frequency of such assessments has yet to be clarified.

As mentioned, a Model of Care has been drafted by the HSE Amyloidosis Working Group 34 which aims to improve standard of care for Irish patients with amyloidosis. It aims to reduce burden of disease by improved awareness and earlier diagnosis in order to prevent less endorgan sequelae. It also aims to avoid patients having to travel abroad (usually to the UK National Amyloid Centre in London) as part of their diagnostic journey by creating a complex care centre or network in Ireland.

With the advent of diseasemodifying treatment, the outlook for patients with this hitherto progressive terminal disease is certainly more hopeful although many challenges remain. An active patient group has been set up (the All Ireland Amyloidosis Support Group), and the first all Ireland ATTR Conference, Emerging from the Shadows, was recently held in Gweedore, Co. Donegal, bringing together patients and their families, amyloidosis experts and other health care professionals interested in learning more about the disease.

References available on request