Prognostic biomarkers and staging in AL amyloidosis

Systemic light-chain (AL) amyloidosis is the most common type of pathological multi-organ amyloid deposition. AL amyloidosis is caused by misfolded free light chains (FLC), which accumulate in various organs like the heart, kidney, and liver, causing significant impairment and eventually leading to failure.¹

In the first article of the ‘Amyloidosis Series’, we focused on the incidence and diagnosis of AL amyloidosis. This second article will discuss the prognosis and risk stratification models for AL amyloidosis and summarize the best current practices for patient evaluation.

Cardiac prognostic biomarkers

In any amyloidosis type, the majority of patients present with heart involvement. In AL amyloidosis specifically, approximately 70–80% of patients have cardiac amyloid deposition, with increasing FLC levels usually linked to the severity of the heart involvement.¹ Thus, the investigation into soluble heart-related biomarkers and their prognostic potential has long been at the center of AL amyloidosis research. Such markers include the cardiac troponin I and T (cTnI, cTnT), which have been some of the first prognostic markers to predict outcomes in patients with AL amyloidosis. It is now widely established that cTnT is a better AL amyloidosis prognostic biomarker than cTnI. Another broadly used cardiac marker for outcome prognosis is the inactive form of the probrain natriuretic peptide (proBNP), also known as NT-proBNP. Soluble cTnT and NT-proBNP levels have provided the basis for the main risk stratification models used in AL amyloidosis.^1-3

Risk stratification models

The most commonly used biomarker risk models are the first universally accepted Mayo 2004 model, the European 2015 modification of Mayo 2004, and the Mayo 2012. Table 1 indicates the original risk factor thresholds of cTnT and NT-proBNP for all these models.

In all three models, patients are classified as Stage 1 if no risk factors are present, Stage 2 if they have one risk factor, or Stage 3 if they present with two risk factors. The European modification of Mayo 2004 model provides an extra subclassification of Stage 3 patients into high- or low-risk (3a vs 3b) by using a second NT-proBNP threshold of 8500 ng/L. The Mayo 2012 model uses an additional risk factor for disease staging, the difference between involved and uninvolved FLC, with Stage 4 patients having all three risk factors (Table 1).^1-3

With the advent of time, cTnT has evolved to the use of the high-sensitivity cTnT (hs-cTnT) marker. This assay, together with the fact that some centers only have available BNP or cTnI for disease prognosis, led to the formulation of conversion thresholds that correspond to the original Mayo-based model cTnT and NT-proBNP staging thresholds. Table 2 shows these thresholds, which can be used for the accurate comparison between biomarker staging.^1,2

Table 1. Original risk factor thresholds of AL amyloidosis staging (cTnT and NT-proBNP)^1-3

Model	Risk factor threshold			Stage*	HR for death (95% CI)3	Median OS, months1
cTnT (μg/L)	NT-proBNP (ng/L)	Other factors
AL, light-chain; BNP, brain natriuretic peptide; cTn(T/I), cardiac troponin T/I; dFLC, difference in involved and uninvolved free light chain; HR, hazard ratio; hs-cTnT, high-sensitivity cTnT; NT-proBNP, N-terminal probrain natriuretic peptide; OS, overall survival. *Stage definitions: Stage 1, no risk factors; Stage 2, presence of one risk factor; Stage 3, presence of two risk factors; Stage 4, all risk factors present. The exception is the European 2015 modification of the Mayo 2004 model, in which Stage 3 patients (two risk factors) are further divided into Stage 3a or 3b dependent on if NT-proBNP > 8500 ng/L; †Alternatively, cTnI ≥ 100 ng/L or hs-cTnT ≥ 50 ng/L; ‡Alternatively, BNP ≥ 400 ng/L.
Mayo 2004	≥ 0.035†	≥ 332	None	1	Reference	130
2	2.5 (1.9–3.5)	54
3	6.7 (5.0–9.1)	10
European 2015 modification of Mayo 2004 (Mayo 3b)	≥ 0.035†	≥ 332	Stage 3 only: NT-proBNP ≥8500 ng/L	1	Reference	130
2	2.6 (1.9–3.5)	54
3a	4.9 (3.6–6.8)	24
3b	11.1 (8.1–15.4)	4
Mayo 2012	≥ 0.025	≥ 1800‡	dFLC ≥180 mg/L	1	Reference	130
2	1.7 (1.2–2.3)	72
3	4.1 (3.1–5.5)	24
4	6.3 (4.8–8.3)	6

Table 2. Conversion table for comparable risk factor thresholds in AL amyloidosis staging^2,3

Model	cTnT (μg/L)	cTnI (μg/L)	hs-cTnT (ng/L)	NT-proBNP (ng/L)	BNP (ng/L)
AL, light-chain; ASCT, autologous stem cell transplantation; BNP, brain natriuretic peptide; cTn(T/I), cardiac troponin T/I; hs-cTnT, high-sensitivity cTnT; NA, not applicable; ND, no data; NT-proBNP, N-terminal probrain natriuretic peptide. *Simple binary troponin T threshold for predicting transplant-related mortality 25% vs 4%.
Mayo 2004	≥ 0.035	≥ 0.1	≥ 50	≥ 332	81
European 2015 modification of Mayo 2004 (Mayo 3b)	≥ 0.035	≥ 0.1	≥ 50	≥ 332	81
> 8500	> 700
Mayo 2012 model	≥ 0.025	ND	≥ 41	≥ 1800	≥ 400
Rounded > 40
Mayo ASCT troponin risk marker*	≥ 0.06	ND	≥ 73	NA	NA
Rounded ≥ 75

A subset of patients with AL amyloidosis will undergo autologous stem cell transplantation (ASCT). For those patients, a threshold of cTnT ≥ 0.06 μg/L or hs-cTnT ≥ 75 ng/L has been shown to predict early transplant-related mortality following ASCT (Table 2).³

Organ-specific biomarkers

Except for the above-mentioned cardiac biomarkers, individual organ-specific markers have also been predictive of disease outcomes in AL amyloidosis patients. For patients with kidney involvement, two main biomarkers have been independently validated by two studies for their predictive value for end-stage renal disease when used in combination.^1,2 These are the estimated glomerular filtration rate (eGFR) from serum creatinine and proteinuria. Patients with both markers affected (eGFR < 50 mL/min and proteinuria > 5 g/24 h) have a risk of ≥ 50% of suffering from end-stage kidney disease within 3 years from diagnosis.¹

Efstathios Kastritis and colleagues presented, at the 62nd American Society of Hematology (ASH) Annual Meeting and Exposition, the validation of the soluble urokinase-type plasminogen activator receptor (suPAR) as a biomarker of renal outcomes in AL amyloidosis. suPAR levels and their variation were independently associated with renal progression and progression to dialysis either when measured at baseline or after 6 months. It was also independent of baseline renal stage, renal progression, and even hematologic response.⁴

Other heart-specific biomarkers that are prognostic of short-term overall survival (OS) in patients with AL amyloidosis are septum thickness, left ventricular longitudinal function, and left ventricular ejection fraction as evaluated by ultrasonography or imaging.¹

Liver-specific prognostic biomarkers are sparse and of lower quality when compared with heart- and kidney-related markers.

Tumor-related prognostic biomarkers

One commonly used tumor-related prognostic indicator of AL amyloidosis is the tumor burden, as assessed by the proportion of bone marrow plasma cell (PC) infiltration. Upon cytology or histology, PC infiltration of > 10–20% is predictive of worse progression-free survival and OS.¹

Serum levels of FLC have also been shown to predict patient outcomes in multiple studies, with the difference between involved and uninvolved FLC (dFLC) being a prognostic of patient OS. The dFLC ≥ 180 mg/L threshold is used as the third risk factor in the risk stratification Mayo 2012 model (Table 1). A high dFLC has been linked to more severe organ involvement and failure, especially of the heart.¹

Other tumor-associated biomarkers include tumor clones genetics with translocation t(11;14), gain of 1q21, deletion of 17p, any chromosomal aberration or trisomies, being all linked to adverse OS in patients with AL amyloidosis.¹

New prognostic biomarkers

To date, the above-mentioned cardiac biomarkers remain the most common prognostic for AL amyloidosis in patients with heart involvement. Nevertheless, there has been a lot of research into novel prognostic markers that could provide more or additional information over the classic cardiac markers hs-cTnT and NT-proBNP. Although still not widely validated and not yet used in routine practice, promising results have been published for the following biomarkers (Table 3 summarizes the currently identified novel biomarkers by univariate and multivariate analyses and their validation strength):

Table 3. Novel prognostic biomarkers for AL amyloidosis and their validation stregnth²

Novel biomarker	N of validation studies	N of patient cohorts	N of patients
AL, light chain; FMD, flow-mediated dilatation; GDF-15, growth differentiation factor 15; LAS, long axis strain; MCF, myocardial contraction fraction; RDW, red blood cell distribution width; sST2, soluble suppression of tumorigenicity-2; vWF:Ag, von Willebrand factor antigen.
GDF-15	2	3	107 + 202 + 73 = 382
vWF:Ag	1	1	111
sST2	2	2	502 + 73 = 575
Osteopontin	2	2	150 +73 = 223
MCF ± LAS	1	1	74
FMD brachial artery	1	2	115 + 55 = 170
RDW	1	1	94

• High levels of the growth differentiation factor 15 (GDF-15) were associated with a higher risk of early mortality and poor OS, independently of NT-proBNP and hs-cTnT, in three independent patient cohorts (GDF-15 ≥ 7.575 ng/mL or GDF-15 ≥ 2.3 ng/mL). Moreover, GDF-15 levels of ≥ 4000 pg/mL were also linked to a higher risk of progression to dialysis, independent of eGFR and proteinuria.²
• The antigen levels of von Willebrand factor antigen (vWF:Ag) may also provide a novel prognostic factor for OS, with patients having a worse OS when vWF:Ag ≥ 230 U/dL, and it was shown to be able to subdivide patients with Stage 3 disease into different prognostic groups, and even further with Stage 3b ultra-high risk disease.²
• Soluble suppression of tumorigenicity-2 (sST2) was found to predict OS, independent of NT-proBNP, serum FLC, or cTnT, in two independent patient cohorts, with sST2 ≥ 30 ng/mL being associated with a worse OS.²
• Osteopontin, associated with cardiovascular events in other diseases, was validated in two separate studies as another prognostic marker for OS in patients with AL amyloidosis. Each study assessed a different threshold level, with osteopontin ≥ 154.7 ng/mL or > 426.8 ng/mL being linked to poorer OS.²
• Increased flow-mediated dilatation of the brachial artery (≥ 4.5%) was associated with an increased risk of early mortality and poor OS in patients with AL amyloidosis.²
• Novel magnetic resonance imaging (MRI)-derived prognostic biomarkers include myocardial contraction fraction (MCF) with or without long axis strain (LAS). Patients with reduced MCF (≤ 55.5%) and LAS (≥ −6.4%) were identified as being at higher risk of mortality.²
• Last but not least, red blood cell distribution width (RDW) was identified as an independent predictor of OS in a small cohort study. Increased levels of RDW ≥ 13.8% were associated with worse OS in patients with AL amyloidosis. Interestingly, even in patients with no cardiac involvement, increased RDW was still associated with worse OS.^2,5

Conclusion

Cardiac related biomarkers, like NT-proBNP and hs-cTnT, provide the best prognostic value for patients with AL amyloidosis and heart involvement. Based on these biomarkers, three Mayo-based risk stratification models exist and are widely used.

Renal staging systems based on eGFR and proteinuria are also very valuable in stratifying patients with renal involvement. Nevertheless, multiple novel biomarkers have shown promising prognostic potential independently of the traditionally used biomarkers, providing hope for new models to discriminate further the prognosis of advanced-stage AL amyloidosis.

Such multi-biomarker risk models will hopefully aid with the management and treatment of all disease stages and facilitate the distinction of patients in need of novel therapeutic regimens.