Single Molecule Real-Time Sequencing of the M Protein (SMaRT M-Seq): Toward Personalized Medicine Approaches in Monoclonal Gammopathies

Introduction

In patients affected by monoclonal gammopathies, tumoral B cells or plasma cells secrete a monoclonal antibody (termed M protein), which can be used to track the presence of the tumor itself. Moreover, the M protein can directly cause potentially life-threatening organ damage, which is dictated by the specific, patient's unique clonal light and/or heavy chain, as in patients affected by immunoglobulin light chain (AL) amyloidosis. Yet, the current paradigm in the diagnosis and management of these conditions treats the M protein as a simple tumor biomarker to be identified/quantified. Patients' specific M protein sequences remain mostly undefined and molecular mechanisms underlying M-protein related clinical manifestations are largely obscure.

Methods

By combining the unbiased amplification of expressed immunoglobulin genes with long-read, single molecule real-time DNA sequencing and bioinformatics analyses, we have established a method to identify the full-length sequence of the variable region of expressed immunoglobulin genes and to rank the obtained sequences based on their relative abundance, thus enabling the identification of the full-length variable sequence of M protein genes from a high number of patients analysed in parallel.

Results

The assay, which we termed Single Molecule Real-Time Sequencing of the M protein (SMaRT M-Seq), has undergone an extensive technical validation. Sequencing of contrived bone marrow samples generated through serial dilutions of plasma cell lines into control bone marrow, as well as sequencing of bona fide bone marrow samples from AL patients and comparison with gold-standard techniques of immunoglobulin gene sequencing showed:

• 100% sequence-accuracy at the individual base-pair level;

• High repeatability (CV<0.8% for sequencing of pentaplicates) in defining the molecular clonal size (i.e. the fraction of total immunoglobulin sequences coinciding with the clonal sequence);

• A high sensitivity in identifying clonal immunoglobulin sequences (10 ^-3 when employing low-coverage sequencing on multiple, pooled samples).

Noteworthy, SMaRT M Seq was applied to a cohort of 86 consecutive patients with AL amyloidosis (17 κ and 69 λ; median BMPC infiltration 9%, IQR 6-13%; median dFLC 176 mg/L, IQR 75-370 mg/L), including cases with small clonal burden and M protein which was undetectable with conventional M protein studies. A full-length sequence of the variable region of the clonal light chain was obtained in all patients (median molecular clonal size of 88.3%, IQR: 70.7 - 93%). The most common κ germline genes were IGKV1-33 and IGKV4-01 (24% each of the 17 κ AL patients), and the most common λ germline genes were IGLV6-57 (26% of the 69 λ AL patients), IGLV2-14 (17%), IGLV3-01 (17%) and IGLV1-44 (10%). The most frequent λ and κ germline genes together (IGLV6-57, IGLV2-14, IGLV3-01, IGLV1-44, IGKV1-33 and IGKV4-01) accounted for 66% of all the clones. Germline gene usage correlated with selected clinical features.

Sequence information was then exploited to improve mass spectrometry-based amyloid typing on fat pad aspirates and to enable the sensitive detection of clonotypic sequences using short-read DNA sequencing of the involved light chain isotype (up to 10 ^-7 dilution).

Conclusions

We have established SMaRT M-Seq as a novel valuable assay to reliably identify the full-length variable sequence of M proteins. SMaRT M-Seq has undergone extensive technical validation, showing high accuracy, repeatability and sensitivity. The latter is determined by the number of reads analyzed per sample. This is in turn dictated by the sequencing output of the employed sequencing platform, and by the number of pooled samples analyzed in a given sequencing round, thus proving to be scalable. Even when analyzing multiple samples on a sequencing platform with low sequencing output, the achieved sensitivity of SMaRT M-Seq significantly exceeds the requirements for the identification of clonal B cells/plasma cells in patients with AL amyloidosis.

Sequencing disease-associated M proteins from large cohorts of patients has the potential to uncover molecular mechanisms of M protein-related clinical manifestations which have remained largely unexplored so far, and could enable approaches of personalized medicine for the sensitive detection of patients' specific M proteins at diagnosis and after anti-clonal therapy.