Hyperdiploidy Is Rare in Patients with AL Amyloidosis – Identification of Major Cytogenetic Groups in Early Monoclonal Plasma Cell Disorders.

AL amyloidosis (AL) is characterized by the deposition of amyloid fibrils in diverse tissues due to an underlying monoclonal plasma cell dyscrasia. In a previous study (Bochtler et al, Blood 2008) we have demonstrated that in AL cytogenetic aberrations were detectable in about 90% of patients (pts). Translocation t(11;14) proved to be the most frequent aberration in AL found in 45% of the pts. In this study we evaluated whether the concept of hyperdiploidy and non-hyperdiploidy as major pathogenetic pathways in monoclonal gammopathy of undetermined significance (MGUS) and multiple myeloma (MM) is also applicable to AL. Our study was based on the largest patient group tested for cytogenetics in AL thus far including 184 pts with AL - among them 21 pts with concomitant MM I. They were assessed for their ploidy status by interphase fluorescence in situ hybridization (FISH). 179 MGUS and MM I pts not requiring therapy served as controls. We used a well established score (Wuilleme et al, Leukemia 2005), which requires extra copies for at least two out of the three probes 5p15/5q35, 9q34 and 15q22 as criterion for hyperdiploidy. The hyperdiploidy frequency was very low in AL with 14% as compared to 32% in MGUS / MM I (p<0.001). Among AL pts those with a concomitant MM I displayed a higher hyperdiploidy frequency than those without (43% versus 10%, p<0.001) suggesting that chromosomal gains reflect progression of the monoclonal plasma cell clone.

Addressing hyperdiploidy probes in detail, we could show that both in the 184 pts. with AL and 179 pts. with MGUS / MM I gains of 11q23, 17p13 and 19q13.3 closely clustered with the three hyperdiploidy defining probes 5p15/5q35, 9q34 and 15q22 (p'0.01 for all probes after adjusting for multiple testing). However, gain of 11q23 was also frequently detected in association with t(11;14). The group with gain of 11q23 subdivides into a t(11;14) positive and a hyperdiploidy positive subgroup in both the AL (p<0.001) and the MGUS / MM I (p<0.001) entities. As revealed by additional probes for 11p15 and 11cen, gain of 11q23 in hyperdiploid pts reflected a gain of the whole chromosome 11 in all tested pts (10 AL and 31 MGUS / MM I). On the contrary, gain of 11q23 in t(11;14) positive pts was merely due to the translocation involving chromosome 11 (with 25 out of 26 AL and 5 out of 7 MGUS / MM I pts displaying a normal diploid status for 11p15 and 11cen). Therefore, gain of 11q23 is a poor indicator of hyperdiploidy in AL, where t(11;14) frequencies are particularly high and hyperdiploidy frequencies are particularly low.

Addressing the cytogenetic clustering of hyperdiploidy with other cytogenetic aberrations we observed a strong inverse association of hyperdiploidy with t(11;14) in both AL and MGUS / MM I (p<0.001 in both entities). Accordingly, both aberrations were allocated to branches separating from each other already at the root in the oncogenetic tree model (see figure 1). Del13q14/t(4;14) and IgH translocations with an unknown partner also separated as distinct branches early from the root. These similar clustering patterns of both AL and MGUS / MM I with 4 major cytogenetic groups suggests common pathogenetic mechanisms in both entities despite their differing hyperdiploidy and t(11;14) frequencies.

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No relevant conflicts of interest to declare.