Beta-2 Microglobulin (B2 M) Genomic Alterations and Absent Protein Expression in Pediatric and Adolescent Classical Hodgkin Lymphoma

Question: Genomic analysis of classical Hodgkin lymphoma (cHL) has been limited by difficulty isolating sufficient numbers of Reed-Sternberg (RS) cells from reactive background tissue. Our group has overcome this by utilizing flow cytometry to isolate RS cells followed by deep sequencing with platform optimized for ultra-low input DNA (Reichel et al, ASH 2013). Whole exome sequencing of cHL cases using this methodology revealed recurrent inactivating alterations in B2 M, including alterations in 2 of 3 pediatric and adolescent cases. In an extended cohort of adult patients, B2 M alterations clustered with the nodular sclerosis (NS) histologic subtype and were associated with superior outcome. The incidence and clinical relevance of B2 M alterations in the pediatric/adolescent group has not been explored.

Methods: To characterize B2 M alterations in pediatric/adolescent cHL, immunohistochemistry for B2 M expression was performed on primary tumor (n = 51) and paired relapse (n = 2) samples in patients age ≤25 y. B2 M status was correlated with histologic subtype, clinical characteristics of disease, and patient outcome.

Results: Absence of B2 M expression was observed in 90% of cases, including those with known mutations in B2 M. Cases with absent B2 M spanned with pediatric/adolescent age range (median 21 y, range 6 – 25 y) with no age difference between B2 M + vs. – groups. Absent B2 M expression clustered with the NS subtype of disease (p < 0.01). In two cases with paired diagnostic/relapse specimens the B2 M status remained constant at both time points (B2 M +, n = 1, B2 M -, n = 1). With a median follow up of 47 months, progression free survival was 75% and 77% in the B2 M + and – groups respectively. Only one death occurred in this cohort, in a B2 M + patient.

Conclusions: Inactivating alterations in B2 M are present in a significant portion of pediatric/adolescent cHL. In contrast to adult cases, B2 M status is not correlated with age. B2 M – cases cluster with the NS histologic subtype and may define a unique molecular subset of disease. Given the excellent outcome in pediatric/adolescent cHL, investigations with larger cohorts are needed to determine the prognostic relevance of B2 M alterations.