The Complete Mutatome� and Clonal Architecture of Del(5q)

Cytogenetic abnormalities are found in around half of MDS patients (pts) and have both clinical impact and may be subtype-defining, e.g. in 5q-syndrome. Interstitial deletion of the long arm of chr.5 [del(5q)] is the most common aberration (almost 20% of cases with abnormal cytogenetics). Del(5q) is heterogeneous, occurring as a sole abnormality or in combination, with the deleted region often truncated within or extended and/or beyond the CDR boundaries.  Isolated del(5q) is frequently shorter and confers a more favorable prognosis with regard to survival and lenalidomide (LEN) responsiveness, while del(5q) in the context of a complex karyotype (CK) imparts a poor prognosis. In addition to chromosomal lesions, somatic mutations can contribute to the pathogenesis of MDS, including del(5q). We theorized that recognition of molecular defects in MDS with del(5q) may clarify the pathogenic mechanisms behind this lesion and help explain the clinical heterogeneity.

We analyzed 225 pts with myeloid neoplasia and del(5q) using WES (n=107 samples) and targeted multiplexed PCR (top 60 most frequently mutated genes) (n=133 samples); serial analysis was performed in 15 pts studied at ≥2 time points, 11 during LEN therapy and 4 upon relapse/progression. A total of 116 samples had a CK with other lesions such as -7/del(7q) found in 31% cases, and 18% had -17/del(17p).  

WES (average depth >60x) was followed by a bioanalytic pipeline, detecting ≥1 mutated gene in 71% of cases.  Candidate somatic alterations were found in 357 genes and selected for further analysis.  When focused on hemizygous mutations within the retained 5q allele, CSNK1A1 mutations were the most common, found in 4 pts, while other genes were only sporadically affected.   Among heterozygous mutations on the non-deleted portion of del(5q) and other chromosomes (Chr), we found several novel mutations, in addition to TP53 (n=26), DNMT3A (n=8), PRPF8 (n=8), RUNX1 (n=5), TET2 (n=5), and ASXL1 (n=4), among others. Furthermore, LOH/haploinsuffciency of genes on 7q (e.g., LUC7L2, CUX1, EZH2 and MLL3) appears to be a common defect seen in pts with non-isolated del(5q), suggesting synergistic functional defects. When functionally grouping gene mutations, DNA methylation family (8 cases) and transcription factor mutations (29 cases) were associated with advanced disease (AD) and a CK.  Heterozygous mutations in TP53 (34%) or deletions involving the TP53 locus (23%) resulted in total of 42% of cases carrying either TP53 LOH or mutation.  TP53 lesions were more common in pts with AD vs. low risk.  (21 vs. 5 p=.0008).  In contrast, TP53 mutations are found in 8-10% of cases of MDS.

A total of 34 pts were treated with LEN and subgrouped into responders (n=17) vs. refractory (n=9) with an overall response rate of 65%. When mutational profiles were compared, the presence of TP53 mutations did not preclude responsiveness to LEN. CK was present in 12% of responders vs. 67% of refractory pts. The most frequent Chr abnormalities were -7/7q (0% vs. 67% in responders vs. refractory) and 17p- (6% vs. 67% in responders vs. refractory) suggestive of their role in LEN resistance. 

In addition to cross sectional analysis, our WES study using paired Germline/tumor samples followed by deep sequencing facilitated analyses of clonal architecture by examining clonal dynamics over time.  Assessment of del(5q) clone size by allelic imbalance combined with clonal burden by VAF allowed us to reconstruct the clonal hierarchy:  in 73% of cases, del(5q) appeared to be the initial defect followed by subsequent mutations (e.g., TP53, DNMT3A, IDH2).  In contrast, in 24% of cases, TP53, RUNX1, JARID2, were the primary defect followed by a subclonal del(5q) events.   Serial samples collected before and after therapy demonstrated that responses were associated with decreased clonal burden for del(5q) but persistence of certain mutations.  In refractory cases, persistent subclonal lesions and the appearance of new lesions were associated with progression. For example, pts with TP53, LAMB4, EPHA6 progressed and acquired additional lesions such as CSMD2 or KCND2, and did not see the disappearance of TP53 alterations upon treatment.    

In conclusion, no unifying somatic defect was found in pts with del(5q) regardless if the deletion event was primary or subclonal.  Most commonly associated lesions were not present on the retained 5q alleles but rather other chr yet modified clinical behavior, including responsiveness to LEN.