Page 233 - Haematologica Vol. 107 - September 2022

LETTER TO THE EDITOR
Pure erythroid leukemia is characterized by biallelic TP53 inactivation and abnormal p53 expression patterns in de novo and secondary cases
Pure erythroid leukemia (PEL) is a rare type of acute mye- loid leukemia (AML) characterized by a neoplastic prolif- eration of immature erythroblasts associated with a complex karyotype and a poor prognosis.1-3 PEL can arise de novo, but more frequently occurs as a therapy-related neoplasm or transformation from myelodysplastic syn- dromes (MDS).4,5 To date, the potential differences be- tween de novo and secondary (therapy-related or MDS-derived) PEL have not been well explored. Recent studies have shown that TP53 mutations are common in PEL1,5 and that p53 overexpression is also frequent.6 Strong p53 expression shown by immunohistochemistry has become an important clue in the initial workup of PEL. However, we have observed some PEL cases lacking p53 expression despite the presence of TP53 mutations. We conducted the current study to investigate TP53 mutation characteristics and p53 protein expression in PEL and to examine whether secondary PEL cases differ from de novo disease. Another aim of this study was to address PEL as an entity that should be histopathologically and geneti- cally defined, versus classifying these cases following the current classification scheme in which a history of prior chemoradiation therapy or MDS is relatively more empha- sized.
We collected 22 cases of PEL, defined by a predominant proliferation of neoplastic erythroblasts that formed sheets in bone marrow, of which 30% or more were pro- normoblasts. The clinical characteristics of our patients are summarized in Table 1. There were 14 men and eight women with a median age of 69 years (range, 37-81). Eleven (50%) patients had a history of chemotherapy for other malignancies (therapy-related), five (23%) had MDS, one (4%) had primary myelofibrosis (PMF), and five (23%) occurred de novo. Among 21 patients with treatment in- formation available, five (24%) did not receive any therapy due to poor performance status or poor response to prior treatments for MDS. The remaining 16 patients received treatments after the diagnosis of PEL (Table 1). Four (25%) patients achieved a complete response with incomplete hematologic recovery; the response in three patients (#3, 12, 20) was transient and one patient (#22) remained in complete response with incomplete hematologic recovery at last follow-up, 4.7 months after diagnosis. None of the patients was eligible for stem cell transplantation. Twenty-one patients had clinical follow-up: 20 had died at last follow-up and one patient (#22) was alive. The
median survival time for this entire cohort was 2.8 months (range, 0.2-7.3); 2.3 months (range, 0.2-7.3) for therapy-re- lated, 2.6 months (range, 0.4-4.9) for patients with a his- tory of MDS, and 3.9 months (range, 2.2-5.5) for de novo PEL.
Targeted next generation sequencing (NGS) with panels composed of genes commonly mutated in myeloid neo- plasms was performed on bone marrow samples from 20 patients (19 using an 81-gene panel and 1 using a 28-gene panel) at the time of PEL diagnosis as previously de- scribed.7 One case was tested for TP53 mutation using Sanger sequencing. In total, 21 cases were tested and all patients had TP53 mutation(s) (Table 2). A total of 25 TP53 mutations were detected: 18 patients had one TP53 mu- tation, two patients (#15 and 17) had two mutations, and one (#5) patient had three mutations. Twenty-two (88%) mutations occurred in the DNA binding domain (exons 5- 8), including 12 in exon 5, one in exon 6, four in exon 7, and five in exon 8. The remaining three mutations oc- curred in exon 4, exon 10, and a splice site, respectively. The types of TP53 mutations included 19 (76%) missense, 1 (4%) nonsense, 1 (4%) splice site, and 4 (16%) small dele- tion. Among the four cases with small deletion mutations, three caused frameshift. The median variant allele fre- quency (VAF) of TP53 mutations was 35% (range, 1-92.3%). One patient (#13) was not assessed for TP53 mutation, but immunohistochemistry showed strong and diffuse p53 ex- pression, suggestive of TP53 mutations. The detailed TP53 mutational profiles are summarized in Table 2. Among the 16 patients who received treatment, 11 (#3, 4, 7, 8, 11, 15, 17, 18, 20-22) had repeat TP53 mutation analysis by NGS after treatment and all showed persistent TP53 mutations. Among the 20 cases tested by NGS, additional gene mu- tations were detected in nine (45%) patients (Online Sup- plementary Table S1), including DNMT3A (n=3; VAF 10.3-29.3%), NRAS (n=2; VAF 5% and 26.6%), TET2 (n=1; VAF <3%), FLT3 (n=1; VAF 1.7%), PRPF40B (n=1; VAF 40.8%), KMT2A (n=1; VAF 15.2%) and GATA2 (n=1; VAF 1.8%). Patient #22 had a history of PMF that was positive for JAK2 V617F (VAF 32%) and negative for TP53. At the time of progres- sion to PEL, JAK2 V617F was detected with a VAF of 1%, and TP53 mutation was acquired (VAF 80.6%).
Twenty cases underwent conventional karyotyping at the time of PEL diagnosis and all (100%) had complex karyo- types (Online Supplementary Table S1). Among 19 cases with karyotype data available, 12 (63%) had -5/5q-, 12
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