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From left to right: Study
authors Ilaria Iacobucci,
Charles Mullighan, Hiroto
Inaba, and Zhaohui Gu
Photo from St. Jude
Children’s Research Hospital

An extensive analysis of mixed phenotype acute leukemia (MPAL) has led to new insights that may have implications for disease classification and treatment.

Researchers believe they have identified new subtypes of MPAL that should be included in the World Health Organization (WHO) classification for acute leukemia.

Each of these subtypes shares genomic characteristics with other acute leukemias, which suggests the new subtypes might respond to treatments that are already in use.

This research has also shed light on how MPAL evolves and appears to provide an explanation for why MPAL displays characteristics of both acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL).

“ALL and AML have very different treatments, but MPAL has features of both, so the question of how best to treat patients with MPAL has been challenging the leukemia community worldwide, and long-term survival of patients has been poor,” said Charles Mullighan, MBBS, MD, of St. Jude Children’s Research Hospital in Memphis, Tennessee.

With these issues in mind, Dr. Mullighan and his colleagues conducted their study of MPAL and described their findings in Nature.

New classifications

The researchers used whole-genome, whole-exome, and RNA sequencing to analyze 115 samples from pediatric patients with MPAL.

The analysis revealed mutations that define the two most common subtypes of MPAL—B/myeloid and T/myeloid—and suggested these subtypes share similarities with other leukemia subtypes.

The researchers found that 48% of B/myeloid MPAL cases carried rearrangements in ZNF384, a characteristic that is also found in cases of B-cell ALL. In fact, the team said the gene expression profiles of ZNF384r B-ALL and ZNF384r MPAL were indistinguishable.

“That is biologically and clinically important,” Dr. Mullighan said. “The findings suggest the ZNF384 rearrangement defines a distinct leukemia subtype, and the alteration should be used to guide treatment.”

The researchers noted that patients with ZNF384r exhibited higher FLT3 expression than patients with other types of B/myeloid or T/myeloid MPAL, so patients with ZNF384r MPAL might respond well to treatment with a FLT3 inhibitor.

This study also showed that cases of B/myeloid MPAL without ZNF384r shared genomic features with other B-ALL subtypes, such as Ph-like B-ALL, which may have implications for treatment.

Another of the researchers’ discoveries was that T/myeloid MPAL and early T-cell precursor ALL have similar gene expression profiles.

The team identified several genes that were mutated at similar frequencies in T/myeloid MPAL and early T-cell precursor ALL, including WT1, ETV6, EZH2, and FLT3. WT1 was the most frequently mutated transcription factor gene in T/myeloid MPAL.

Based on these findings, the researchers said the WHO classification of acute leukemia should be updated to include:

  • ZNF384r acute leukemia (either B-ALL or MPAL)
  • WT1-mutant T/myeloid MPAL
  • Ph-like B/myeloid MPAL.

Evolution of MPAL

The researchers’ analyses also revealed leukemia-initiating genetic alterations in early hematopoietic progenitors.

The team said this and other findings—including the common genomic features of ZNF384r MPAL and B-ALL—suggest the ambiguous phenotype of MPAL results from alterations in immature hematopoietic progenitors.

“These findings suggest that the founding mutation occurs early in blood cell development, in some cases in hematopoietic stem cells, and results in an acute leukemia with features of both myeloid and lymphoid cells,” said study author Thomas Alexander, MD, of the University of North Carolina at Chapel Hill.

“One previous theory was that the reason you have two different cancer types within the same patient is that they acquire different mutations that drive them to become AML or ALL, with genomically distinct tumors within the same patient. That doesn’t seem to be the case from our data. Our proposed model is that the mutations occur earlier in development in cells that retain the potential to acquire myeloid or lymphoid features.”

This research was supported by the National Cancer Institute, the National Institutes of Health, Cookies for Kids’ Cancer, and other organizations.

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