**CORRESPONDENCE**

**OPEN**

CYTOGENETICS AND MOLECULAR GENETICS

# Response to the Comments from the Groupe Francophone de Cytogénétique Hématologique (GFCH) on the 5th edition of the World Health Organization classification of haematolymphoid tumors

© The Author(s) 2023

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*Leukemia* (2023) 37:1170–1172; https://doi.org/10.1038/s41375-023-01872-6

## TO THE EDITOR:

We thank Nguyen-Khac et al. [1], who, on behalf of the Groupe Francophone de Cytogénétique Hématologique (GFCH), provide thoughtful comments on the summary of the 5th Edition of the World Health Organization (WHO) Classification of Haematolymphoid Tumors (WHO-HAEM5) [2, 3] and express that the new edition is a reflection of "*the most up-to-date-knowledge on the topic*". The comments in the Letter to the Editor touch on several important aspects of WHO-HAEM5, with emphasis on genetic aspects of the classification. As a representative group of editors who primarily oversaw topics raised in the Letter, we provide this response on behalf of the rest of our esteemed colleagues who contributed to the classification. We take the opportunity to also add some explanatory comments on certain issues tangentially related to the topics raised by the GFCH.

## CHROMOSOMAL ABNORMALITIES IN DISEASE NAMES

The GFCH states that "*chromosomal abnormalities [...] have been removed from the WHO-HAEM5 classification of most of the hematological malignancies with defining genetic abnormalities caused by this type of defect—especially acute leukemia*". We would like to clarify that although the nomenclature of genetically defined entities in WHO-HAEM5 no longer includes chromosomal translocations, the importance of chromosomal abnormalities and their detection using chromosomal banding analysis has not been diminished at all. Indeed, chromosomal abnormalities are discussed in detail in essentially all sections in which they are relevant, as can be seen in the beta version of the classification available online (https://tumourclassification.iarc.who.int). We completely agree with the GFCH that chromosome banding analysis continues to contribute fundamental knowledge in the management of patients with hematological cancers. Indeed, WHO-HAEM5 consistently emphasizes the notion that a variety of diagnostic tools may be helpful to detect a particular disease-defining abnormality. However, on balance, the WHO-HAEM5 editorial board determined that including chromosomal abnormalities alongside disease-defining genetic alterations in the name of a disease type would add unnecessary complexity and redundancy.

## FUSION AND OTHER TERMINOLOGY USED FOR DEREGULATED GENES

The GFCH proposes that the term "juxtaposition" is more suitable than "fusion" when referring to the deregulation of an intact oncogene. Here we should note that this comment effectively takes issue with the guidelines of the HUGO Gene Nomenclature Committee (HGNC) [4], which is independent from WHO-HAEM5 but has been adopted as a standard therein. Notwithstanding, we do agree with the GFCH that the HGNC recommendations in this regard lead to the loss of important information, as different pathogenetic mechanisms cannot be distinguished. Namely, alterations leading to bona fide fusion genes, i.e., a fusion transcript translated into a fusion protein from an in-frame link of two partial genes, are not differentiated from enhancer-hijacking mechanisms that juxtapose a full coding region of a physiologically existing gene next to the regulatory regions of another gene resulting in dysregulated expression of the gene and its encoded full-length protein. This distinction is certainly highly meaningful from a cancer genetics and molecular pathogenesis point of view. Alas, such genetic precision is often lost in routine practice. For example, commercial assays for the detection of enhancer-hijacking mechanisms use the term "fusion assay" (like dual-color dual fusion FISH assays e.g., for the named IG::MYC and IGH::CCND1 "fusions"). In addition, some enhancer-hijacking mechanisms, like several IGH translocations, have been shown to express chimeric transcripts joining both partner loci, though these are not bona fide gene fusions. Thus, the term "gene fusion" has been adopted in the WHO Classification as an umbrella term for structural chromosomal variants contributing to onco-genesis through bona fide gene fusion or enhancer-hijacking mechanisms.

Along similar lines, we acknowledge that the term "rearrangement" is applied more widely than defined *sensu strictu*. The term "rearrangement" narrowly defined refers to the genomic assembly of gene segments to a functional gene as typical for *IG* and *TR* loci. In this regard, reporting *IG* or *TR* rearrangements always needs to include information on the pattern of rearrangement, i.e., monoclonal, oligoclonal or polyclonal. Acknowledging the terminology commonly used in daily practice, here also the WHO-HAEM5 adopts the term "rearrangement" as an umbrella term for all kinds of pathogenic structural genomic variants affecting a given gene, including chromosomal translocations, inversions, copy number variants, etc. As this can lead to some ambiguity regarding the type of genomic variants (e.g., changes in gene
dosage vs. gene regulation) and their consequences (e.g., bona
fide gene-fusion vs. enhancer hijacking), the reader is referred to
the exact definitions of the meaning for a given entity within
respective sections of the Blue Book.

We would like to take the opportunity to also point that the
WHO-HAEM5 follows the HGNC guidelines regarding italicizing
immunoglobulin (IGH, IGK, IGL) and T-cell receptor (TRA, TRB, TRD,
TRG) loci [4]. In contrast, mutated gene products, i.e., mutated
proteins, are not italicized.

CLONAL CYTOPENIA OF UNDETERMINED SIGNIFICANCE
(CCUS) AND MYELODYSPLASTIC NEOPLASMS (MDS)

Formal inclusion of clonal haematopoiesis of indeterminate
potential (CHIP) and clonal cytopenia of undetermined signifi-
cance (CCUS) as distinct disease types in WHO-HAEM5 provides an
opportunity to improve the demarcation between these precursor
conditions and myelodysplastic neoplasms (MDS). Within this
framework, CCUS includes cases with somatic mutations in
myeloid malignancy-associated genes meeting the criteria of
CHIP and/or clonal chromosomal abnormalities in a patient with
one or more cytopenia(s) but without morphologic dysplasia. On
the other hand, the prerequisites for the diagnosis of MDS
fundamentally include cytopenia(s) and morphologic dysplasia.
The authors and editors of WHO-HAEM5 contend that this
framework will continue to identify patients with bona fide
MDS, while avoiding over-diagnosis of patients with precursor
myeloid states and rare cases of transient clonal chromosomal
abnormalities [5].

The CCUS/MDS diagnostic framework in WHO-HAEM5 does
align in principle with the proposal by Brett et al. [6]. Indeed,
WHO-HAEM5 establishes that any non-recurrent clonal abnormal-
ality may define CCUS in the absence of morphologic dysplasia.
WHO-HAEM4R and other more recent proposals have made
exceptions to this premise and allow bypassing the dysplasia
requirement when certain cytogenetic alterations are detected,
including: complex karyotype; 5q deletion or loss of 5q due to
unbalanced translocation; monosomy 7, 7q deletion, or loss of 7q
due to unbalanced translocation; or inv(3q)/t(3;3). The WHO-
HAEM5 editors and authors contend that such an exception is
unnecessary, since cases harboring such abnormalities hardly ever
occur without concomitant dysplasia and other clear stigmata of
MDS. Indeed, the detection of such abnormalities in a patient who
does not clearly meet the diagnostic criteria for MDS should raise
concern and prompt careful diagnostic evaluation and close
follow up. The detection of inv(3q)/t(3;3) should prompt assess-
ment for MECOM rearrangement, which, if present, would support
a diagnosis of acute myeloid leukemia with MECOM rearrange-
ment. We do concur with the authors that it would not be
practical to refer to clonal abnormalities in "myeloid cells".

B-CELL PROLYMPHOCYTIC LEUKEMIA

The authors raise several questions regarding B-cell prolympho-
cytic leukemia (B-PLL) and raise concern that it is no longer
recognized as an entity in WHO-HAEM5. Nguyen-Khac et al. [1]
would not be surprised to know that this topic generated vivid
and thoughtful discussions. In the end, the consensus among
oncologists, haematopathologists, and geneticists was that B-PLL
lacked sufficient distinction to warrant retaining it as a distinct
disease type. We acknowledge of course that WHO-HAEM4R had
already removed MCL with nucleolated cells resembling pro lymph-
ocytes from B-PLL. This left CD5-positive B-cell lymphoid
proliferations with >15% prolymphocytes divided into two
categories: atypical CLL (CLL/PLL) with ≤55% prolymphocytes;
and B-PLL with >55% prolymphocytes. The latter category is also
heterogeneous; it includes cases that have prolymphocytes that
are morphologically identical to those seen in CLL and other cases

characterized by a uniform population of medium to large
lymphoid cells with prominent nucleoli. Here it should be noted
that the 55% prolymphocyte count used to distinguish between
atypical CLL and B-PLL had been set arbitrarily and its value is
likely impossible to validate at present. Moreover, so-called B-PLL
cases were highly heterogeneous in terms of immunophenotype
and genetic features. MYC aberrations, TP53 disruption, trisomy 12,
del(8p), and under-representation of del(13q) show strong
resemblance to poor-risk CLL, albeit they remain distinct from
diffuse large B-cell lymphoma-type Richter transformation [7].
Together, these findings support classifying such cases as
prolymphocytic transformation of CLL.

This leaves a group of CD5-negative, mostly splenic, lymphomas
with prominent nucleoli that could not be grouped with more
common diseases. These were grouped together as a new
" placeholder" entity called splenic B-cell lymphoma/leukemia with
prominent nucleoli (SBLPN). This new entity includes some cases of
splenic marginal zone lymphoma, CD5-negative cases of B-PLL
and HCLv. The rationale for including HCLv in SBLPN is that it
classically shows prominent nucleoli but does not have clinical,
immunophenotypic, or genetic features of HCL. We acknowledge
that this is based on the evaluation of only a total of 15 cases with
HCLv [8]. Of these, five had mutation and/or deletion of TP53
compared to none in HCL. The presence of TP53 mutation/
deletion was strongly associated with poor outcomes in these
patients, making TP53 the most clinically relevant disease driver in
this entity. Interestingly, cases formerly labelled as CD5-negative
B-PLL also have prominent nucleoli with frequent TP53 disruption
conferring poor prognosis. TP53 and MYC alterations are classically
associated with progression and/or transformation, and their
frequent occurrence in SBLPN may point to a category of
"progressed" lymphomas distinct from "transformation", as
discussed in the Blue Book section on transformed indolent
lymphomas.

Our approach to B-PLL and SBLPL has aimed to be systematic
and evidence-based while also being pragmatic. Namely, we had
to consider the clinical perspective that splitting rare poor-risk
patient groups into distinct disease entities adds barriers to clinical
trial design and eligibility, whereas relating them—where appro-
priate—to more established entities (such as MCL, CLL) may
ultimately fill the evidence gap. These classification challenges
reflect the inherent difficulty in deriving clear disease types from
small patient groups and heterogeneous genetic studies. In this
context, we acknowledge that SBLPN is best regarded as a
placeholder type that will require additional studies to identify
distinctive features that could inform future editions of the
classification.

CONCLUDING REMARKS

In conclusion, we thank the GFCH for highlighting the value of
WHO-HAEM5 classification and for raising important issues worthy
of continued discussions. Our response is intended to provide
some clarification regarding the points raised and shed light on
some related decision processes that shaped the WHO-HAEM5
classification. We would like to conclude by pointing the GFCH
and the readers to additional details in the "epilogue" of the
summary paper by Alaggio et al. [3] in which we discuss the
limitations and the opportunities of any classification, including
WHO-HAEM5.

DISCLAIMER

The content of this paper represents the personal views of the
authors and does not represent the views of the authors'
employers and associated institutions. This work is intended to
provide a preview and summary of content whose copyright
belongs solely to the International Agency for Research on Cancer/
World Health Organization. Any or all portions of the material in
this work may appear in future International Agency for Research
on Cancer/World Health Organization publications.

Reiner Siebert ID ¹✉, Anna Schuh ID ², German Ott³, Ian A. Cree ID ⁴,
Ming-Qing Du ID ⁵, Judith Ferry⁶, Andreas Hochhaus ID ⁷,
Kikkeri N. Naresh ID ⁸, Eric Solary ID ⁹ and Joseph D. Khoury ID ¹⁰
¹Institute of Human Genetics, Ulm University and Ulm University
Medical Center, Ulm, Germany. ²Department of Oncology, University
of Oxford, Oxford, UK. ³Department of Clinical Pathology, Robert-
Bosch-Krankenhaus, and Dr. Margarete Fischer-Bosch Institute of
Clinical Pharmacology, Stuttgart, Germany. ⁴International Agency for
Research on Cancer (IARC), World Health Organization, 25 Avenue
Tony Garnier, CS 90627, 69366 Lyon, France. ⁵Division of Cellular and
Molecular Pathology, Department of Pathology, University of
Cambridge, Cambridge, UK. ⁶Department of Pathology,
Massachusetts General Hospital and Harvard Medical School, Boston,
MA, USA. ⁷Hematology/Oncology, Universitätsklinikum Jena, Jena,
Germany. ⁸Section of Pathology, Clinical Research Division, Fred
Hutchinson Cancer Center, Seattle, WA, USA. ⁹Department of
Hematology, Gustave Roussy Cancer Center, Université Paris-Saclay,
Villejuif, France. ¹⁰Department of Pathology and Microbiology,
University of Nebraska Medical Center, Omaha, NE, USA.
✉email: reiner.siebert@uni-ulm.de

REFERENCES

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2. Khoury JD, Solary E, Abla O, Akkari Y, Alaggio R, Apperley JF, et al. The 5th edition of the World Health Organization Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms. Leukemia. 2022;36:1703–19.

3. Alaggio R, Amador C, Anagnostopoulos I, Attygalle AD, Araujo IBO, Berti E, et al. The 5th edition of the World Health Organization Classification of Haematolymphoid Tumours: Lymphoid Neoplasms. Leukemia. 2022;36:1720–48.

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6. Brett VE, Lechevalier N, Trimoreau F, Dussiau C, Dimicoli-Salazar S, Coster L, et al. The presence of a chromosomal abnormality in cytopenia without dysplasia identifies a category of high-risk clonal cytopenia of unknown significance. Genes Chromosom Cancer. 2023;62:139–51.

7. Chapiro E, Pramil E, Diop M, Roos-Weil D, Dillard C, Gabillaud C, et al. Genetic characterization of B-cell prolymphocytic leukemia: a prognostic model involving MYC and TP53. Blood. 2019;134:1821–31.

8. Hockley SL, Morgan GJ, Leone PE, Walker BA, Morilla A, Else M, et al. High-resolution genomic profiling in hairy cell leukemia-variant compared with typical hairy cell leukemia. Leukemia. 2011;25:1189–92.

AUTHOR CONTRIBUTIONS

SDK is standing member of the WHO Classification of Tumors editorial board. RS, AS, GO, MQD, AH, KNN, and ES are expert members of the Haematolymphoid Tumors 5th edition blue book editorial board. All listed authors edited and approved the paper.

FUNDING

Open Access funding enabled and organized by Projekt DEAL.

COMPETING INTERESTS

All authors underwent IARC clearance for potential competing interests regarding this work.

ADDITIONAL INFORMATION

Correspondence and requests for materials should be addressed to Reiner Siebert.

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