1. B-ALL with DUX4 Rearrangement
B-ALL with
DUX4 rearrangement is one of the newer described subtypes of B-ALL.
DUX4-rearranged B-ALL comprises 16% of the B-other cases or 4% of all pediatric B-ALL
[28][1]. In B-ALL in AYA in Japan,
DUX4,
ZNF384, and
MEF2D fusion genes account for about 40% of Ph-negative cases
[29][2]. In B-ALL in Malaysia and Singapore,
DUX4-rearranged B-ALL is the third-most-common subtype
[130][3]. This leukemia has a favorable prognosis, similar to B-ALL, with high hyperdiploidy and
ETV6::RUNX1 fusion, despite the presence of high MRD levels
[130,131][3][4].
The
DUX4 gene encodes a homeobox-containing protein and is located within a subtelomeric D4Z4 repeat region on 4q and 10q. The gene is present in 11–100 copies on each allele and is epigenetically silent in somatic tissues. The
DUX4 rearrangement occurs most frequently with
IGH and less frequently with the
ERG gene. In the
IGH::
DUX4 fusion, a segment of the
DUX4 gene is relocated to
IGH, leading to the overexpression of DUX4. This rearranged form of
DUX4 binds with a genetic region in the ETS-family transcription factor
ERG (ETS-related gene), which leads to the expression of an
ERG protein fragment that inhibits normal ERG function and causes leukemic transformation.
ERG deletions are frequent secondary alterations in
DUX4-rearranged B-ALL
[28,29,132][1][2][5]. Also,
IKZF1 deletions co-occur with
ERG deletions in
DUX4-rearranged B-ALL. As the prognosis of
IKZF1 deletions depends on the co-occurring mutations in B-ALL, the usually adverse prognosis of
IKZF1 deletions can be overcome in these patients by chemotherapy based on MRD evaluation
[133][6].
DUX4 rearrangements in B-ALL are complex and different from those in
CIC::DUX4 fusion-positive (non-Ewing) round-cell sarcoma (sarcoma described in
[134][7])
[28][1]. The complexity of the genetic rearrangement is likely to be the reason why these abnormalities were not detected in the pre-genomics era. The gene expression profile for
DUX4-rearranged B-ALL is distinctive
[28][1]. Flow cytometry showed strong (aberrant) surface expression of CD371 on the leukemic cells in
DUX4-rearranged B-ALL, which, when combined with the expression of CD2, diagnosed all cases of this type of B-ALL
[135][8]. CD371 is predominantly expressed on myeloid cells
[135][8] and is not expressed on mature lymphocytes (see image in
[36][9]).
DUX4-rearranged leukemic cells may also express CD66c, and the co-expression of CD66c and CD2 was almost exclusively found in
DUX4 fusion-positive B-ALL
[136][10]. An immunohistochemical stain for detecting
DUX4 fusions showed immunohistochemical positivity in five of six molecularly-positive cases and negativity in three of three molecularly-negative cases
[137][11].
DUX4-rearranged B-ALL leukemic cells may switch to monocyte-like cells, which is a feature of CD371 expression
[135[8][11],
137], and this switch does not lead to a worse outcome
[138][12].
The WHO-HAEM5 diagnostic criteria require RNA or DNA sequencing by NGS to diagnose this type of B-ALL. The desirable criteria include confirming the
DUX4 gene rearrangement, the presence of CD371 expression on leukemic cells by FCI, or both
[36][9]. It is noteworthy that while RNA sequencing can diagnose
DUX4 fusions, the most extensive study of
DUX4-rearranged B-ALL patients examined by whole-genome sequencing (WGS) in a single clinical trial in the U.K. showed that whole-transcriptome sequencing alone could not be relied upon to identify all
DUX4-rearranged B-ALL cases in the absence of WGS. These investigators established an automated bioinformatics pipeline that improved the detection of
DUX4 fusions by WGS
[139][13].
2. B-ALL with ZNF384 Rearrangement
The zinc finger protein 384,
ZNF384, gene is located on the chromosomal locus 12p13.31. The gene encodes for a zinc finger transcription factor that is ubiquitously expressed in the bone marrow and other tissues. The transcription factor appears to bind and regulate the promoters of the extracellular matrix genes
[140][14].
ZNF384 rearrangements may occur with at least ten different gene partners in about 5% of childhood B-ALLs, 10% of adult B-ALLs, and 48% of mixed-phenotype acute leukemia, B/myeloid-type
[141,142][15][16].
In Japan,
ZNF384-related fusion genes were identified in 4.1% of 291 B-ALL or about 9% of B-other ALL patients. All
ZNF384-related gene fusions, including
TCF3::ZNF384 and
EP300::ZNF384, showed weak or negative CD10 expression with aberrant CD13 and CD33 expression. But the clinical features differed depending on the specific fusion gene. Higher cell counts, younger age (median age five years), and more frequent relapses were present in
TCF3::ZNF384-positive than in
EP300::ZNF384-positive B-ALL patients. The latter group of B-ALL patients had a median age of 11 years
[30,142][16][17]. FISH with break-apart probes or genomic sequencing (RNA or DNA) is required to diagnose the cryptic
ZNF384 rearrangement
[36][9].
3. B-ALL with MEF2D Rearrangement
Myocyte-enhancer factor 2 (Mef2) transcription factors are necessary for early B-cell development
[143][18].
MEF2D, located on 1q22, encodes one of these transcription factors.
MEF2D was found to be rearranged in about 5% of pediatric B-ALL without recurring genetic abnormalities.
MEF2D can rearrange with multiple genes (
BCL9,
CSF1R,
DAZAP1,
HNRNPUL1, and
SS18), with
BCL9, located on 1q21, being the most frequent.
MEF2D::BCL9-rearranged B-ALL presents at a median age of 14 years. Morphologically, the leukemic cells appear to be mature B-cell leukemia-like cells with high expression of HDAC
[144][19]. They have a characteristic immunophenotype with weak or absent CD10, CD38 positivity, and cytoplasmic IgM positivity. The cytogenetic rearrangement is cryptic by karyotyping, and diagnosis requires FISH, gene expression profiling, or genomic sequencing. There is resistance to chemotherapy, with very early relapse in this high-risk leukemia
[32,35,144][19][20][21].
4. B-ALL with PAX5alt and B-ALL with PAX5 p.P80R
The PAX5 gene encodes for a transcription factor that regulates numerous genes essential for normal B cell development. B-ALL with PAX5alt and B-ALL with PAX5 p.P80R refer to two distinct types of B-ALL. Both of these types of B-ALL harbor molecular genetic abnormalities in PAX5, which lead to a loss of the normal PAX5 protein, initiating a precursor B lymphoblastic leukemia.
B-ALL with
PAX5 p.P80R is unique because this subtype of B-ALL is characterized by a single point mutation in PAX5 instead of the other types of abnormalities that are common in B-ALL, such as deletions and translocations. This point mutation, c.239C>G, p.P80R, causes a substitution of proline to arginine in the DNA-binding domain of
PAX5. In a cohort of 170 adult B-ALL cases that were negative for the known genetic abnormalities in B-ALL, gene expression data profiling showed four clusters corresponding to B-ALL with rearranged
ZNF384,
DUX4,
KMT2A, and
BCR::ABL1-like features
[145][22]. A fifth cluster in this study comprised 14 patients with
PAX5 p.P80R and lacked any fusion gene. Sanger sequencing identified 16 additional cases with
PAX5 p.P80R in another cohort
[145][22]. Cytogenetics showed structural rearrangements of 9p or 7p, including dic(9;20) and der(7;9). The second allele was deleted or inactivated, leading to biallelic loss of PAX5
[34,145][22][23]. Mutations of genes in the RAS pathway were also present
[34,145][22][23].
B-ALL with
PAX5alt includes leukemia-causing genetic abnormalities other than
PAX5 p.P80R. This type of B-ALL has a gene expression profile distinct from that of B-ALL with
PAX5 p.P80R
[34][23]. It comprises about 3–5% of childhood ALLs and 9.6% of adult B-ALLs.
In contrast, B-ALL with
PAX5 p.P80R comprises about 1% of childhood B-ALLs and up to 5% of adult B-ALLs. By FCI, B-ALL with
PAX5 p.P80R shows a pro-B immunophenotype, with low CD20 and high CD45 expression on the leukemic cells. The leukemic cells are CD13-negative, CD33-positive, and CD2-positive and show stronger intensity CD10 expression than in
KMT2A-rearranged B-ALL. The prognosis of B-ALL with
PAX5 p.P80R is better than that of B-ALL with
PAX5alt abnormalities
[33,34,131][4][23][24]. The diagnosis of these subtypes requires genomic sequencing.
5. B-ALL with MYC Rearrangement
This rare leukemia occurs in <1% of children, 1–2% of AYA, and 2–3% of adult B-ALLs
[36][9]. These cases have a precursor B-ALL immunophenotype, including no expression of surface immunoglobulins, but they harbor
MYC rearrangement. According to gene expression profiling, these leukemias cluster with precursor B cells and other B-ALLs, but not with Burkitt leukemia
[146][25]. In adults, these leukemias are considered high-risk B-ALLs with poor prognoses
[131][4]. Children with
MYC-rearranged B-ALLs are usually treated with Burkitt lymphoma therapy, with a better outcome than adults with
MYC-rearranged B-ALL
[36][9].
6. B-ALL with NUTM1 Rearrangement
NUTM1 rearrangement is more frequent in infants (about 3–5%) than in children (0.4–0.9%) with B-ALL, and this rearrangement has not yet been detected in adults with B-ALL
[147][26].
The nuclear protein in the testes (NUT) is normally located in post-meiotic spermatogenic cells, wherein a global increase in hyperacetylation occurs for spermatogenesis. The NUT Midline carcinoma family member 1 (
NUTM1) gene (also known as
NUT), located on 15q14, was first discovered as a part of the fusion gene in a rare and aggressive carcinoma called NUT carcinoma
[148,149][27][28]. NUT carcinoma harbors a reciprocal t(15;19)(q14;p13.1) translocation between
NUTM1 on chromosome 15q14 and the BET family gene
BRD4 on chromosome 19p13.1, leading to an in-frame
BRD4::NUT fusion oncogene driven by the
BRD4 promoter
[149][28]. Subsequently, with the increased evaluation of tumors by genomic sequencing approaches, the
NUTM1 gene was found to also be present with other fusion partner genes in different types of cancers, including sarcomas and B-ALL
[150][29]. These fusions lead to aberrant NUTM1 overexpression, and the altered global chromatin acetylation might confer sensitivity to histone deacetylase inhibitors and possibly to bromodomain inhibitors for
NUTM1::BRD9 fusion cases
[36][9].
Intriguingly, while NUT carcinoma is a highly aggressive cancer,
NUTM1-rearranged B-ALL has a favorable prognosis. This type of B-ALL occurs in infants and comprises 21.7% to 30% of non-
KMT2A-rearranged (or
KMT2A germline) B-ALL in infants
[147,151][26][30]. Among nine
NUTM1-rearranged B-ALL patients with a median age of 8.8 months,
ACIN1 (
n = 5),
CUX1 (
n = 2),
BRD9 (
n = 1), and
ZNF618 (
n = 1) were identified as fusion partners
[151][30]. Interestingly, this same study also identified other
KMT2A-germline infant B-ALL patients with a median age of about 11 months who harbored
PAX5 fusion; those patients had a poor prognosis
[151][30].
By immunophenotype, the leukemic cells in
NUTM1-rearranged B-ALL may be CD10-positive or CD10-negative, in contrast with CD10-negative leukemic cells in
KMT2A-rearranged B-ALL
[147,151][26][30]. However,
KMT2A-rearranged B-ALL may also be positive for CD10
[136][10], indicating that CD10 expression alone cannot be used to distinguish these two subtypes of ALL. The diagnosis can be made by FISH using a break-apart
NUTM1 probe or RNA or DNA sequencing
[36,151][9][30].
7. B-ALL with ETV::RUNX1-like Features
This leukemia subtype comprises about 1–3% of childhood ALL
[28][1] and about 2% of adult B-ALL
[131][4]. Similar to Ph-like B-ALL, B-ALL with
ETV::RUNX1-like features lacks the
ETV6::RUNX1 fusion, but the gene expression profile is similar to that of B-ALL with
ETV::RUNX1 fusion (see figure in
[36][9]).
FCI shows the leukemia cells are CD24-positive and CD44-negative or low. However, note that this immunophenotype is not specific to this subtype of B-ALL and was also identified in B-ALLs with other genetic subtypes diagnosed by gene expression profiling
[31]. Molecular analysis reveals combined
ETV6 and
IKZF1 alterations (rearrangements and deletions) in this type of leukemia
[28][1]. Further, recent genomic studies showed biallelic
ETV6 inactivation
[139][13] and the APOBEC mutational signature in
ETV6::RUNX1-like childhood B-ALL patients
[139,152][13][32]. Of note, B-ALL with
ETV6::RUNX1-like features may also arise in patients with germline
ETV6 alterations
[152][32].