Chromosome 21

1
Chromosome 21 and lymphoproliferation
Todd Druley MD/PhD
2
Case 1

• KS was an 8 hour old baby boy born by spontaneous
  vaginal delivery at an OSH. Delivery was
  uncomplicated. The child had dysmorphic features
  consistent with Downs Syndrome,
  hepatosplenomegaly and a III/VI cardiac murmur on
  exam.
• CXR revealed cardiomegaly. CBC WBC 34.4 with
  52 blasts, Hemoglobin 14.3 g/dL, plts 258,000.
  The pt was transferred to SLCH for further
  evaluation.
• Bone marrow aspiration biopsy showed, The
  striking megakaryocytic hyperplasia in the core
  biopsy and the expression of megakaryocytic
  antigens on the blasts in the aspirate are the
  typical features of transient myeloproliferative
  disorders (TMPD) associated with Down's
  Syndrome. (Dr. R. Burack)
• Bone marrow contained 19 blasts which were CD34
  and CD117 positive. Cytogenetics demonstrated a
  clone with trisomy 21 and another clone with
  tetrasomy 21.

3
Case 2

• MM was a 2 yo girl with a PMHx including Downs
  syndrome, TOF and AV canal s/p repair,
  developmental delay s/p bilateral watershed
  strokes, and tracheal reconstruction s/p
  tracheostomy due to tracheal rings.
• Pt brought to ED after two weeks of fever,
  agitation and increased work of breathing not
  improved with two courses of enteral antibiotics.
  Cardiac echo suggestive of AV vegetation. Pt
  dxd with endocarditis and started on cefotaxime,
  vancomycin and gentamicin.
• CBC showed WBC 29.9 with 40 blasts, hemoglobin
  15.4 g/dL and platelets 56,000. Pt had diffuse
  petechiae on exam.
• Bone marrow biopsy and aspirate revealed 98
  blasts that were Tdt and CD79a positive
  confirming a diagnosis of pre-B ALL.

4
Downs syndrome and cancer

• Downs syndrome (DS) patients have a 10-20 fold
  higher risk of developing leukemia (Hasle, 2000).
• The incidence of transient myeloproliferative
  disorder (TMD) in DS newborns ranges from 10-25
  (Zipursky et al., 1999)
• A common complication of TMD is the development
  of acute megakaryocytic leukemia (AMKL FAB M7),
  which is estimated at 13-33 (reviewed in
  Zipursky, 2003)
• Overall, DS patients have a nearly 500-fold risk
  of developing AMKL and a 20-fold risk of
  developing ALL.
• Intriguingly, solid cancers (particularly breast
  CA) occur less frequently in patients with DS.
  Notable exceptions include retinoblastoma and
  germ cell tumors. (Stage, 1998)

5
TMD and AMKL

• Somatic mutations in exon 2 of GATA-1, leading to
  a truncated protein GATA-1s that lacks the
  transcriptional activation domain, are seen in a
  majority of DS patients with both TMD and/or
  AMKL.
• Crispino (2003) Blood 6/6 patients
• Rainis (2003) Blood 16/17 patients
• Hitzler (2003) Blood 9/12 patient
• Groet (2003) Lancet 10/10 patients
• GATA 1-6 are zinc-finger transcription factors
  that regulate many genes involved with
  hematopoiesis. GATA-1 is located on the
  X-chromosome.

6
TMD and AMKL

• For mature megakaryocyte differentiation to
  occur, it appears that GATA-1 must dimerize with
  RUNX-1.
• RUNX-1 is a hematopoietic transcription factor on
  chromosome 21.
• A target of chromosomal translocation in 20 of
  AML and thought to be involved in 25 of
  childhood ALL.

7
TMD and AMKL

• Immunoprecipitation experiment demonstrating the
  physical association between GATA-1 and RUNX1.
  (Elagib et al, Blood 2003)
• In the same system, the GATA-1s mutation was
  shown to abrogate the interaction between GATA-1
  and RUNX1.

8
TMD and AMKL

• The current model hypothesizes that a lack of
  GATA-1 activity (due to the N-terminal mutation)
  combined with the increased dose effect of RUNX-1
  in trisomy 21 promotes proliferation of abnormal,
  poorly differentiated megakaryoblasts.

9
Other Genes on Chromosome 21 APP

• Baldus et al (PNAS 2004, 3915) found three genes
  on chromosome 21 that were overexpressed in AML
  patients with complex karyotypes.
• The Amyloid ? Precursor Protein (APP) on 21q21
  had the highest mRNA expression, not solely due
  to copy number.
• APP is a transmembrane glycoprotein that has been
  implicated in the pathogenesis of Alzheimers and
  dementia in adults with DS.
• In neurons, wildtype APP has been shown to block
  the pro-apoptotic action of p53 (Xu, 1999).
• Expression profiling identified APP as one of the
  most differentially expressed genes in lymphoid
  malignancies (Baldus, 2004).

10
Other Genes on Chromosome 21 ETS2

• ETS2 is a transcription factor with locus at
  21q22, whose expression has been shown to induce
  apoptosis in the presence of normal p53. (Xu et
  al, PNAS 1999)
• Baldus et al demonstrated amplification of ETS2
  in conjunction with frequent loss of the p53
  locus in DS-AML derived cells.
• Aberrant expression of several ETS proteins has
  been observed in various types of human
  malignancies. (Reviewed in Dittmer, 2003)
• These findings suggest that amplification of ETS2
  in the presence of abnormal p53 expression or
  function, could lead to deregulation of
  hematopoiesis (or other cell lineages).

11
Other Genes on Chromosome 21 ERG

• ERG, also on 21q21, is a transforming oncogene
  expressed on endothelial cells and hematopoietic
  stem cells.
• Overexpression of ERG does not correlate to
  increased copy number and drives hematopoiesis
  toward megakaryocyte differentiation.
• ERG can dimerize with GATA1s to promote clonal
  megakaryoblastic proliferation. (Rainis et al,
  Canc Res 2005)
• Rainis et al demonstrated the highest levels of
  ERG expression in trisomy and/or tetrasomy 21 ALL
  clones, however, expression analysis by Baldus et
  al showed that amplification of ERG was not
  simply due to copy number.

12
Altered response to chemotherapy

• Cystathione ? synthase
• Found on 21q22.3.
• Twelve-fold in vitro increase in expression in DS
  myeloblasts. Correlates with increased patient
  sensitivity due to increased rate of metabolism
  of ara-C to ara-CTP (Taub 1999, 2000).
• Thought to contribute to MTX toxicity due to
  altering folate metabolism and trapping 5-CH3
  THF.
• Reduced folate carrier Expressed at highest
  levels in hyperdiploid ALL blasts with 4 copies
  of chromosome 21. Level of expression was
  inversely proportional to MTX levels (Belkov,
  1999)

13
Chromosome 21 genes of interest

• TIAM (tumor invasion and metastasis factor)
  involved in leukemogenesis
• Phosphoribosyl aminoimidazole synthetase
• Phosphoribosyl glycinamide synthetase
• Phosphoribosyl glycinamide formyl transferase
  (GART)
• Interferon ?/? receptor (IFNAR)
• Cytokine family 2-4 (CFR2-4)
• SON gene with significant homology to myc

14
Conclusions

• Downs syndrome predisposes to myeloid and
  lymphoid malignancies.
• Progression to malignancy in DS appears to be a
  multifactorial phenomenon dependent upon the
  presence of a third chromosome 21, but not solely
  based on gene copy number.
• Somatic mutations in exon 2 of the GATA-1 gene
  occur more frequently in DS and deregulate
  megakaryocyte differentiation.
• Overexpression of RUNX-1 without dimerization to
  GATA-1 promotes clonal megakaryoblast expansion.

15
Conclusions

• Overexpression of enzyme-encoding genes on
  chromosome 21 alter the DS myeloblast response to
  traditional chemotherapy.
• Overexpression of ETS2 in the setting of altered
  p53 function could potentially play a role in
  malignant transformation of multiple tissue
  types.
• Many other genes found on chromosome 21 may
  potentially contribute to malignant
  transformation in Downs Syndrome.