3195

TLX1

HOX11|TCL3;T-cell leukemia homeobox 1;TLX1;T-cell leukemia homeobox 1

T-cell leukemia homeobox protein 1|T-cell leukemia/lymphoma protein 3|homeo box-11 (T-cell leukemia-3 associated breakpoint, homologous to Drosophila Notch)|homeobox protein Hox-11|proto-oncogene TCL-3

10q24

protein-coding

Clonal genetic aberrations in tumour cells provide critical information for the development of new diagnostic and therapeutic strategies for patients. In paediatric T-cell acute lymphoblastic leukaemia (T-ALL) chromosomal translocations are present in 30-35% of cases. HOX11 and the closely related HOX11L2 genes play a key role in T-ALL. HOX11 is aberrantly activated by either of the two chromosomal translocations, t(7;10) and t(10;14). In this study, HOX11 expression levels were measured by real-time quantitative reverse-transcriptase polymerase chain reaction. We show that leukaemic blasts from 15/76 (19.7%) paediatric T-ALL patients expressed the HOX11 gene at high level and 22/76 (28.9%) at low level, yet the reported frequency for chromosomal rearrangement of 10q24 is 4-7%. Direct cytogenetic analysis revealed that only 2/16 specimens that showed HOX11 expression exhibited abnor-malities at 10q24. These results confirmand extend our previously published findings, and implicate mechanisms other than gross chromosomal translocations for the deregulation of HOX11. Analysis of clinical outcome for the whole study group showed a trend for better outcome forpatients with leukaemic blasts expressing HOX11 at high level. A statistically significant difference in clinical outcome was found in a subgroup of 20 patients treated for high-risk disease on CCG-1901 from the children's cancer Group, where HOX11 expression in leukaemic blasts conferred a prognostic advantage (P=0.01).

The molecular mechanisms involved in disease progression and relapse in T-cell acute lymphoblastic leukemia (T-ALL) are poorly understood. We used single nucleotide polymorphism array analysis to analyze paired diagnostic and relapsedT-ALL samples to identify recurrent genetic alterations in T-ALL. This analysis showed that diagnosis and relapsed cases have common genetic alterations, but also that relapsed samples frequently lose chromosomal markers present at diagnosis, suggesting that relapsed T-ALL emerges from an ancestral clone different from the major leukemic population at diagnosis. In addition, we identified deletions and associated mutations in the WT1 tumor suppressor gene in 2 of 9 samples. Subsequent analysis showed WT1 mutations in 28 of 211 (13.2%) ofpediatric and 10 of 85 (11.7%) of adult T-ALL cases. WT1 mutations present in T-ALL are predominantly heterozygous frameshift mutations resulting in truncation of the C-terminal zinc finger domains of this transcription factor. WT1 mutations are most prominently found in T-ALL cases with aberrant rearrangements of the oncogenic TLX1, TLX3, and HOXA transcription factor oncogenes. Survival analysisdemonstrated that WT1 mutations do not confer adverse prognosis in pediatric andadult T-ALL. Overall, these results identify the presence of WT1 mutations as a recurrent genetic alteration in T-ALL.

tumor suppressor genes on the X chromosome may skew the gender distribution of specific types of cancer. T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy with an increased incidence in males. In this study, we report the identification of inactivating mutations and deletions in the X-linked plant homeodomain finger 6 (PHF6) gene in 16% of pediatric and 38% of adult primary T-ALL samples. Notably, PHF6 mutations are almost exclusively found in T-ALL samples from male subjects. mutational loss of PHF6 is importantly associated with leukemias driven by aberrant expression of the homeobox transcription factor oncogenes TLX1 and TLX3. Overall, these results identify PHF6 as a new X-linked tumor suppressor in T-ALL and point to a strong genetic interaction between PHF6 loss and aberrant expression of TLX transcription factors in the pathogenesis of this disease.

Based on cytogenetic analysis, chromosomal translocations involving band 10q24 occur in 4-7% of T-cell acute lymphoblastic leukemia (T-ALL). The HOX11 gene is located in this chromosomal band and is activated by translocations t(10;14) (q24;q11) and t(7;10) (q35;q24). Ectopic expression of the HOX11 gene appears tobe involved in the development of T-cell tumors. The aim of this study was to determine the frequency of HOX11 activation in pediatric ALL patients and to correlate gene expression with ALL immunophenotype. None of 53 B-lineage ALLs was positive for HOX11 expression, however, Northern blot and RT-PCR analysis revealed that four of 12 T-ALLs (33%) showed expression of the gene. In order toassess whether HOX11 expression is present in other pediatric malignancies we examined a panel of 20 tumor cell lines established from solid tumors and leukemias, but none of them showed expression of HOX11. Using our RT-PCR method we confirmed that HOX11 expression is not detectable in normal T-cells. These findings indicate that HOX11 expression in pediatric ALL is exclusive to T-ALL and does not occur in B-lineage ALL. The frequency detected by molecular techniques was significantly higher than the frequency reported in the literature based on cytogenetic analysis. These results support the notion that ectopic expression of the HOX11 homeobox gene is a crucial step in T-cell tumorigenesis.