T cell acute lymphoblastic leukemia (T-ALL).

T cell precursor.

Less than 5% among a series of non selected adult and pediatric T-ALLs (n = 3 out of 92). Six cases were described, all of them children, and 5 out of 6 being under 3 years old (1.1, 1.3, 1.8, 2.5, and 2.9 years old, respectively), which is very young for T-cell leukemia. The t(6;7) translocation could therefore be relatively common in this very low range of age.

Lymphoblasts

The prognosis is yet to be evaluated.

t(6;7)(q23;q34) may be barely detectable by chromosome banding techniques alone.

Involvement of the TCRB locus and the MYB locus can be demonstrated using flanking FISH probes.

No fusion gene
The t(6;7)(q23.3;q34) translocation results in juxtaposition of TRB regulatory sequences to the MYB-AHI1 locus. It results in deregulated expression of C-MYB, as demonstrated by skewed allelic expression.

C-MYB is a transcription factor involved in hematopoiesis. In T-cell differenciation, discrete threshold levels of MYB activity regulate transition through distinct stages, suggesting that a deregulated expression could disturb the maturation process and play a role in oncogenesis.
A potential role of AHI1 deregulation as a cofactor has to be evaluated.
Of note, the same locus at 6q23.3 is also involved in short tandem duplications of a about 230 kb genomic region which includes the C-MYB gene (about 10% T-ALL in children and adults). This somatic abnormality can be detected by array-CGH, genomic Q-PCR or fiber-FISH, but not or hardly by standard metaphasic or interphasic FISH.