Involvement of the putative hematopoietic transcription factor SCL in T- cell acute lymphoblastic leukemia

Abstract

The SCL gene, initially discovered at the site of a translocation breakpoint associated with the development of a stem cell leukemia, encodes a protein that contains the highly conserved basic helix-loop- helix (bHLH) motif found in a large array of eukaryotic transcription factors. Recently, we have described a nonrandom, site-specific SCL rearrangement in several T-cell acute lymphoblastic leukemia (ALL) cell lines that juxtaposes SCL with a distinct transcribed locus, SIL. The SIL/SCL rearrangement was found in leukemic blasts from 11 of 70 (16%) newly diagnosed T-cell ALL patients, a prevalence substantially higher than that of the t(11;14) translocation, which has previously been reported as the most frequent nonrandom chromosomal abnormality in T- cell ALL. We did not detect the SIL/SCL rearrangement in the leukemic blasts from 30 patients with B-cell precursor ALL, indicating that the rearrangement was specific for T-cell ALL. Analysis of RNA from these patients indicated that an SIL/SCL fusion mRNA was formed, joining SIL and SCL in a head-to-tail fashion. The fusion occurs in the 5′ untranslated region (UTR) of both genes, preserving the SCL coding region. The net result of this rearrangement is that SCL mRNA expression becomes regulated by the SIL promoter, leading to inappropriate SCL expression. The resultant inappropriate expression of this putative transcription factor may then contribute to leukemic transformation in T-cell ALL.

Simultaneous SIL‐TAL1 RT‐PCR detection of all tald deletions and identification of novel tald variants

“…DNA detection of tal d1 and tal d2 rearrangements was performed on 1 mg DNA using the LK1 (specific for sil db1 [GCT CCT ACC CTG CAA ACA GA]) primer and LK2 (specific for tal db1 [CAG AAG GGC AGC AAA CAA AC]) [T a 60ЊC] or LK3 (specific for tal db2 [ATG TAT GGA ACG TCA CTC AG]) [T a 56ЊC] primers (Fig 1), as previously described (Aplan et al, 1992a;Macintyre et al, 1992).…”

“…Sequence analysis of the SIL-TAL1 PCR products showed alternative splicing from SIL exon 1 (1a herein) to exons 3 or 4 (Aplan et al, 1992a) and to a newly described SIL exon 1b. Only one SIL-TAL1 RT-PCR band was demonstrated by Huang et al (1995), partly due to the fact that they used an exon 3 internal hybridization probe, which would only detect form II and III transcripts.…”

“…Detection of tal d1 and tal d2 is most commonly performed by PCR from DNA. Detection of the rare variants (Aplan et al, 1992a;Bash et al, 1993;Breit et al, 1993b) (each of which have been described only once) requires either further primer pairs or Southern screening. SIL-TAL1 RT-PCR analysis detected all 13 known tal d1 (10 T-ALL, three cell lines) and a tal d2 positive control, thus demonstrating an absence of false negativity.…”

“…It also detected a novel tal d7 in a case of childhood ALL (6 years old). This represents the third SIL breakpoint to be identified, sil db3 (sil db1 (Aplan et al, 1990); sil db2 (Aplan et al, 1992a)) and the sixth TAL1 breakpoint, tal db6 . Although tal d7 was detected from DNA with our LK1 and LK2 primers, it would not have been detected by those used by Jonsson et al (1991) or Breit et al (1993a, b).…”

Chromosomal translocations cause deregulated BCL6 expression by promoter substitution in B cell lymphoma.

“…Conversely, in the t(3;14) translocations, the IgH genes provide promoter and 5' untranslated sequences which substitute for the corresponding BCL6 sequences, a mechanism that we have called promoter substitution. While translocations involving promoter substitutions have not been observed before in B cell malignancies (for a review see Rabbitts, 1994), one example exists in T cell-derived acute lymphoblastic leukemia in which the chromosome lp33 deletion can join SIL exon 1 to TAL-] exon 3 in a head-to-tail fashion (Aplan et al, 1992;Baer, 1993) In all three cases studied here, IgH promoter regions (IS, or IY3) were found fused to BCL6 transcripts, while in two DLCL biopsies (KC1445 and SM1444; Figure 3), additional fusion transcripts initiated from further upstream D or V-D sequences could also be identified. Although the 5' termini and the abundance of the latter transcripts could not be characterized, the recurrent selection of I promoters, together with the fact that 13-BCL6 fusion transcripts represent the most abundant BCL6 RNA species in Ly8 cells ( Figure 4B), suggest that the recruitment of I promoters may be the most common and therefore the most biologically significant consequence of the Ig-BCL6 recombination.…”