Summary Mixed Lineage Leukemia (MLL) fusion proteins cause oncogenic transformation of hematopoietic cells by constitutive recruitment of elongation factors to HOX promoters, resulting in over-expression of target genes. The structural basis of transactivation by MLL fusion partners remains undetermined. We show that the ANC1 Homology Domain (AHD) of AF9, one of the most common MLL translocation partners, is intrinsically disordered and recruits multiple transcription factors through coupled folding and binding. We determined the structure of the AF9 AHD in complex with the elongation factor AF4, and show that aliphatic residues which are conserved in each of the AF9 binding partners form an integral part of the hydrophobic core of the complex. NMR relaxation measurements show AF9 retains significant dynamic behavior which may facilitate exchange between disordered partners. We propose that AF9 functions as a signaling hub which regulates transcription through dynamic recruitment of co-factors in normal hematopoiesis and in acute leukemia.

Introduction: The scapula tip free flap has been described for reconstruction of short mandible defects with extensive soft tissue needs. The versatility of this flap has not been extensively described. Methods: Retrospective case series of patients who underwent mandibulectomy and reconstruction with the scapula tip free flap from 2005 to 2016. Outcomes include bony union, complications, dental rehabilitation, and donor site morbidity. Results: A total of 120 patients were identified. Average harvested bone was 7.7 cm, with 54% undergoing one osteotomy, and 76.7% harvested as a chimeric flap. Radiographically, average inset bone was 6.6 cm. Complete or partial bony union was observed at 74.4% of proximal and 82.6% of distal osteotomies. A 95% of reconstructions met criteria for dental implants. Mean disabilities of the arm, shoulder and hand score was 21.2. Conclusions: The scapular tip is an excellent option for reconstruction of segmental mandible defects with the option of osteotomy, excellent bony union rates, low donor site morbidity, and potential for dental rehabilitation.

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