Cyclin-dependent kinase 4/6 (CDK4/6)-specific inhibitors, such as palbociclib, have shown clinical efficacy, but primary or secondary resistance has emerged as a problem. To develop more effective therapeutic approaches, investigation is needed into the mechanisms of resistance or adaption. Here, it is demonstrated that CDK2 compensates for loss of CDK4 activity to rescue palbociclib-arrested breast cancer cells, suggesting that inhibition of both kinases is required to achieve durable response. In addition, a novel strategy is described to inhibit tyrosine phosphorylation of p27Kip1 (CDKN1B) and simultaneously inhibit both CDK2 and CDK4. p27Kip1 is a required assembly factor for cyclin-CDK4 complexes, but it must be phosphorylated on residue Y88 to open or activate the complex. The Brk-SH3 peptide, ALT, blocks p27 Y88 phosphorylation, inhibiting CDK4. Nonphosphorylated p27 is no longer a target for ubiquitin-mediated degradation and this stabilized p27 now also inhibits CDK2 activity. Thus, ALT induction inhibits both the kinase that drives proliferation (CDK4) and the kinase that mediates resistance (CDK2), causing a potent and long-lasting cell-cycle arrest. ALT arrests growth of all breast cancer subgroups and synergizes with palbociclib to increase cellular senescence and to cause tumor regression in breast cancer xenograft models. The use of ALT demonstrates that both CDK4 and CDK2 need to be inhibited if long-term efficacy is to be achieved and represents a novel modality to inhibit breast cancer cells. Modulating tyrosine phosphorylation of p27 impacts both proliferative (CDK4) and resistance (CDK2) mechanisms in breast cancer and suggests that phospho-p27 status may serve as a biomarker for patients that are responsive to CDK4/6 inhibition. .

Cyclin D and cyclin-dependent kinase 4 (cdk4) are overexpressed in a variety of tumors, but their levels are not accurate indicators of oncogenic activity because an accessory factor such as p27Kip1 is required to assemble this unstable dimer. Additionally, tyrosine (Y) phosphorylation of p27 (pY88) is required to activate cdk4, acting as an "on/off switch." We identified two SH3 recruitment domains within p27 that modulate pY88, thereby modulating cdk4 activity. Via an SH3-PXXP interaction screen, we identified Brk (breast tumor-related kinase) as a high-affinity p27 kinase. Modulation of Brk in breast cancer cells modulates pY88 and increases resistance to the cdk4 inhibitor PD 0332991. An alternatively spliced form of Brk (Alt Brk) which contains its SH3 domain blocks pY88 and acts as an endogenous cdk4 inhibitor, identifying a potentially targetable regulatory region within p27. Brk is overexpressed in 60% of breast carcinomas, suggesting that this facilitates cell cycle progression by modulating cdk4 through p27 Y phosphorylation. p27 has been considered a tumor suppressor, but our data strengthen the idea that it should also be considered an oncoprotein, responsible for cyclin D-cdk4 activity.C yclin D1-cyclin-dependent kinase 4 (cdk4) complexes promote the G 0 /G 1 -phase transition, and as such their activity is tightly regulated by a variety of mechanisms, including the transcription and translation of the mitogen sensor cyclin D1 and positive and negative regulatory phosphorylation of cdk4 (1, 2). The best-characterized substrate of cyclin D-cdk4 is the G 1 gatekeeper, retinoblastoma (Rb), and deregulation of cdk4 potentially accelerates Rb phosphorylation and cell cycle transitioning, promoting cancer development (3). Cyclin D1 and cdk4 are overexpressed in a variety of human cancers, and in mouse models, loss of either cdk4 or cyclin D1 prevents the development of certain oncogene-driven tumors, further evidence of their involvement (4-6). However, the levels of cyclin D or cdk4 in a tumor may not be reliable measures of activity, due to the fact that a third protein, an assembly factor such as p27Kip1 or p21Cip1, is required both for the stabilization and then the subsequent activation of this complex (1, 7).Independently of its ability to assemble cyclin D-cdk4 complexes, p27 acts as a bona fide "switch" turning cyclin D-cdk4 complexes on or off, which in turn modulates cell cycle entry or exit (8, 9). Tyrosine (Y) phosphorylation of p27 on residues Y74, Y88, and Y89 opens the cyclin D-cdk4-p27 ternary complex, rendering it able to phosphorylate substrates such as Rb (9-14). Cyclin D-cdk4-p27 complexes isolated from cells in G 0 lack Y phosphorylation on p27 and are catalytically inactive, while complexes isolated from proliferating cells are Y phosphorylated and active. Y88 and Y89 are part of the 3-to-10 helix, which has been shown to insert into the cdk ATP binding cleft (15). When not phosphorylated, residues Y88 and Y89 (Y88/Y89) sequester within this binding pocket and block cdk4 activity (p2...

This study aimed to analyze the functional and radiological outcomes of lumbar decompression in patients with degenerative lumbar scoliosis (DLS). Overview of Literature: Patients with DLS have symptoms related to lumbar canal stenosis (LCS) and those due to compensated spinal imbalance. Whether the deformity is the cause of pain or is an adaptive change for the ongoing LCS remains debatable. The extensive surgery for deformity correction along with spinal fusion is reported to have high perioperative morbidity and complication rate. Methods: This retrospective analysis involved 51 patients who underwent lumbar decompression for LCS associated with DLS from October 2006 to October 2016. The magnitude of the curve was determined using Cobb's angle and lumbar lordosis (D12-S1) on the preoperative and final follow-up, respectively. The Visual Analog Scale (VAS) and modified Oswestry Disability Index (mODI) scores at the preoperative and final follow-up indicated the functional outcome. Statistical analyses were performed using Student t-test. Results: All 51 patients were included in the statistical analyses. The mean patient age at presentation was 63.88±7.21 years. The average follow-up duration was 48±18.10 months. The average change in the Cobb's angle at the final follow-up was statistically insignificant (1°±1.5°, p=0.924; 20.8°±5.1° vs. 21.9°±5.72°). The mean change in lumbar lordosis at the final follow-up was statistically insignificant (3.29°±1.56°, p=0.328; 30.2°±7.9° vs. 27.5°±7.1°). There was statistically insignificant worsening in the back VAS scores at the final follow-up (4.9±1.9 vs. 6.0±1.2, p=0.07). There was statistically significant improvement in the leg pain component of the VAS score at the final follow-up (5.8±1.05 vs. 2.6±1.2, p<0.001). There was statistically significant improvement in the mODI scores at the final follow-up (p<0.001). Conclusions: Lumbar decompression in DLS is associated with good functional outcome, especially when the symptoms are related to LCS. Curve progression following lumbar decompression is very less at mid-term and is similar to that in the natural course of the disease.

Study DesignThis retrospective study was conducted including 18 patients who underwent posterior-only stabilization and fusion procedure for pseudoarthrosis in the ankylosed spine from October 2007 to May 2015.PurposeThis study aimed to describe the treatment outcomes in 18 patients with Andersson lesion (AL) who were managed using the posterior-only approach.Literature ReviewAL is an unstable, localized, vertebral, or discovertebral lesion of the spine. It is observed in patients with ankylosing spondylitis. The exact etiology of this disorder remains unclear, and the treatment guidelines are not clearly described.MethodsWe analyzed 18 patients with AL who were treated with posterior long segment spinal fusion without any anterior interbody grafting or posterior osteotomy. Pre- and postoperative radiography, computed tomography, and recent follow-up images were examined. The pre- and postoperative Visual Analog Scale score and the Oswestry Disability Index score were evaluated for all patients. Whiteclouds’ outcome analysis criteria were applied at the follow-up. Moreover, at study completion, patient feedback was collected; all the patients were asked to provide their opinion regarding the surgery and were asked whether they would recommend this procedure to other patients and them self undergo the same procedure again if required.ResultsThe most common site was the thoracolumbar junction. The symptom duration ranged from 1 month to 10 years preoperatively. Most patients experienced fusion by the end of 1 year, and the fusion mass could be observed as early as 4 months. Pseudoarthrosis void of up to 2.5 cm was noted to be healed in subsequent imaging. In addition, clinically, the patients reported good symptomatic relief. No patient required revision surgery. Whiteclouds’ outcome analysis score at the latest follow-up revealed good-to-excellent outcomes in all patients.ConclusionsALs can be treated using the posterior-only approach with long segment fixation and posterior spinal fusion. This is a safe, simple, and quick procedure that prevents the morbidity of anterior surgery.

To evaluate the accuracy of three-dimensional (3D) printed patient-specific templates (PSTs) for placement of pedicle screws (PAs) in patients undergoing revision surgeries for complex kyphoscoliosis deformity with sublaminar wires in situ. Overview of Literature: Revision kyphoscoliosis correction surgery in pediatric patients is a challenging task for the treating surgeon. In patients with sublaminar wires in situ, the native anatomical landmarks are obscured, thus making the freehand screw placement technique a highly specialized task. Hence, the concept of using PSTs for insertion of PAs in such surgeries is always intriguing and attractive. Methods: Five consecutive patients undergoing revision deformity correction with sublaminar wires in situ were included in this study. Patients were divided in two groups based on the technique of PA insertion. A total of 91 PAs were inserted using either a freehand technique (group A) or 3D printed templates (group B) (34 vs. 57). The placement of PAs was classified according to a postoperative computed tomography scan using Neo's classification. Perforation beyond class 2 (>2 mm) was termed as a misplaced screw. The average time required for the insertion of screws was also noted. Results: Mean age, surgical time, and blood loss were recorded. The change in mean Cobb's angle in both groups was also recorded. The difference in rates of misplaced screws was noted in group A and group B (36.21% vs. 2.56%); however, the mean number of misplaced PAs per patient in group A and group B was statistically insignificant (6.5±3.54 vs. 4.67±1.53, p=0.4641). The mean time required to insert a single PA was also statistically insignificant (120±28.28 vs. 90±30 seconds, p=0.3456). Conclusions: Although 3D printed PSTs help to avoid the misplacement of PAs in revision deformity correction surgeries with sublaminar wires in situ, the mean number of misplaced screws per patient using this technique was found to be statistically insignificant when compared with the freehand technique in this study.