Cyclin-dependent kinase-4 (CDK4) is a protein-serine kinase involved in the cell cycle. Human cell division is regulated primarily at the G1-to-S or the G2-to-M boundaries within the cell cycle. The sequential activation of cyclin-dependent kinases and their subsequent phosphorylation of critical substrates promote orderly progression through the cell cycle. The complexes formed by CDK4 and the D-type cyclins (e.g., D1, 168461; D2, 123833; D3, 123834) are involved in the control of cell proliferation during the G1 phase. CDK4 is inhibited by p16, also known as cyclin-dependent kinase inhibitor-2 (CDKN2A; 600160).

Hanks (1987) isolated the CDK4 gene by screening a HeLa cell cDNA library with probes designed to recognize clones encoding protein-serine kinases. Zuo et al. (1996) noted that the CDK4 protein comprises 303 amino acids.

Zuo et al. (1996) determined that the CDK4 gene contains 8 exons and spans 5 kb.

Demetrick et al. (1994) mapped the CDK4 gene to chromosome 12q13 by fluorescence in situ hybridization. CDK2 maps to the same band. By fluorescence in situ hybridization, Mitchell et al. (1995) mapped CDK4 to 12q14 and concluded that it is distal to GLI (165220) and CHOP (126337), which they placed at 12q13.3-q14.1; and proximal to MDM2 (164785), which they placed at 12q14.3-q15.

Harbour et al. (1999) presented evidence that phosphorylation of the C-terminal region of RB1 (614041) by CDK4/CDK6 (603368) initiated successive intramolecular interactions between the C-terminal region and the central pocket. The initial interaction displaced histone deacetylase from the pocket, blocking active transcriptional repression of E2F (189971) by RB. This facilitated a second interaction that led to phosphorylation of the pocket by CDK2 (116953) and disruption of pocket structure. These intramolecular interactions provided a molecular basis for sequential phosphorylation of RB by CDK4/CDK6 and CDK2. CDK4/CDK6 was activated early in G1, blocking active repression by RB. Cyclin E (see 123837) and CDK2 were activated near the end of G1.

Stepanova et al. (1996) found that CDC37 (605065) and HSP90 (see 140571) associated preferentially with the fraction of CDK4 not bound to D-type cyclins. Pharmacologic inactivation of CDC37/HSP90 function led to reduced stability of CDK4.

Modiano et al. (2000) found that 5 of 16 healthy individuals expressed CDK4 mRNA, protein, and activity in unstimulated peripheral blood T cells and that these T cells proliferated directly in response to interleukin-2 (IL2; 147680) in the absence of mitogens. In cells from these individuals, CDK4 expression and activity were resistant to protein kinase inhibitors, unlike stimulated cells from individuals lacking basal CDK4 expression. The phenotype of the T cells of these individuals was comparable to that observed in a human IL2-dependent T-cell line. Modiano et al. (2000) proposed that CDK4 activity may be a useful marker for cytokine responsiveness in T cells.

In primary epidermal cells, Lazarov et al. (2002) found that oncogenic RAS (190020) transiently decreased CDK4 expression in association with cell cycle arrest in the G1 phase. CDK4 coexpression circumvented RAS growth suppression and induced invasive human neoplasia resembling squamous cell carcinoma. Tumorigenesis was dependent on CDK4 kinase function, with cyclin D1 required but not sufficient for this process. In facilitating escape from G1 growth restraints, RAS and CDK4 altered the composition of cyclin D and cyclin E complexes and promoted resistance to growth inhibition by INK4 (600160) cyclin-dependent kinase inhibitors. These data identified a new role for oncogenic RAS in CDK4 regulation and highlighted the functional importance of CDK4 suppression in preventing uncontrolled growth.

Matsuura et al. (2004) showed that SMAD3 (603109) is a major physiologic substrate of the G1 cyclin-dependent kinases CDK4 and CDK2. Except for the retinoblastoma protein family, SMAD3 was the only CDK4 substrate demonstrated to that time. Matsuura et al. (2004) mapped CDK4 and CDK2 phosphorylation sites to thr8, thr178, and ser212 in SMAD3. Mutation of the CDK phosphorylation sites increased Smad3 transcriptional activity, leading to higher expression of the CDK inhibitor p15 (600431). Mutation of the CDK phosphorylation sites of Smad3 also increased its ability to downregulate the expression of c-myc (190080). Using Smad3 knockout mouse embryonic fibroblasts and other epithelial cell lines, Matsuura et al. (2004) further showed that Smad3 inhibited cell cycle progression from G1 to S phase and that mutation of the CDK phosphorylation sites in Smad3 increased this ability. They concluded that CDK phosphorylation of SMAD3 inhibits its transcriptional activity and antiproliferative function.

Lee et al. (2014) reported that in mice, insulin activates Ccnd1 (168461)/Cdk4, which in turn increases Gcn5 (KAT2A; 602301) acetyltransferase activity and suppresses hepatic glucose production independently of cell cycle progression. Through a cell-based high-throughput chemical screen, Lee et al. (2014) identified a Cdk4 inhibitor that potently decreases Pgc1a (PPARGC1A; 604517) acetylation. Insulin/Gsk3b (605004) signaling induces Ccnd1 protein stability by sequestering Ccnd1 in the nucleus. In parallel, dietary amino acids increase hepatic Ccnd1 mRNA transcripts. Activated Ccnd1/Cdk4 kinase phosphorylates and activates Gcn5, which then acetylates and inhibits Pgc1a activity on gluconeogenic genes. Loss of hepatic Ccnd1 results in increased gluconeogenesis and hyperglycemia. In diabetic models, Ccnd1/Cdk4 is chronically elevated and refractory to fasting/feeding transitions; nevertheless, further activation of this kinase normalizes glycemia. Lee et al. (2014) concluded that insulin uses components of the cell cycle machinery in postmitotic cells to control glucose homeostasis independently of cell division.

Zhang et al. (2018) showed that PDL1 (605402) protein abundance is regulated by cyclin D-CDK4 and the cullin 3 (603136)-SPOP (602650) E3 ligase via proteasome-mediated degradation. Inhibition of CDK4 and CDK6 in vivo increases PDL1 protein levels by impeding cyclin D-CDK4-mediated phosphorylation of SPOP and thereby promoting SPOP degradation by the anaphase-promoting complex activator FZR1 (603619). Loss-of-function mutations in SPOP compromise ubiquitination-mediated PDL1 degradation, leading to increased PDL1 levels and reduced numbers of tumor-infiltrating lymphocytes in mouse tumors and in primary human prostate cancer specimens. Notably, combining CDK4/6 inhibitor treatment with anti-PD1 (600244) immunotherapy enhances tumor regression and markedly improves overall survival rates in mouse tumor models. Zhang et al. (2018) concluded that their study uncovered a novel molecular mechanism for regulating PDL1 protein stability by a cell cycle kinase and revealed the potential for using combination treatment with CDK4/6 inhibitors and PD1-PDL1 immune checkpoint blockade to enhance therapeutic efficacy for human cancers.

The restriction (R) point marks the point in the cell cycle when cells become independent of mitogen signaling and CDK2 activity becomes self-sustaining through a feedback loop between cyclin A2 (CCNA2; 123835)/CDK2 and RB1, leading to an irreversible commitment to proliferation. Cornwell et al. (2023) demonstrated that mitogen signaling maintained CDK2 activity in S and G2 phases of the cell cycle, and that, in the absence of mitogen signaling, some post-R-point cells exited the cell cycle and entered a G0-like state instead of irreversibly committing to proliferation. Further analysis indicated that mitosis and cell cycle exit were 2 mutually exclusive fates, and that competition between the 2 determined whether cells continued to proliferate or exited the cell cycle. As a result, the decision to proliferate was fully reversible, even when cells were in post-R state, because CDK2 activation and RB1 phosphorylation were reversible in all post-R cells after loss of mitogen signaling. CDK4/CDK6 promoted cyclin A2 synthesis in S/G2, and cyclin A2 stability was the primary contributor to cell cycle exit. Cells were dependent on mitogens and CDK4/CDK6 activity to maintain CDK2 activity and RB1 phosphorylation throughout the cell cycle. The R-point irreversibility phenomenon was observed in the absence of mitogens, because in most cells, the half-life of cyclin A2 was long enough to sustain CDK2 activity throughout G2/M to reach mitosis. The results implied that there is no single point when cells are irreversibly committed to proliferation that can be defined by a single molecular event, but rather that it is determined by the cell's proximity to mitosis, as well as the cyclin A2 level when mitogen signaling is lost.

Susceptibility To Cutaneous Malignant Melanoma 3

In a human cutaneous malignant melanoma (CMM3; 609048) cell line, Wolfel et al. (1995) identified a mutation in the CDK4 gene (R24C; 123829.0001). The same mutation was found in 1 additional melanoma among 28 melanomas analyzed.

Zuo et al. (1996) identified germline R24C mutations in affected members of 2 unrelated families with malignant melanoma.

Soufir et al. (1998) identified an R24H (123829.0002) mutation in a French family with malignant melanoma.

In a large Norwegian pedigree first reported by Grimstvedt (1969), Molven et al. (2005) identified the R24H mutation. Molven et al. (2005) stated that whereas approximately 20% of melanoma-prone families bear a mutation in the CDKN2A locus (600160), mutations in the CDK4 locus are much rarer, having been linked to the disease in only 6 families worldwide.

Autosomal Recessive Primary Microcephaly 31

In 5 patients from 2 unrelated consanguineous families with autosomal recessive primary microcephaly-31 (MCPH31; 621507), Verdu Schlie et al. (2025) identified 2 homozygous loss-of-function mutations in the CDK4 gene (123829.0003 and 123829.0004). The mutation in the first family (family A) was found by whole-exome sequencing; the patient in the second family (family B) was identified through the GeneMatcher program after undergoing exome sequencing. The mutations, which were confirmed by Sanger sequencing, segregated with the disorder in both families. Each was found at a very low frequency in the heterozygous state in gnomAD (v4.1.0), with no homozygotes. Western blot analysis of fibroblasts derived from 2 patients with different mutations showed undetectable levels of full-length CDK4 protein in patient-derived cells, consistent with a loss of function. Fibroblasts derived from 2 patients with different mutations showed impaired proliferation and increased accumulation in the G0/G1 phase of the cell growth cycle compared to controls. The abnormalities were associated with decreased phosphorylation of the RB1 gene (614041). These defects, including phosphorylation of RB1, could be rescued by complementation with wildtype CDK4. The mutations had no detectable effects on S phase, DNA replication, or mitosis. The findings indicated that disruption of the canonical function of CDK4 in regulating the G1-to-S transition in the cell growth cycle through phosphorylation of RB1 in G1 underlies the cell proliferation defect that results in microcephaly.

Zou et al. (2002) noted that Cdk4-null mice were viable, but exhibited diabetes mellitus due to degeneration of pancreatic beta cells, as well as growth retardation and infertility due to severe hypoplasia and dysfunction of the pituitary. Embryonic fibroblasts from Cdk4-null mice initially proliferated at normal rates, but they displayed a 4- to 5-hour delay in reentry into the cell cycle following quiescence. Zou et al. (2002) found that Cdk4 was required for Ras-mediated transformation, and Cdk4 disruption led to senescence that was independent of Arf (600160) or p53 (191170). Senescence was associated with increased Cdkn1a (116899) stability.

Malumbres et al. (2004) found that Cdk6-null mice were viable and developed normally, although hematopoiesis was slightly impaired. Embryos defective for Cdk4 and Cdk6 died during the late stages of embryonic development due to severe anemia. However, these embryos displayed normal organogenesis, and most cell types proliferated normally. In vitro, embryonic fibroblasts lacking Cdk4 and Cdk6 proliferated and became immortal upon serial passage. Quiescent Cdk4/Cdk6-null cells responded to serum stimulation and entered S phase with normal kinetics, although with lower efficiency. These results indicated that D-type cyclin-dependent kinases are not essential for cell cycle entry and suggested the existence of alternative mechanisms to initiate cell proliferation upon mitogenic stimulation.