Mutations in Cullin 4B result in a human syndrome associated with increased camptothecin-induced topoisomerase I-dependent DNA breaks

Abstract

CUL4A and B encode subunits of E3-ubiquitin ligases implicated in diverse processes including nucleotide excision repair, regulating gene expression and controlling DNA replication fork licensing. But, the functional distinction between CUL4A and CUL4B, if any, is unclear. Recently, mutations in CUL4B were identified in humans associated with mental retardation, relative macrocephaly, tremor and a peripheral neuropathy. Cells from these patients offer a unique system to help define at the molecular level the consequences of defective CUL4B specifically. We show that these patient-derived cells exhibit sensitivity to camptothecin (CPT), impaired CPT-induced topoisomerase I (Topo I) degradation and ubiquitination, thereby suggesting Topo I to be a novel Cul4-dependent substrate. Consistent with this, we also find that these cells exhibit increased levels of CPT-induced DNA breaks. Furthermore, over-expression of known CUL4-dependent substrates including Cdt1 and p21 appear to be a feature of these patient-derived cells. Collectively, our findings highlight the interplay between CUL4A and CUL4B and provide insight into the pathogenesis of CUL4B-deficiency in humans.

Essential role of the CUL4B ubiquitin ligase in extra-embryonic tissue development during mouse embryogenesis

“…Despite the largely overlapping functions of CUL4A and CUL4B, recent studies also revealed distinct roles for the two CUL4 family members in DNA-damage response, male meiosis, and response to environmental toxins [12][13][14][15]. While CUL4B compensates for the loss of CUL4A [12], CUL4A expression, which is unaffected by CUL4B mutations [16], is apparently unable to rescue the neuronal and developmental deficiencies found in CUL4B patients with XLMR. Notably, the loss of CUL4B function is not lethal in humans [2][3][4][5].…”

Human DNA Ligase I Interacts with and Is Targeted for Degradation by the DCAF7 Specificity Factor of the Cul4-DDB1 Ubiquitin Ligase Complex

“…The complexes formed by cullins, Cul4A and Cul4B, with DDB1 have been implicated in several different aspects of genome maintenance, including the repair of DNA damage caused by UV light (27)(28)(29), replication licensing (30 -32), DNA damageand stress-activated signaling (33,34), replication-associated nucleosome assembly (35), and DNA polymerase ␦ (Pol␦) subunit structure (36,37). Furthermore, mutations in CUL4B have been identified in humans with mental retardation, macrocephaly, and peripheral neuropathy with cell lines from these patients exhibiting defects in the repair of camptothecin-induced DNA single-strand breaks (38).…”

Akt Phosphorylates Wnt Coactivator and Chromatin Effector Pygo2 at Serine 48 to Antagonize Its Ubiquitin/Proteasome-mediated Degradation

“…4B). Two Cul4 paralogs, Cul4A and Cul4B, exist in mammals; they share 82% sequence identity, both interact with DDB1, and they have been shown to play redundant roles in some contexts (55,56,70,71). Overexpression of Cul4A led to reduced FLAG-Pygo2 protein level in a dosage-dependent manner (Fig.…”