Homologs of breast cancer genes in plants

Abstract

Since the initial discovery of genes involved in hereditary breast cancer in humans, a vast wealth of information has been published. Breast cancer proteins were shown to work as tumor suppressors primarily through their involvement in DNA-damage repair. Surprisingly, homologs of these genes can be found in plant genomes, as well. Here, we want to give an overview of the identification and characterization of the biological roles of these proteins, in plants. In addition to the conservation of their function in DNA repair, new plant-specific characteristics have been revealed. BRCA1 is required for the efficient repair of double strand breaks (DSB) by homologous recombination in somatic cells of the model plant Arabidopsis thaliana. Bioinformatic analysis indicates that, whereas most homologs of key components of the different mammalian BRCA1 complexes are present in plant genomes, homologs of most factors involved in the recruitment of BRCA1 to the DSB cannot be identified. Thus, it is not clear at the moment whether differences exist between plants and animals at this important step. The most conserved region of BRCA1 and BARD1 homologs in plants is a PHD domain which is absent in mammals and which, in AtBARD1, might be involved in the transcriptional regulation of plant development. The presence of a plant-specific domain prompted us to reevaluate the current model for the evolution of BRCA1 homologs and to suggest a new hypothesis, in which we postulate that plant BRCA1 and BARD1 have one common predecessor that gained a PHD domain before duplication. Furthermore, work in Arabidopsis demonstrates that - as in animals - BRCA2 homologs are important for meiotic DNA recombination. Surprisingly, recent research has revealed that AtBRCA2 also has an important role in systemic acquired resistance. In Arabidopsis, BRCA2 is involved in the transcriptional regulation of pathogenesis-related (PR) genes via its interaction with the strand exchange protein RAD51.

Keywords: Arabidopsis; BARD1; BRCA1; BRCA2; BRCC36; DNA repair; breast cancer; homologous recombination.

Figure 1

Structural comparison of the human and Arabidopsis BRCA1 and BARD1. BRCA1 and BARD1 proteins of Homo sapiens and Arabidopsis thaliana: Despite their difference in length (1863 aa in humans and 941 aa in Arabidopsis), these proteins have a very similar composition with common RING, P300/CBP interaction and BRCT domains. HsBRCA1 harbors a coiled-coil domain that is absent in AtBRCA1, whereas we found a larger than normal PHD in Arabidopsis BRCA1 and BARD1, but not in the human protein. BRCT, BRCA1 C-terminal; CBP, CREB binding protein; P300, histone acetyltransferase p300; PHD, plant homeodomain; RING, really interesting new gene.

Figure 2

Alignment of the PHD domains and their C-terminal extension. Shown here is an alignment of all the extended PHD domains found in the putative BRCA1 and BARD1 homologs of plants. The C4HC3 motif residues that are typical for PHD fingers are marked with an asterisk. It is clearly visible that the N-terminal area upstream of the typical PHD motif is also highly conserved. In fact, it is the most highly conserved domain in the plant BRCA1 and BARD1 homologs. The only organisms for which we could not find a PHD were Chlamydomonas reinhardtii and Volvox carteri. The PHD in Selaginella moellendorffii and both Ostreococcus species lack one conserved cysteine in this motif.

Figure 3

Model of the evolution of BRCA1 and BARD1 in animals and plants. We propose that there was a single predecessor for the animal and plant BRCA1 and BARD1 genes. After the division of the animal and plant kingdoms, the single plant gene acquired a PHD domain. Afterward, there must have been an independent duplication of the plant and animal proteins, leading to BRCA1 and BARD1 in both kingdoms. Because the plant gene already had a PHD prior to its duplication, we can find that domain in all BRCA1 and BARD1 plant proteins.

Figure 4

Evolution of BRCA1 and BARD1 in plants. This tree shows the phylogeny of the plant BRCA1 and BARD1 proteins calculated with the Minimum Evolution method. It is clearly visible that there is a common predecessor to both BRCA1 and BARD1. (A) Depicts the putative point where the PHD domain was acquired. (B) Shows the point of duplication and separation of the BRCA1 and BARD1 proteins.

Figure 5

The three BRCA1 complexes in humans. The three human complexes in which the BRCA1–BARD1 heterodimer was found are called the A, B, and C complexes. The A complex is involved in DNA repair via homologous recombination. The heterodimer is needed to load RAD51 on single-stranded DNA, where it then catalyzes the strand invasion. The B complex has a role in the G1/S cell cycle checkpoint, and the C complex is involved in the G2/M checkpoint and 5′ end resection, which is the initial step after a DNA double strand break. See the text for details. (Proteins with a known homolog in Arabidopsis are colored, and the names are written in bold; ABRA1, Abraxas1; BARD1, BRCA1 associated RING domain protein 1; BRCA1, breast cancer susceptibility gene 1; BRCA2, breast cancer susceptibility gene 2; BRCC36, BRCA1/BRCA2 containing complex subunit 36; BRCC45, BRCA1/BRCA2 containing complex subunit 45; CtIP, CtBP interaction partner; FANCJ, Fanconi anemia complementation group J; MLH1, mutL Homolog 1; MRE11, meiotic recombination 11 homolog A; MSH6, mutS Homolog 6; NBA1, new component of the BRCA1 A complex; NBS1, Nijmegen breakage syndrome 1; PALB2, partner and localizer of BRCA2; RAD50, radiation sensitive 50; RAD51, radiation sensitive 51; TopBP1, topoisomerase 2 binding protein 1)

Figure 6

Recruitment of the BRCA1 A complex to a DSB in mammals. (A) After a double strand break (DSB) is recognized, ATM phosphorylates the histone variant H2AX in proximity to the DSB, which is subsequently called γ-H2AX. (B) Next, the mono-ubiquitination of the phosphorylated H2AX occurs. It is unclear which protein facilitates this, but it is possible that RNF8 is responsible for the initial ubiquitination together with the E2 ligase UBC13 and MMS2. Another hypothesis is that BMI1 mono-ubiquitinates γ-H2AX before RNF8 is recruited, and RNF8 elongates the K63-linked ubiquitin chain. (C) With the help of HERC2, RNF168 is recruited and further elongates the ubiquitin chain, again with UBC13 and MMS2. (D) The ubiquitin chain is then recognized by RAP80, a part of the BRCA1 A complex. This complex then orchestrates DNA repair by bringing RAD51 to the DSB. (Proteins with a known homolog in Arabidopsis are colored and the names are written in bold; ABRA1, Abraxas; ATM, ataxia telangiectasia mutated; BARD1, BRCA1 associated RING domain protein 1; BRCA1, breast cancer susceptibility gene 1; BRCA2, breast cancer susceptibility gene 2; BRCC36, BRCA1/BRCA2 containing complex subunit 36; BRCC45, BRCA1/BRCA2 containing complex subunit 45; MDC1, mediator of DNA-damage checkpoint protein 1; NBA1, new component of the BRCA1 A complex; PALB2, partner and localizer of BRCA2; RAD51, radiation sensitive 51)

Figure 7

Structure of human and Arabidopsis BRCA2. The human BRCA2 and the two Arabidopsis BRCA2 proteins differ in terms of their length (3418 aa compared to 1151/1155 aa). Despite that difference, almost all the important domains of the human protein are conserved in plants. The only exceptions are the missing TR2 region and the reduced amount of BRC repeats in Arabidopsis. (OB, oligonucleotide binding; TR2, terminal repeat 2)