The genetic material of every mammalian cell is stored in the nucleus. In healthy organisms, the nucleus, as a rule, is round in shape, provided by the stability of the nuclear envelope and the nuclear lamina. The latter is a network of proteins sandwiched between the inner nuclear envelope and DNA, and largely shapes the shape of the nucleus.
Anomalies in the shape of the nuclei are often observed in human diseases such as cancer. In particular, nuclear abnormalities called micronuclei (small nuclear structures near the main nucleus) and nuclear vesicles (protrusions of the main nucleus) can have a major impact on the integrity of genomic DNA. These abnormalities have historically been used as a diagnostic and prognostic tool for several types of cancer such as breast cancer. However, the exact molecular mechanisms of their formation are not fully understood.
The absence of MOF causes a stochastic shape loss of the kernel.
In their latest study, the laboratory of Max Planck director Asifa Akhtar discovered the spontaneous accumulation of micronuclei and nuclear vesicles in primary cells after the loss of an epigenetic regulatory enzyme called MOF. This well-studied enzyme facilitates access to genetic material by modifying histones, which are the proteins around which DNA is wound in the nucleus. “MOF deposits acetyl groups on histones. This opens up the chromatin, the packaging of DNA, and allows gene activity and therefore protein coding. We wondered what the link might be between the classical epigenetic regulator MOF and abnormal nuclear vesicles. and microkernels,” explains Asifa Akhtar.
To investigate this question, his team led by graduate student Adam Karoutas conducted an unbiased identification of the full spectrum of proteins that are acetylated by MOF in the cell. Their analysis showed that MOF targeted not only histone proteins, but also the nuclear plate protein Lamin A/C. In addition, the group identified the MOF-associated NSL complex responsible for the acetylation of Lamin A/C. Lamin A/C is one of the building blocks of the nuclear lamina with a mesh structure that acts as a viscoelastic shell for the nucleus. The study showed that the loss of lamin A/C acetylation leads to softening of the nuclei, which are subject to mechanical stress and eventually destruction. Under these conditions, the probability of the formation of nuclear bubbles and micronuclei is much higher.
Genomic decay in the nucleus
“These nuclear bubbles and individual micronuclei pose a threat to the integrity of the genetic material of cells. When we sequenced the genomic DNA of primary cells lacking MOFs, we found a battlefield,” says Adam Karutas, first author of the publication.
The researchers saw that the collapse of the nuclear architecture destroys parts of the cellular chromosomes while other parts remain completely intact. Separate segments of chromosomes were broken at many points and incorrectly assembled. Entire parts were missing and others were duplicated or incorrectly combined. Scientists and clinicians use the term “chromotripsis” (chromo is a chromosome, and trypsis is a gap in Greek) to describe this genomic catastrophe. “These deaths are associated with congenital diseases and occur in 20-30% of cases of very aggressive cancer. However, for the first time, we were able to observe chromotripsy in primary cells that lacked only MOF,” explains Adam Karutas.
The desperate attempt by cellular DNA repair mechanisms to reassemble the fragments and repair damage results in mutations that eventually drive cell division and death out of control. In an attempt to understand the molecular mechanisms underlying this phenomenon, the researchers studied the epigenetic landscape of nuclear abnormalities. To their surprise, they found that it is completely different in bubbles and nuclear micronuclei than in their neighboring main nuclei.
Epigenetic damage control mechanism
“While the core nuclei showed a balance of “loosely packed” and “densely packed” regions of DNA called euchromatin and heterochromatin, respectively, we observed that the DNA in micronuclei and nuclear vesicles was predominantly in a heterochromatin state. It is believed that this denser form of DNA suppresses the activity of genes in the first place,” says Adam Karutas.
The scientists called this recently observed phenomenon heterochromatin enrichment in nuclear anomalies (HENA). Just as HENA decorates the hand, histone modifications coat the DNA of nuclear anomalies. “Most strikingly, HENA affects gene transcription. It stops the process of using DNA as a template for making RNA. Thus, genes that are “stuck” in nuclear abnormalities and potentially damaged simply switch. We hypothesize that HENA may play a protective role in cells that accumulate nuclear abnormalities and can become carcinogenic,” Asifa Akhtar explains.
Prevent the path to disaster
The scientists hope that future work will help determine the role of the HENA epigenetic mechanism in diseases associated with nuclear abnormalities, such as cancer or aging. Another promising line of research is the pharmacological alteration of lamin A/C acetylation. “We tested histone deacetylase inhibitors. These are epigenetic drugs already used in the treatment of cancer. They work by increasing overall protein acetylation. In our first tests on mammalian cells, we observed an increase in lamin A/C acetylation, which eliminated nuclear abnormalities compared to cells. no MOF,” says Adam Karoutas. “How MOF-dependent acetylation of Lamin A/C is regulated in living organisms is unclear. Thus, a full characterization of this discovery in mouse models in various tissues or during aging would be the next logical step and offers an exciting opportunity to unlock therapeutic potential in the future. Asifa Akhtar says