ORC1 Protein in DNA Replication and Centrosome Regulation, Linked to Rare Dwarfism

Orc1 protein mutations may explain Meier-Gorlin syndrome (MGS), an extreme dwarfism, with small brain size, and other abnormal growth characteristics

Scientists at Cold Spring Harbor Laboratory (CSHL) have linked gene mutations found in patients with Meier-Gorlin syndrome (MGS) with specific cellular dysfunctions that may give rise to this rare condition.

Although only 53 cases of Meier-Gorlin syndrome have been reported in the medical literature since the first patient was described in 1959, it is a malady whose mechanisms are bringing to light new functions for some of the cellular processes common to all life.

Pathology related to MGS is traced to the fundamental
process called mitosis in which cells replicate
their genetic material and prepare to divide into
two identical "daughter" cells.

Bruce Stillman, Ph.D., CSHL president, professor,and cancer biologist has made seminal discoveries over three decades that helped reveal the choreography of how chromosomes duplicate within cells. Dr Stillman's research suggests how mutant versions of a protein called Orc1 contributes during mitosis in two distinct ways to MGS. The research is published online ahead of print in Genes & Development.

Orc1 is the largest of six proteins that form parts of a cellular machine called the origin recognition complex, or ORC. In human cells, ORC attaches to DNA at specific locations whenever a cell is preparing to duplicate and go through cell division. These locations are called replication start sites or origins.

When ORC and several other helper proteins attach, each of the assembled groups of proteins is said to form a pre-replication complex (pre-RC).

When pre-RCs have attached at all of the "start" sites throughout the genome - a process called licensing - cells can then begin to copy the double helix of DNA to produce two copies, one for each daughter cell. ORC enables a cell to keep track of its DNA replication which is essential as the genetic material must be copied once, and only once.

Origin recognition complex (ORC) proteins
have many roles within the cell.
Not only are they involved in DNA replication,
a number of them are also known to be involved
in regulation of a cellular organ called the centrosome.

Floating inside the cell's watery cytoplasm, the centrosome
helps regulate the cell's progression through the cell-cycle.
It ensures that the duplicated chromosomes are segregated
evenly to the daughter cells.

During replication, the centrosome organizes threadlike
microtubules that form a delicate spindle segregating the two identical sets of chromosomes - the centrioles.

Serving as anchor points for those "threads" the centrioles, just like DNA, must be licensed to insure only one copy is made per cell division. The centrosome also helps establish cell outgrowths, such as axons in brain cells and cilia in many other cells of the body.

Stillman and Manzar Hossain, Ph.D., have followed up clues in recent research linking mutations in the genes encoding proteins of the pre-RC, including Orc1, with mutations in Meier-Gorlin syndrome patients. In a study published this February 2012, a multinational research team noted that MGS patients with mutations in Orc1 were the shortest and had the smallest brain size.

The new study by Hossain and Stillman describes how Orc1 mutations cause cellular dysfunctions that contribute directly to the most severe instances of dwarfism and small brain size.

Stillman also notes that while Orc1 MGS individuals
have a relatively small brain,
they display normal intelligence,
suggesting the enormous potential
of the human brain.

In their dissection of Orc1, Hossain and Stillman discovered that different domains of the Orc1 protein control centrosome copy number and DNA replication; and that both are involved in MGS pathology.

The CSHL scientists suggest that centrosome reduplication as well as dysregulation of DNA replication (in which Orc1 is also involved) are directly associated with the more severe manifestation of dwarfism and microcephaly, or abnormally small brain size, seen in the most pronounced MGS cases.

For the first time, they observed that when the Orc1-encoding gene is mutated as observed in MGS patients, the role normally played by the Orc1 protein in preventing the centrosome from re-duplicating itself is disturbed. By causing defects in the duplication of centrosomes, processes such as cell division and cell signaling can go awry, leading to the severe growth defects found in Orc1 MGS patients.

How tissues come to be the size that they are is still not well understood. Studying how mutations in centrosomes affect tissue and body size may lead to insights into body and brain size regulation, Stillman suggests.

"Meier-Gorlin syndrome mutations disrupt an Orc1 CDK inhibitory domain and cause centrosome reduplication" appears online a head of print on August 14, 2012 in Genes & Development. The authors are: Manzar Hossain and Bruce Stillman. The paper can be viewed online at: http://genesdev.cshlp.org

This research was supported by a grant from the National Cancer Institute [CA13106].

About Cold Spring Harbor Laboratory Founded in 1890, Cold Spring Harbor Laboratory (CSHL) has shaped contemporary biomedical research and education with programs in cancer, neuroscience, plant biology and quantitative biology. CSHL is ranked number one in the world by Thomson Reuters for impact of its research in molecular biology and genetics. The Laboratory has been home to eight Nobel Prize winners. Today, CSHL's multidisciplinary scientific community is more than 360 scientists strong and its Meetings & Courses program hosts more than 12,500 scientists from around the world each year to its Long Island campus and its China center. Tens of thousands more benefit from the research, reviews, and ideas published in journals and books distributed internationally by CSHL Press. The Laboratory's education arm also includes a graduate school and programs for undergraduates as well as middle and high school students and teachers. CSHL is a private, not-for-profit institution on the north shore of Long Island. For more information, visit www.cshl.edu.

Original article: http://www.cshl.edu/Article-Stillman/protein-involved-in-dna-replication-centrosome-regulation-linked-to-dwarfism-and-a-proportionally-s