A group of scientists has taken another step toward understanding the unsolved Mendelian diseases, according to a recent study. By studying the inheritance of a protein known as SMCHD1, which is coded by the SMCHD1 gene, they discovered that Mendelian diseases are inherited from either parent due to gene mutations in the developing egg or sperm.
A team of National University of Singapore (NUS) scientists led by Assistant Professor Xue Shifeng from the Department of Biological Sciences discovered that mutations in the SMCHD1 gene can cause diseases such as facioscapulohumeral muscular dystrophy (a muscle degenerative disorder) and Bosma arhinia microphthalmia syndrome (BAMS), which causes nose and eye abnormalities.
Discoveries that researchers made
The researchers discovered that SMCHD1 from mothers regulates the expression of a group of genes known as the HOX genes in offspring, which determines the position of body parts in an embryo along its axis from head to tail. The researchers also discovered that inactivating SMCHD1 in female zebrafish causes changes in HOX gene expression, resulting in skeletal defects in their offspring.
The study led by NUS researchers, in collaboration with A*STAR, Yale-NUS and Aix-Marseille University, was published in Nature Communications 23 June 2022.
In mammals, SMCHD1 is important in female X-inactivation, a process in which one of the X chromosome copies is randomly selected and disabled. This makes studying the role of the SMCHD1 gene inherited from mothers difficult because inactivating the SMCHD1 gene is lethal for female mammals.
Zebrafish is use as model organism
The research team decided to use zebrafish, a vertebrate commonly used as a model organism in biomedical research, to circumvent this issue. Zebrafish lack X-inactivation, allowing the team to study the role of the SMCHD1 gene inherited from mothers. The researchers inactivated the SMCHD1 gene in zebrafish to study how it will affect gene expression and structural development in zebrafish offspring.
Observation of the experiment done on zebrafish
NUS scientists observed that SMCHD1 protein is placed into the egg by the mother. The inactivation of the SMCHD1 gene in female zebrafish caused alterations in HOX gene expression in their fertilised eggs. HOX genes play an important role in ensuring the specific patterns and identities of different body parts in the baby. The loss of the SMCHD1 gene resulted in premature activation of HOX genes resulting in skeletal patterning defects in the zebrafish offspring.
Asst Prof Xue and her team demonstrated a new concept that gene products such as proteins from the mother’s egg can control gene expression occurring in the developing embryo. Factors that control gene expression produced by the mother in the developing egg can set up the conditions for proper gene activation after the egg is fertilised with a sperm. Through further lab studies, the team found that the same principle applied to mammals.
Interpreting unsolved genetic diseases
The results of the study could change the way unsolved Mendelian diseases are interpreted. Based on this study, some genetic abnormalities in parents could manifest in their children, opening possibilities of explaining birth defects seen in children by examining the genetic make-up of their parents.
