22 Aug 2011

Hirschsprung’s (HSCR) disease (congenital megacolon) is a birth defect affecting 1 in 3000 babies in Asia. These babies suffer from severe constipation and intestinal obstruction because nerve cells which co-ordinate bowel movement are absent. The reasons for the absence of nerve cells in the bowel are unclear, and therefore treatment remains unsatisfactory. The University of Hong Kong Li Ka Shing Faculty of Medicine has conducted a study that integrates genetics, statistics and bioinformatics with experimental biology to investigate the mechanisms by which a small number of genes interact with each other to cause HSCR disease. With the mouse models, the study identified that specific DNA changes in Patched (PTCH1) and Delta-like 3 (DLL3) genes (genes that work coordinately to control the sequential waves of neural differentiation and formation of the nerve cells in bowel) will lead to premature development of nerve-supporting cells, thus increase the risk of having HSCR by 278%. The novel finding provides important breakthrough for understanding the pathology of this genetic disease. The research is just published in “Journal of Clinical Investigation”, the top journal in the field of developmental biology.

Professor Paul Tam Kwong-hang, Chair of Pediatric Surgery, Department of Surgery, The University of Hong Kong Li Ka Shing Faculty of Medicine who led the study says, “Neural stem cells normally develop first into nerve cells and then into nerve supporting cells in embryos; in Hirschsprung's disease, premature development of nerve-supporting cells reduces stem cells available for nerve cell development, thus causing HSCR disease. The findings shed light on the behaviour of neural stem cells in general and pave the way for cell-transplant therapy for Hirschsprung's disease in particular.”

Another leading researcher of the study, Dr Elly Ngan Sau-wai, Assistant Professor of the Department of Surgery, The University of Hong Kong Li Ka Shing Faculty of Medicine remarks, “Getting insights into human biology allows us to have a better understand of genetic disease mechanisms, which will pave a way for improving diagnosis and treatments in the long run.”

About Hirschsprung’s (HSCR) disease
Babies born with HSCR disease will die unless the portion of the bowel with no nerve cells is surgically removed. In Hong Kong, over 120 affected babies have been operated in the last ten years. Nevertheless, the functional outcome of surgery is variable and a significant number of patients still suffer from life-long complications, leading to not only tremendous psychosocial impact on the patients, but also a heavy financial burden to the health care system.

HSCR disease is attributed to a failure in formation of nerve cells in the intestine. During development, neural stem cells in the bowel receive varied signals from their environment and differentiate into a wide-range of nerve cells, which subsequently form a neural network to control the bowel movement. At least two waves of differentiation (neurogenesis and gliogenesis) are involved. In general, neurogenesis starts first to generate varied types of neurons, which is then followed by gliogenesis, formation of the supporting cells (glia) for the neurons. This differentiation process has to be precisely regulated, in order to establish a functional network. DNA changes in the genes implicated in this process may disturb the functional neural network formation, leading to HSCR disease.

Study results
For a better understanding of the genetic causes of HSCR, the HKU research team has conducted a genetic study to explore the complete DNA content of these patients and has found new genes implicated in the disease. The work was published in “Proceedings of the National Academy of Sciences” in 2009.

Based on the data of 200 patients and 408 healthy individuals generated in previous study in 2009, HKU researchers employed another sophisticated computational and statistics methods to identify a discrete number of genes which interact with each other to cause HSCR disease. It was revealed that specific DNA changes in PTCH1 and DLL3 confer higher risk to HSCR. These two genes encode functional proteins mediating two key developmental signaling pathways, Hedgehog and Notch, respectively. Therefore, it is conceivable that coordination between these two signal pathways is crucial for generation of nerve cells in the bowel.

To demonstrate this, two mouse models were established, in which removal of PTCH1 induced robust DLL expression and turned on the Notch pathway, leading to premature gliogenesis and reduction of stem cells in mutant bowels. DLL bridged Hedgehog and Notch pathways to coordinate neuronal and glial cell formation. In addition, the researchers also observed this functional implication of Hedgehog and Notch on neural differentiation in a human stem cell model. Thus, DNA changes in PTCH1 and DLL3 may affect the differentiation of neural stem cells in the bowel, giving rise to HSCR disease.

Collectively, HKU researchers have defined PTCH1 and DLL3 as HSCR susceptibility genes and suggest that Hedgehog/Notch-induced premature gliogenesis may represent a new disease mechanism for HSCR.

About the HKU research team
This study was performed at Department of Surgery, HKU, and was led by Professor Paul Tam Kwong-hang, and Dr Elly Ngan Sau-wai. Genetic analysis was performed in collaboration with Dr Maria-Mercedes Garcia-Barcelo of the Department of Surgery, and Professor Sham Pak-chung, Dr Stacey S. Chemy and Dr Benjamin Yip Hon-kei of the Department of Psychiatry, HKU. Other researchers include Dr Lau Sin-ting, Dr Vincent Lui Chi-hang, Dr Kenneth Wong Kak-yuen, Dr Sham Mai-har, Mr Eric Sat Wai-yin and two postgraduate students (Carmen Kwok Ka-man and Poon Hiu-ching).

Acknowledgement
This work was supported by a seed funding grant for basic research from the University of Hong Kong, and by research grants (HKU775710, HKU773909, HKU765407, HKU775907, and HKU752806) from the Research Grants Council.