Hepatitis Virus Found to Attack Kidney

Hepatitis is the inflammation of the liver, caused by one of several viruses. The commonest is hepatitis A, which may occasionally lead to acute liver failure. The hepatitis B virus is much more ferocious: it can cause death in the short term, and increase the risk of liver cirrhosis and cancer in the long term. Hepatitis B virus infection and related diseases are thus major health hazards. Of the 200 million chronic carriers of the hepatitis B virus, 80 per cent live in Asia, and many of these are infected during childhood. In Hong Kong, a vigorous programme of vaccination at birth is likely to reduce the incidence drastically in the next generation, but the situation is less rosy in the less developed parts of Asia.

Researchers in CUHK noted a few years ago that, in one type of kidney disease, the basement membrane of the patient's glomeruli becomes thickened, leading to loss of protein and later, to kidney failure. (Glomeruli are bundles of capillary tubes in the kidney, in which waste material is separated from the blood circulation and eventually passed out of the body as urine.) It appears that this kind of membranous structural change is sometimes associated with hepatitis infection, a possibility first reported some 20 years ago. Is this really the case? What is the mechanism linking liver disease to kidney damage? What light would it shed on hepatitis and on kidney diseases? These were the questions that Prof. K.N. Lai, together with his colleagues Dr. Philip K.T. Li and Dr. John S.L. Tam, set out to investigate, with the support of a grant of HK$260,000 awarded by the Research Grants Council in 1991.

The fact that the hepatitis B virus finds its way into the kidney is not strange; after all, the virus exists in the blood, and all blood eventually passes through the kidney, which acts as a filtering station. The mystery is how this tiny virus, small enough to pass through the capillary, can lead to overall tissue inflammation in the kidney.

The hypothesis is that the hepatitis B virus provokes the manufacture of antibodies. (The agent that induces antibodies is called an antigen.) The antibodies act by capturing the viruses and forming immune complexes; these relatively large complexes may become deposited in the glomeruli, and eventually alter the tissue structure. For so many immune complexes to be deposited, the virus must first replicate in the kidney or in some other organ. But where exactly does it replicate? Prof. Lai started by investigating the biochemistry and pathology of the diseased kidney, hoping to find clues on the mechanism by which the hepatitis B virus can injure the kidney.

The most important element in cell division and replication is the DNA, a double-helix structure in the cell nucleus. Genetic information is contained in the two strands forming the double helix, which are complementary to each other, segment for segment. The RNA is usually a single strand, also complementary to the DNA, and is therefore able to transfer the information from the original DNA template. It does so by excising small segments, called messenger RNA, or mRNA, which are small enough to pass through the wall of the cell nucleus and carry the genetic information outside the nucleus. This is how each of us grow from a single cell, and this is basically how a virus replicates itself. A virus contains not much besides the DNA genetic information.

Thus, if the hepatitis B virus is really replicated in the kidney, then one should be able to find in the kidneys of these patients traces of the DNA, RNA and mRNA associated with the hepatitis

Using PCR techniques to detect mRNA of the hepatitis virus in kidney biopsies from patients. Lane 2 is a positive control of the relevant RNA; it is clear that the same RNA is detected in biopsies located in lanes 5, 6, 7, and 8.

B virus. The discovery of DNA alone would not be decisive, since they could be manufactured elsewhere and brought to the kidney. But RNA or mRNA will stay put after they have performed their duty to transfer DNA information. So the site where they are found will definitely be the site where the virus is replicated. So the investigators set out to look for RNA and mRNA.

After careful research and study, Prof. Lai's team has been able to demonstrate the presence of immune complexes related to hepatitis B e antigen (HBeAg) in the diseased glomeruli. DNA of the hepatitis B virus has also been found in the nearby renal tubules, where urine is concentrated.

However, the amount of RNA found is extremely small and cannot be detected directly. Normally, RNA breeds DNA. Through a technique called reverse transcription, researchers can synthesize small copies of DNA out of the small quantity of RNA found. And through polymerase chain reaction triggered by suitable enzymes, this small quantity of DNA can multiply about 10,000 fold to produce a copious amount, from which the presence or absence of RNA can be detected chemically. Through such a mechanism researchers successfully proved the presence of RNA and mRNA of the hepatitis B virus in the kidney.

The finding suggests that the virus replicates in the tubules of the kidney. The HBeAg that is then shed into the blood circulation induces antibody formation. The chemical complex formed by the HBeAg, its antibody, and the virus circulate in the blood and are finally trapped by the glomeruli. The trapping of the complexes leads to the thickening of basement membranes and local inflammation.

Equipped with the new knowledge they have just discovered, Prof. Lai and his colleagues have now set out to study the role of hepatitis B virus in hepatitis B carrier patients with immunoglobulin A (IgA) nephropathy.