Up to 1993, three research teams had managed to produce "models" of CF in mice(29). Each of these "models" is considered separately.

NORTH CAROLINA MOUSE
In August 1992, a research team from North Carolina reported on their work with a gene in mice (identified in 1991 as being equivalent to the CFTR gene in humans), in which they disrupted the statement of the gene in embryonic stem cells by incorporating part of a different gene(30). After several attempts, they managed to transmit the new gene to offspring and to breed from them. Three types of mice resulted -

1) CFTR (+/+) - normal parent mice, in which the inherited gene from healthy unmodified parents both carried the normal CFTR gene;
2) CFTR (+/-) - mice which only inherited the disrupted gene from one modified
parent;
3) CFTR (-/-) - mice inheriting the disrupted gene from both of the modified parents.

In the CFTR (-/-) mice, the CFTR gene was severely disrupted - so the animal "model" became known as a *knock-out" mouse(31).

Despite its limitations, the North Carolina CFTR (-/-) mouse was greeted with enthusiasm. Colins and Wilson from Michigan University wrote in the journal, `Nature`, in August 1992, that it would, "Undoubtedly have significant repercussions in the rigorous search for a better understanding and improved treatments for this debilitating disease, and stands as a significant milestone in the midst of a flurry of recent stunning advances in CF research", but added, "Similarities between the pathology of the mouse model and the human disease did not extend well to the reproductive, pancreatic, hepato-biliary [liver] and respiratory systems"(32).

For each of these body systems, the findings in the CFTR (-/-) mouse differed from those recognised in the human clinical situation.

Reproductive System
In the CFTR (-/-) mouse, there was no evidence of abnormality in the reproductive tract, and up to the time of publication of the first reports on CFTR (-/-) mice, one long-surviving (-/-) male had fathered three litters(33).

The results of animal experiments were in complete contrast to the clinical situation, where 95% of male human CF sufferers are infertile(33).

Pancreatic System and Liver
Except for inflammation of the gall bladder, there was no conclusive evidence in CFTR (-/-) mice of pancreatic or liver disease(34). On examination, only two out of five (40%) mice showed pancreatic changes, and there was no sign of focal bilary cirrhosis(35).

In humans, pancreatic changes are seen in 85% of CF patients, and focal bilary cirrhosis is the major pathological change in the liver of 14% to 43% of CF sufferers(35).

Respiratory System
`Science`, in August 1992, reported that CFTR (-/-) mice, "don`t have the mucus-dogged lungs and persistent lung infections that plague human CF sufferers. That could be because the lungs of mice are fundamentally different from those of humans - they have fewer mucus secreting glands and cells overall.."(35).

Baringa (the author of the above-cited article) also referred to a comment made by Boucher that the lungs of CF mice have some of the same signs as the lungs of human CF sufferers(36) but Baringa also conceded, "Despite these similarities, symptoms of the CF mice are hardly identical to those of human CF"(36).

Further to this, Koller (leader of the North Carolina team) is quoted as saying that "it would be very unlikely to get a mouse model that is identical to humans"(37). The reason is (according to Baringa) "because there are just too many subtle physiological
differences between the two species" [i.e. mice and humans - SAV](38).

In humans, lung damage is not apparent in newborn babies with cystic fibrosis. Abnormal secretions begin to appear in the first weeks of life. The earliest changes are dilation of the sub-mucus gland ducts and secretory structures. Functional abnormality in the lung does not first appear until later, with mucus obstruction of the small airways. This is followed by chronic bacterial infection in the first few years of life(39).

Lung infections kill 95% of human CF patients(40,41)

The North Carolina team reported "our CFTR (-/-) mice die early in life of gastrointestinal obstruction, and not from pulmonary infections"(42).

CAMBRIDGE TEAM MOUSE
Less than a month after the North Carolina group had published a paper in `Science` (on 21st August 1992),  a British team at Cambridge University gave a brief description of their version in the `Lancet`.

Jointly funded by the Cystic Fibrosis Trust and the Wellcome Trust, the Cambridge team had inserted a foreign genetic element into the CFTR gene of embryonic stem cells of mice, producing another strain of mouse which carried a disrupted CFTR gene. When carrying the gene from both parents, most of the mice died with their stomachs and intestines full of milk - suggesting poor absorption of food. Some mice which survived the prenatal period developed a distended gut, blocked by clogged mucus - but in other mice, the obstruction only occurred later(43).

The Cambridge researchers believed that intestinal blockage in the mice was similar to the early signs of CF in humans, and stated, "This mutation in the CFTR locus provides an animal model for the most common autosomal recessive disease in north Europeans",adding, "phenotypes that more closely parallel the milder forms of cystic fibrosis, e.g. infertilty only.  The availability of these mutant mice should greatly
facilitate tests of new drugs and gene therapies"(43).

Continuing their work with their supposed mouse "model" of CF, the Cambridge team published more data in May 1993 - describing the symptoms they had observed, and found that their mice (in which genes that determine a particular characteristic are identical) showed a remarkable similarity to the characteristics of the North Carolina mice(44).

Intestinal Blockage
The Cambridge researchers reported on intestinal obstruction of the small intestine (a condition known as meconium ileus), which was not in accordance with findings in most human CF patients - a fact which the researchers, themselves, acknowledged, "Whereas some 80% of the CF mice develop meconium ileus, only about 10% [of human] CF patients are present with these symptoms at birth", and added, "The reason for this difference [between mice and human] is not known"(44).

Pancreatic System
In the Cambridge studies, only 50% of the CF mice showed any sign of blockage in the pancreatic ducts, and pancreatic abnormalities were only seen in mice which were more than 17 days old. In older mice, the changes were not severe enough to alter enzyme secretion(44).

In humans, pancreatic changes are seen in 85% of CF sufferers(45).

Respiratory System
In mice, killed at up to four weeks of age, there was no sign of mucus accumulation in the lungs(46).

In humans, mucus accumulation in the lungs is one of the recognised major causes of death in CF patients(47,48).

Some of the results of the Cambridge animal experimenters were similar to those of the North Carolina group(49). But the Cambridge team also reported, "we also find blockage of pancreatic ducts in some [of our CF] animals, and lachrymal gland[responsible for tear secretion] pathology changes not yet reported [by the North Carolina group] for CF mice"(49).

EDINBURGH MOUSE
Four weeks after the paper published by the North Carolina group, and five days after brief description of the Cambridge team`s CF mouse, researchers from Edinburgh reported, in `Nature` (17th September 1992), on their method of disrupting the CFTR gene of embryonal stem cells by inserting a foreign gene. The work on the production of the Edinburgh team mouse was jointly funded by the Medical Research Council, the Cystic Fibrosis Trust and the Agricultural and Food Research Council(50).
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Stem cells (with the disrupted CFTR gene) were injected into the embryo of another mouse, and then placed back into a female mouse to develop. When the embryos had matured to adult mice, they were mated, and produced three litters - which were named as follows -