BSAVA News

The problem of inherited diseases: Do we have the answers? DISEASES that are inherited or have a major genetic component are extremely important in small animal practice. There are now more than 300 inherited disease entities recognised in dogs and one new inherited disease can be added to the list approximately monthly. Recent advances in molecular genetics have led to the identification of the gene mutations responsible for many inherited diseases in humans and also of genetic factors that lead to increased susceptibility to disease. We can expect similar advances to follow, albeit at a

slower rate, in small animal medicine.

It is recognised that the occur- rence of disease results from the interaction of the genetic status of the animal with environmental factors. Some diseases may be genetic in basis with modifica- tion by environmental factors, whereas others may be predomi- nantly environmental but possi- bly associated with a genetic predisposition. There are several examples where defects known to be due to a particular gene muta- tion vary significantly in features such as age of onset, rate of pro-

THE veterinary profession has, over the years, worked hard with committed dog breeders to improve the health and welfare of pedi- gree dogs, as evidenced by the BVAKennel Club hip dysplasia and eye schemes. With recent scientific advances in molecular biology gradually filtering their way through to small animal veterinary medicine, the tools we need to make a greater impact in this area are slowly becoming available.

The BSAVA holds a fruitful liaison with The Kennel Club in the form of the BSAVAKennel Club Scientific Committee, which offers advice and encouragement to breeders and to members of the veter- inary profession who wish to gather information about disease problems in specific breeds and to design programmes to control their prevalence.

We intend to publish a series of articles in the BSAVA News each month until the end of this year to keep members informed of the hard work that is being undertaken in this important area of small animal veterinary medicine. The first in the series, which appears above, is written in a way that I hope will increase the understand- ing of new technologies in molecular genetics and the impact they are having, and may have in the future, on our ability to screen for inherited disease problems.

JONATHAN ELLIOTT Chairman, BSAVA Scientific Committee

Examining an X-ray of a dog with h ip dysplasia. The identification of the causal gene mutations, and the eradication of the defect, i s a great challenge for pol<vgenic conditions such as h ip dysplasia

gression or severity. This varia- tion may be due to the genetic background on which they are expressed and, or, environmental factors.

The maintenance of purebred dogs within isolated breed pools coupled with certain breeding practices can influence the fre- quency of occurrence of inherited defects. Successful show dogs are often used extensively at stud, leading to the presence of their genes in a relatively high propor- tion of subsequent generations. This can have beneficial effects by bringing out desirable charac- teristics, but it can also have deleterious effects if a widely used stud dog turns out to be a carrier of a recessively inherited

CONTENTS The problem of inherited diseases 299 Making our presence felt at Crufts 302 Petsavers Malassezia project nears completion 303 Weight loss benefits Petsavers 304 Drawing in the funds 304 Squashing the opposition 305 Going Dutch . . . 305 Jerusalem: Something for everyone 306 News in brief 306

Editor: Frances Barr Division of Companion Animals, University of Bristol, Langford House, Bristol. Avon BS18 7DU.

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disease. In such an instance, the number of carriers of this defect can increase considerably within a couple of generations making it much more likely that two car- riers are mated and produce affected offspring. This is the so- called popular sire effect. The practice of line breeding also increases the likelihood of two carriers of recessive disease being mated. Inbreeding is also import- ant in encouraging the persist- ence of polygenic traits.

Is a particular disease inherited?

An inherited basis for a disease may be suspected where there is breed specificity for the disease, or even more strongly where it occurs most frequently in partic- ular breeding lines. An accurate diagnosis and definition of the disease or syndrome is essential and should be followed by screening for the disease within the affected breed or family to allow pedigree analysis in an attempt to ascertain the mode of inheritance. It may not be pos- sible to establish the mode of inheritance without particular animals being test mated. Such a practice carries its own moral and possibly legal implications.

How does a gene defect cause disease?

Genes are sections of DNA arranged on chromosomes which code for the production of a spe- cific protein such as a metabolic enzyme, structural protein or a factor to control development. Alterations in the gene sequence (mutations) may alter the result- ing product (eg, stopping produc- tion of the protein or changing the amino acid sequence of the resulting protein). What effect any particular gene mutation has depends on the normal function of the gene product and how this is altered by the mutation. For example, in Samoyeds with a familial nephropathy, a single nucleotide substitution in the gene coding for a5 chain of type IV collagen has been demonstrat- ed to give rise to a defect in this protein, which results in defec- tive structure of the glomerular basement membrane. The base- ment membranes in affected dogs fragment and split during the first year of life and the animals develop signs of renal disease.

Mendelian genetics

The dog genome is spread over

Generalised progressive retinal atropy in a miniature long-haired dachshund. Most of the generalised progress- ive retinal atrophies are autosomal recessive diseases (photo courtesy o f Dr S . M. Crispin, University of Bristol)

78 chromosomes (38 pairs of autosomes and two sex chromosomes). Where a gene locus is on the X chromosome it is X-linked and when on the auto- somes it is described as autosomal. As chromo- somes are in pairs, each individual has two copies of each of the genes positioned on the autosomes (they receive one of each pair of autosomes from each parent). For any particular DNA locus there may be several potential differences that exist in a given population. When these differences affect genes they account for the

genetic variability between indi- viduals and if the differences are deleterious (as mentioned above) they may be responsible for genetic disease. An individual who has a difference between the two DNA copies at a particular locus is described as being heterozygous for that locus. Those who have two identical copies of the locus under consid- eration are homozygous.

Modes of inheritance

0 Autosomal dominant - The gene involved is present on an autosome and only one copy of the gene needs to be mutated to produce the disease. When both copies are mutated a more severe manifestation may result. Auto- soma1 disease passes directly from one generation to the fol- lowing and never skips a genera- tion. It is relatively easy to eradicate dominant disease because the presence of the defective gene always causes the phenotype.

The picture of autosomal domi- nant inheritance is complicated in instances where there is partial penetrance. With partial pene- trance, fewer than 100 per cent of the heterozygote animals (ie, those with one copy of the mutant gene in question) develop the clinical manifestation of the disease. When a condition shows complete penetrance, 100 per cent of heterozygotes will express the phenotype. Whether the dis- ease that is partially penetrant is expressed or not in the hetero- zygous state probably depends on the background genotype and, or, environmental factors. Identify- ing the mode of inheritance of characteristics that are partially penetrant is more difficult than those that are fully penetrant as they may skip generations in a similar way to recessive disease.

0 Autosomal recessive - This is the commonest mode of inheri- tance of single gene traits in dogs. The frequency of recessive defects increases when the inbreeding of

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lines carrying the mutant gene is increased. The mutation involves a gene carried on one of the auto- somes. To allow expression of the defect both copies of the gene must be mutant - that is, the affected individual will be homozygous for the particular defect. Animals that are hetero- zygous are described as carriers and, although they have one mutant copy of the gene in ques- tion, they do not show any out- ward sign of the disease.

A characteristic of autosomal recessive disease is that it may skip one or more generations and it is not until two carriers are mated that a percentage of the offspring will be expected to develop the disease (ie, to be homozygotes). The identification of carriers is crucial in the eradi- cation of autosomal recessive defects. Examples of autosomal recessive diseases include most of the generalised progressive retinal atrophies.

0 X-linked recessive - The genes responsible for X-linked dis- orders are on the X chromosome. Male mammals have one X chro- mosome (which they receive from their mothers) and one Y chromosome. If the one X chro- mosome of the male contains the mutant gene this is expressed. Females with a mutation on only one of their X chromosomes do not express the disease pheno- type but are carriers. The disease phenotype is therefore expressed predominantly in males. While it is possible for females to be homozygous for the mutation and therefore to develop disease, this is usually very rare. Examples of X-linked recessive diseases are haemophilia A and haemophilia B. X-linked dominant disease can also occur but is not common.

0 Sex-limited inheritance - This mode of inheritance is due to a locus on an autosome but is only expressed in one sex. For ex- ample, cryptorchidism is obvi- ously seen only in males but the genes for this condition are

Fluorescent in situ hybridisation stoining of a metaphose spread of canine chromosomes using canine cosmic probes [photo courtesy of Dr P. Fisher, Animal Health Trust, and Dr E. Nacheva, Department of Haernatology, University of Cambridge)

carri,ed by both males and females.

0 Polygenic defects - More than one gene locus influences the expression of certain defects. Environmental influence prob- ably plays an important role in the expression of polygenic defects. Recent investigations suggest that some defects (such as some congenital cardiac defects in humans) that were thought to be polygenic are mainly due to altered expression of one major gene. True polygenic defects pre- sent the greatest challenge to identification of the causal gene mutations and also to the eradi- cation of the defect (eg, hip dysplasia).

Control of inherited disease

To be successful, schemes to eradicate inherited disease have to rely on accurate diagnosis of the disease phenotype, if possible prior to sexual maturity, and also knowledge of the mode of in- heritance. Schemes to screen for inherited disease have long been in existence - for example, the BVA/KC/ISDS Eye Scheme and the BVA/KC Hip Dysplasia Scheme. The success of such schemes requires that as many animals as possible within the breed in question are screened (ie, not just those from affected breeding lines). Certain condi-

tions screened for under the eye scheme are not identifiable until after sexual maturity and the majority are recessively inherit- ed. To reduce significantly the incidence of such diseases, test- ing under the scheme needs to be part of a concerted effort on behalf of breeders to control the defect in question.

As already stated, autosomal dominant disease is relatively easy to control whereas the exist- ence of phenotypically normal carrier dogs for recessive diseases makes eradication more difficult. Some carriers can be identified on pedigree analysis - for ex- ample, the offspring of an affected dog or the parents of an affected dog will have at least one mutant copy of the gene. Unfortu- nately, many carriers cannot be identified by pedigree analysis so other methods are required. These are detailed below.

0 Test mating - Test mating may be the only way of ascertaining that a breeding animal is not a carrier of a particular condition. It has been used successfully in the past as part of the control scheme of some diseases. Test mating, though, is time consum- ing, expensive, introduces moral arguments and is impractical if the disease in question is not apparent until later in life or the animals which are homozygous

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for the condition do not survive to be bred from.

0 Objective measurement of a gene product (by direct or func- tional assay) - Carriers of some metabolic disorders can be iden- tified by biochemical assay either for the level of the gene product or accumulation of a metabolic substrate. An example of such a condition is fucosidosis, an autosomal recessive defect in which there is a mutation in the gene for the lysosomal enzyme fucosidase which renders the enzyme protein non-functional. Carriers of the mutant gene, which externally appear normal, can be assayed for fucosidase enzyme activity levels and would, according to the theory of gene dosage, be expected to have only 50 per cent of the normal level of fucosidase. In reality, the situation is not so straightforward because, due to the normal varia- tion in fucosidase levels, there is some overlap between the normal dog population and the carrier population.

0 Direct detection of the gene mutation - The ideal way to allow eradication of a recessive defect is to identify the specific gene defect responsible and to develop a test for that mutation which can be performed on a sample of DNA (eg, from a blood sample). An example of this is the recent identification of the gene defect responsible for the rod-cone dysplasia type 1 form of generalised progressive retinal atrophy in Irish setters and the development of a DNA-based test for this mutation. This allows screening of the breeding popula- tion for the mutation and offers the chance of complete eradica- tion of the defect.

0 Detection of DNA polymor- phisms closely linked to mutant genes - DNA-based tests can also be used to identify carriers where the actual gene mutation has not been established. To allow this, close linkage between the mutant

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locus and a polymorphic (vari- able) DNA marker is required. If the two loci are very close to- gether on the chromosome one particular polymorphism of the marker will almost always be inherited with the mutant gene copy and, if this polymorphism does not occur in the normal population, it can be used to demonstrate the presence of the mutation. Mapping of the dog genome to provide evenly spread polymorphic markers for use in such linkage work is under way and linkage of markers to disease loci has already been demonstrated.

Conclusions

As the modern methods of molecular genetics and genome mapping are applied to the in- vestigation of canine disease, the

genetic cause of a greater number of inherited diseases will be iden- tified, allowing development of specific DNA-based diagnostic tests. Additionally, the presence of markers linked to many of the disease loci will be demonstrated, allowing identification of animals likely to be carrying the disease locus.

DNA testing is not the sole answer to inherited disease and large-scale phenotypic screening programmes will remain the main- stay of our approach to many inherited diseases for some time to come. However, in the future, a combination of all these meth- ods will allow breeding towards a healthier dog population.

DR S. PETERSEN-JONES Department of Clinical

Veterinary Medicine University of Cambridge

Making our presence felt at Crufis

A view f rom the side: the n e w BSAVA stand at Crufts with (from left to right) Mr Colin Price [Chairman of Petsavers), Mr Peter Renwick [DVOphthal, visiting specialist), Mr Richard Harvey [Chair- man, Members Services Committee) and c1 member of the public (photo courtesy of Jo Wills)

CRUFTS is enormous. For four hectic 13-hour days, the dog- showing fraternity gathers at the National Exhibition Centre in Birmingham. The trade exhibition is big and the public attend in droves. The sheer density of stands, people and dogs has to be experienced to be believed. It was into this melee that the BSAVA dipped its collective toe last year and, undaunted, returned this March.

The stand was sited between Discover Dogs and the Special Events Ring, an area generously donated by The Kennel Club. In addition to Crufts, we also hope to have stands at the National and Supreme cat shows. Our aim is to promote people's perception of the veterinary profession as competent, capable and caring, and also to highlight the role of Petsavers and the Clinical Stud- ies Trust Fund to the dog- and cat-owning public. Many of those who visited the stand had never