- PAPERS

Palmoplantar hyperkeratosis in Irish terriers: evidence of autosomal recessive inheritance An abnormal development of the epidermis of the footpad was

observed in Irish terriers. At the age of six months, the affected

animals developed smooth parchment-like footpads. The pad

epidermis then hardened and grew lateral cone-like protrusions

of up to 5 mm in diameter. Fissures and cracks developed and

these predisposed the animal to secondary infection. The repeated

occurrence in subsequent generations led to the assumption of

a hereditary form of hyperkeratosis. Evidence for an autosomal

recessive mode of inheritance was derived from a retrospective

analysis of the breeder’s records. The clinical, histopathological

and ultrastructural features of the disease are presented and

the genetic transmission and its implications discussed.

H. BINDER, S. ARNOLD, c. SCHELLING*, M. SUTER? AND

P. WILD?

Journal of small Animal Practice (2000) 41, 52-55

Department of Reproduction, *Institute of Veterinary Biochemistry and tlnstitute of Veterinary Anatomy, Faculty of Veterinary Medicine, University of Zurich, Winterthurerstrasse 260, 8057 Zurich, Switzerland

?Institute of Veterinary Pathology, Faculty of Veterinary Medicine, University of Bern, Langgassstrasse 122, 3012 Bern, Switzerland

Cases of footpad hyperkeratosis running in families have previously been reported for Irish terriers, Kerry blue terriers and Dogues de Bordeaux, although no specific mode of inheritance has been demon-

strated (Kral and Schwartzman 1964, Paradis 1992, Scott and others 1995). Footpad hyperkeratosis is characterised by excessive proliferation of callus skin with alterations restricted to the pad area. Affected dogs need permanent care to avoid the risk of the footpads developing infection-prone crevices which might lead to intermittent severe lameness. Preventive care involves daily foot soaks and, occasion- ally, antibiotic therapy. Secondary footpad hyperkeratosis can be associated with several diseases, such as canine distemper, ichthyosis, systemic lupus erythematosus and zinc deficiency. Also, idiopathic forms of hyperkeratosis occur in older dogs which tend to develop more focal alterations at the periphery of the pads (Paradis 1992).

The present paper describes the situa- tion in one kennel, based on the breeder’s records and recent cases examined. The affected.Irish terriers did not suffer from any obvious predisposition to hyperkerato- sis until, by the age of six months, the proliferation of footpad epidermis became apparent. The homogeneity of clinical features and the fact that all dogs were

FIG 1. Hindpaws of Irish terriers. (A) Normal Irish terrier; (B) littermate with hyperkeratosis. Profiles of affected nails are circular rather than U-shaped

JOURNAL OF SMALL ANIMAL PRACTICE VOL 4 1 FEBRUARY 2000

related led to the hypothesis of hereditary palmoplantar hyperkeratosis.

Clinical features The diagnosis of footpad hyperkeratosis is impossible before the age of four months. Puppies are therefore sold and adapted to their new homes before the disease becomes apparent. Initial signs usually develop at between 18 and 24 weeks of age without any clinically manifest signs. Only experienced breeders realise a tendency of the affected puppies to develop smooth parchment-like footpads. Soon the pad epidermis thickens and becomes indurated and inelastic. Within months, the rims of the pads grow lateral cone-like protrusions up to 5 mm in diameter. The pad surhce becomes hard and expanding fissures and cracks predispose the animal to secondary infections which cause lameness. Nails grow slightly faster and their profile is round rather than U-shaped (Fig 1). Affected animals generally avoid hard floors, dry sand or gravel, and symptoms

appear to be less severe during winter. The altered footpad epidermis needs

permanent attention and lifelong care. Daily soaking of the paws in 50 per cent propylene glycol combined with frequent filing of the footpads to remove surplus keratin and daily anointment prevents fis- sures and relieves minor sores. Extended inflammation of the pads is an indication for antibiotic treatment and the appli- cation of surgical dressing or moccasins. Also, the fast-growing nails need frequent trimming.

MATERIALS AND METHODS

Biopsies were taken from the footpads of two affected dogs for ultrastructural investigation, and compared by electron microscopy with those from unaffected dogs.

Mode of inheritance was investigated by examination of pedigree dara and statistical analysis. The assumption of recessive inher-

FIG 2. Photomicrographs of the footpad epidermis. (A) Affected Irish terrier shows marked irregular epidermal hyperplasia characterised by multiple, thickened and blunted, partly fused papillae with irregular network of rete ridges. The orthokeratotic cornified layer has multiple ruptures. The hyperplastic stratum granulosum shows marked hypergranulosis. (B) Control dog epidermis with regular papillae and rete ridges, covered by a thick, compact orthokeratotic cornified layer

itance was evaluated by the method of direct maximum likelihood (ML). Carrier dogs were identified by afflicted offspring. Estimation of the expected frequency of hyperkeratotic dogs in affected litters was based on this information, The frequency of diseased puppies was checked addition- ally with the a priori method and Xz-test- ing. Both methods account for truncate binomial distribution under incomplete ascertainment (Wieser and Willner 1333).

Histopathological and ultrastructural evaluation Histological examination revealed moder- ate, irregular and papilliform hyperplasia of the epidermis with a well developed stratum granulosum and marked diffuse orthokeratotic hyperkeratosis (Fig 2). In the affected cases, the overlying stratum corneum is compact but more severely ruptured and discohesive than that of a normal terrier footpad epidermis. This could indicate differing consistency of the stratum in hyperkeratotic pads. The stra- tum corneum is presumably less resistant and more easily disrupted by the mechani- cal stress of histological processing. Epider- mal papillae were pronounced, moderately extended, blunted and partly fused. Hypergranulosis in the epidermis was evident. There were, however, no signs of epidermolysis within the epidermis and no inflammatory infiltrates.

Electron micrographs of the epidermis of affected footpads revealed no abnormal structures (Fig 3). Keratin filaments were normal in size and amount, and kerato- hyaline granules and lamellar bodies were present. Ultrastructural aberrations, as found with other keratodermas, could not be identified.

Mode of inheritance The affected animals arose from inbred matings as well as from outcross matings involving sires of diverse origin. The genealogy of the investigated lineage of

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hyperkeratotic Irish terriers covers 13 gen- erations, including 42 litters. Genealogical information was mainly based on the breeder’s records and retrospectively veri- fied by questioning the breeder and owners on signs, behaviour and cause of death of suspected animals. A total of nine affected males and seven affected females were identified, partly retrospectively. Afflicted puppies were identified in nine litters; eight of them with a total of 15 afflicted puppies are shown in the pedigree analysis (Fig 4 ) . The first suspected dog was born in 1976. The recurrent affliction of related animals in subsequent generations and the pattern of incidence with no evidence of horizontal transmission suggests that the disorder may be inherited. Because of the almost balanced gender ratio (nine males, seven females), sex-linked inheritance can be rejected.

The indices (a to f ) denote different kennels of origin of stud dogs. Under the assumption of recessive autosomal inheri- tance, carriers are identified by affected progeny. Analysis of the pedigree reveals that stud dogs producing afflicted off- spring originated in four out of the six kennels. This could indicate multifactorial inheritance rather than a single gene defect unless the deleterious gene is frequent throughout the entire breed. Suspicion of the latter was derived from similar keratin defects observed in humans and deter- mined to be from a single gene (Lucker and others 1994). Considering the fre- quent exchange of breeding stock among kennels and the relatively small breeding population in pedigree dogs, those stud dogs may well have related ancestry. Although no information about earlier generations is available, a high frequency of the deleterious gene in the Irish terrier breed could be explainable by the founder effect.

The estimation of the frequency of all hyperkeratotic puppies by the ML method, with correction for truncate bino- mial distribution under incomplete ascer- tainment (Wieser and Willner 1993), yielded a figure of q = 0.23 t 0.12 for

FIG 3. Electron micrograph of a granular cell adjacent to keratlnised cells (top) containing keratin filaments (arrows) and keratohyaline granules (g) of normal form, size and distribution. x 17,600

affected puppies. This ML estimate corre- sponds closely to the theoretical expecta- tion. Assuming Mendelian inheritance of an autosomal recessive gene, the expected frequency of affected puppies is 25 per cent. The additional examination of proband frequency with the a priori method confirms the above result by show- ing no significant difference between observed and expected frequencies in affected families (Table 1 ) . The authors therefore suggest the acceptance of the autosomal recessive mode of inheritance

for heritable footpad hyperkeratosis in Irish terriers.

DISCUSSION

The analysis of the reported 16 cases of footpad hyperkeratosis in a family of Irish terriers strongly supports the hypothesis of this dermatosis being transmitted by an autosomal recessive mode of inheritance. The accuracy of the ML-estimate (q = 0.23 * 0.12) is rather low but a close

54 - JOURNAL OF SMALL ANIMAL PRACTICE VOL 41 FEBRUARY 2000

4 -7-6 Ii -&- female male

0 not affected

affected

0 0 supposedcarrier

0 not afflicted

FIG 4. Pedigree sequence of Irish terriers featuring relevant litters only. The indices identify the different kennels (a to f) of outcross stud dogs

correspondence between the calculated (q = 0.23) and expected (q = 0.25) frequency of totally afflicted puppies in proband families is evident. The verifica- tion of the frequency distribution in individual litters, employing the a priori method (Wieser and Willner 1993) and contingency testing, also yielded a close agreement benveen expected and observed frequencies (Table 1).

Further elucidation of the genetic back- ground of footpad hyperkeratosis in Irish terriers requires identification and map- ping of the disease locus. Keratin genes previously identified as defective in other species are likely to be candidates for simi- lar defects in the dog, even if the clinical expressions are species-specific. A signifi- cant number of heritable forms of para- or hyperkeratoses are known in both humans and dogs. The histopathological findings in the footpads of affected Irish terriers are similar to the alterations caused in human subjects by a subgroup of inherited, but genetically rather heterogeneous, kerato- dermas, the non-epidermolytic palmo- plantar keratodermas (Lucker and others 1994). The focal palmoplantar epidermo- lysis, labelled ‘Unna-Thost’, that is caused by subfunctional type-1 collagen, is very similar. However, given ultrastructural differences existing between this condition in terrier footpads and ‘Unna-Thost’, the conclusion of a deficiency identical with the human form is not justified.

In humans, candidate genes for the group of non-epidermolytic keratodermas

seem to be located close to, or within, one of the keratin gene clusters residing on the long arms of chromosomes 12 and 17 (Rogaev and others 1993, Kimonis and others 1994, Lind and others 1994, Shamsher and others 1995). Analysis has been initiated by the isolation of cosmid clones of the two putative canine keratin clusters, to allow evaluation for polymor- phism and subsequent linkage analysis. After the physical localisation of the locus candidate, genes within the locus will be analysed for mutations (Keller 1997).

If the disease described is an autosomal recessive defect then affected individuals must be homozygous and most likely result from inbreeding. Affected offspring also arose from outcross matings with six out of eight sires from four different ken- nels. Available pedigrees covering several generations show no common ancestry for some introduced stud dogs that produced hyperkeratotic puppies. Therefore, the fre- quency of the defective gene would have to be high in the Irish terrier population. This corresponds with the statement of Paradis (1 992) that footpad hyperkeratosis in Irish terriers was widely recognised in Europe. Controversy lies in the fact that, despite assumed high frequency and wide recogni- tion, no other breeders have as yet reported the occurrence of footpad hyperkeratosis in their recent dogs.

Clinical signs of footpad hyperkeratosis appear in subadult dogs. If a breeder becomes aware of the problem, afflicted dogs are eliminated from breeding stock

and mating between carriers is minimised by avoiding combinations that previously produced affected offspring. Hence, the observed frequency of diseased dogs may be lower in the breed than would be expected if the true genetic contamination of the breeding stock was reflected. Hiding of evidently affected dogs, and therefore removing evidence of carrier stud dogs, promotes concealed dissemination of the genetic defect. The elimination of the dele- terious gene requires the complete record- ing of breeding stock and, if a reliable test becomes available, the testing of all animals and exclusion from breeding of carriers.

Acknowledgement The authors acknowledge the financial support by the animal welfare organisation ‘Zurcher Tierschutz’.

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