Abstract
This paper documents the first reported case of fucosidosis in a cat. The cat presented with signs of forebrain and cerebellar dysfunction and a magnetic resonance imaging scan of the brain suggested a degenerative or metabolic disease process. A fine needle aspirate of grossly normal lymph nodes revealed vacuolated lymphocytes and a renal biopsy of an irregular shaped kidney identified vacuolated tubular epithelial cells. A white cell lysosomal enzyme screen revealed negligible α-fucosidase activity. Fucosidosis should be considered in the differential diagnosis of young cats with cerebellar dysfunction and must be added to the list of lysosomal storage diseases affecting the cat.
A 2-year-old neutered female domestic shorthair cat was referred to the Small Animal Teaching Hospital at Liverpool University with a 10-day history of progressive incoordination and a generalised tremor; the owner commented that the cat tended to lunge at her food and also noticed her pelvic limbs tended to slide outwards when standing. Physical examination was largely unremarkable. Neurological examination showed her to be slightly obtunded and ataxic, with a hypermetric thoracic limb gait and a wide based pelvic limb stance. Proprioceptive paw positioning was normal in all four limbs but the hopping responses were slow. Spinal and cranial nerve reflexes were normal but the menace responses were reduced in both eyes; a retinal examination revealed no abnormalities. Based on these findings diffuse and bilaterally symmetrical dysfunction of both the forebrain and cerebellum was suspected. Differential diagnoses considered included degenerative disorders, metabolic conditions, neoplasia, infectious or inflammatory conditions and developmental anomalies. There was no history of exposure to toxins and the cat was fed a normal commercial diet. The most likely aetiologies were thought to be metabolic and degenerative conditions. Routine haematology was unremarkable but biochemistry revealed a slight azotaemia (urea 11.6, reference interval (RI) 3.5–6 mmol/l; creatinine 132, RI 30–110 μmmol/l) with a urine specific gravity of 1.017. The azotaemia gradually resolved with fluid therapy.
The cat was sedated with medetomidine hydrochloride (Domitor; Pfizer; 0.01 mg/kg IM) and butorphanol tartrate (Torbugesic; Fort Dodge; 0.2 mg/kg IM) to perform an ultrasound scan of the abdomen. This revealed markedly abnormal renal architecture, with both kidneys appearing rounded in shape and showing poor corticomedullary definition. There was a heterogenous echogenicity throughout the parencyhmal tissue and, therefore, a renal biopsy was performed. The liver appeared normal and there was no abnormal vasculature.
The cat was then anaesthetised (propofol to effect and sevoflurane) and a magnetic resonance imaging (MRI) scan of the brain was performed (Tesla Siemens Magnetom Harmony, Siemens AG). This revealed extensive and diffuse hyperintensity of the white matter on T2-weighted images, resulting in poor white and grey matter distinction in both the forebrain and cerebellum (Fig 1 A, D and F); there was also poor definition of the sulci. There was bilaterally symmetrical hyperintensity on T2-weighted images in the caudate nucleus, rostral thalamus and the ventral region of the internal capsule (Fig 1D and F). On T1-weighted images, there was bilaterally symmetrical hyperintensity in the internal capsule and rostral thalamus (Fig 1C and E). There was no contrast enhancement after administration of gadobenate dimeglumine (Multihance; Bracco; 0.2 ml/kg) and cerebrospinal fluid analysis was unremarkable.
MRI images of brain. Sagittal T2-weighted image of the affected cat (A) and a normal cat (B) (TR 2650 ms, TE 91 ms, 2.5 mm slices). (C and E) Transverse T1-weighted (TR 511 ms, TE 14 ms, 3 mm slices) and (D and F) T2-weighted (TR 5230 ms, TE 112 ms, 3 mm slices) images of the affected cat at the level of the optic chiasm (C and D) and hypophysis (E and F). On T2-weighted scans there is poor distinction between white and grey matter throughout the brain (A, D and F), most notably in the cerebellum (A), with the white matter being hyperintense and the grey matter hypointense to normal cortical tissue. The rostral thalamus is hyperintense compared with surrounding tissue (D). On T1-weighted images there is a bilaterally symmetrical hyperintensity in the rostral thalamus, internal capsule and basal nuclei (C). A comparison between the T2-weighted sagittal images of the affected cat (A) and the normal cat (B) highlights the marked MRI changes in the cat with fucosidosis. (G and H) Renal biopsy showing diffuse vacuolation of renal tubular epithelial cells.
The combination of brain and renal abnormalities was felt suspicious for a metabolic or degenerative disease, including lysosomal storage diseases (LSDs). In view of this, fine needle aspirates of grossly normal lymph nodes were taken and revealed low numbers of lymphocytes possessing small, discrete, clear cytoplasmic vacuoles. A blood smear also revealed that a few lymphocytes were vacuolated, though the majority appeared normal, and analysis of the renal biopsy showed mild to moderate diffuse vacuolation of the tubular epithelium (Fig 1G and H).
A sample of blood collected in ethylenediaminetetraacetic acid (EDTA) was used to perform a white cell lysosomal enzyme screen using a laboratory at a local paedriatric hospital (Willink Laboratory, Royal Manchester Children's Hospital, UK). This revealed a negligible α-fucosidase activity when compared with the reference values (Table 1). Other lysosomal enzymes were not markedly different from the reference values and a diagnosis of fucosidosis was thus made. The cat progressively deteriorated and was euthanased 6 months after initial presentation; a post-mortem examination was not possible.
White cell lysosomal enzyme assay. The reference values used were from human data. There is a marked reduction of α-fucosidase activity in the clinically affected cat compared with a clinically normal cat.
Units: white cells μmol/g/h (μmol substrate lysed/g protein in purified white cell sample/h).
LSDs are a group of rare inherited metabolic disorders that result from defects in lysosomal enzyme function causing an accumulation of substrate in the cell's lysosome, which is seen histologically as cytoplasmic vacuolation within the cells; ultimately causing cellular dysfunction. Numerous LSDs have been identified in man, many of which have been recognised in domestic animals. 1 Clinical signs usually begin in the first year, and clinical manifestations vary. Both the peripheral and central nervous systems can be affected and changes can also occur in ocular, skeletal and connective tissues. LSDs tend to be progressive and ultimately result in death or, in the domestic species, euthanasia.
Many LSDs have been identified in the cat, including gangliosidosis GM1 and GM2, sphingomyelinosis (Neimann–Pick), globoid cell leukodystrophy, metachromatic leukodystrophy, mucopolysaccharidoses II and V1, mucolipidosis II, mannosidosis, glycogenesis types II and IV and ceroid lipofuscinosis. 2 This is the first report of a cat with fucosidosis, a storage disease most commonly associated with the English Springer Spaniel. 3 The disease in humans and dogs is inherited in an autosomal recessive manner and affected dogs have a marked deficiency in α-l-fucosidase activity, usually around 0–5% of the control mean. The disease in the Springer Spaniel commonly presents with cerebellar signs, similar to this case. Unlike many LSDs, dogs with fucosidosis are usually clinically normal until about 18 months old. This is similar to the juvenile form of fucosidosis in humans, 4,5 which is typified by an adolescent onset and to the cat in the current report, which was 2 years old when the first clinical signs were noted. In dogs the disease is slowly progressive, usually resulting in death or euthanasia by 3–4 years of age; again this reflects the pattern of disease that we identified.
Diagnosis in animals of LSDs can be challenging. Affected animals tend to be immature at the onset of disease and demonstrate progressive clinical signs. However, dogs with fucosidosis and ceroid lipofuscinosis usually develop neurological dysfunction when older than this and the cat documented in the current report presented with a relatively recent, 6,7 acute onset of incoordination. The majority are inherited in an autosomal recessive mode; hence, there is no gender predilection, with the notable exception of Hunter (mucopolysaccharidosis type II) and Fabry (α-galactosidosis) diseases. 8,9 In dogs, although LSDs can affect any breed, there is a definite breed predisposition for certain LSDs, for example, fucosidosis in English Springer Spaniels, 6 ceroid lipofuscinosis in English Setters and globoid cell leukodystrophy in West Highland White Terriers. 10,11 This is not true for cats, in which LSDs have been identified in many non-pedigree as well as pedigree cats.
Procedures that aid diagnosis include examination of leukocytes from lymphoid tissue aspirates and in blood smears; a liver, kidney or muscle biopsy can help to identify vacuolated cells. Cell vacuolation is not specific for any particular LSD but the cell distribution may aid diagnosis; for example, dogs with glycogenoses have evidence of storage material accumulation in skeletal and cardiac muscles as well as nervous tissue, 12 whereas mucopolysaccharidoses frequently cause bone abnormalities alongside neuronal dysfunction. 1 Of particular interest in this case are the abnormal findings on MRI, which gave a strong indication of a metabolic disease process. In human medicine, there are many reports concerning the imaging findings in LSDs. In human fucosidosis a generalised white matter hyperintensity on T2-weighted images throughout the cerebrum and cerebellum has been recorded, 13,14 as in the current case. A bilateral decrease signal intensity of the thalami in T2-weighted images has been found in certain storage diseases, 15 including fucosidosis in which a T1-weighted hyperintensity of the thalamus has also been observed. 16 In the current case, there was a bilaterally symmetric T1-weighted hyperintensity in the internal capsule and rostral thalamus. The MRI findings in this case show similarities to those found in a cat with GM2 gangliosidosis. 17 In this report there was hyperintensity of the internal capsule on T1-weighted images and hyperintensity of the white matter of the whole forebrain in T2-weighted images. The marked changes noted in this case report and the aforementioned case indicate that MRI is a useful technique to assist diagnosis.
If a metabolic disorder is suspected, analysis of urine for abnormal excretion of metabolites or storage products can help to direct screening for LSDs. A lysosomal enzyme analysis can then be performed on leukocytes to assay the activities of a selection of lysosomal enzymes, with affected individuals having severely depleted enzyme activity, usually in the region of 0–5% of normal. In this case, urinalysis was not performed because it was anticipated that a lysosomal enzyme screen might yield a specific diagnosis. Molecular genetic tests are also available for diagnosing several LSDs, including canine fucosidosis, ceroid lipofuscinosis and gangliosidosis.
Histopathological findings in both humans and dogs with fucosidosis include vacuolation of neurons and microglia at all levels of the brain and spinal cord, with vacuolated mononuclear cells scattered throughout the leptomeninges. 18,19 There is diffuse neuronal loss and evidence of demyelination centrally and peripheral nerves may show infiltration of subepineural and endoneural spaces by vacuolated cells. Vacuolated epithelial cells may be found in many extraneural tissues including respiratory, biliary, epididymal, pancreatic and renal tubular epithelia and vacuolated macrophages are often found in the spleen and bone marrow.
The LSDs in domestic animals are very similar to their counterparts in humans, making animal models very popular for testing various therapies. Attempts at treatment have been made by using oral enzyme replacement therapy and by intravenous injection of various DNA vectors. 20 However, penetration of the blood–brain barrier by such enzymes is often too poor to significantly benefit the brain. 20 Some studies have attempted to overcome this by bone marrow transplantation, 21 whilst others have attempted to directly inject a DNA vector into the brain; with a good veterinary example being the use of adeno-associated virus injection to treat cats with mannosidosis. 22 More recently, gene therapy targeting of the endothelium has been shown to be an effective way of infiltrating the central nervous system with the replaced enzyme. 23 However, both experimental studies in dogs and clinical experience in human patients suggest that treatments are only likely to be beneficial when used at an early stage of the disease. 24,25 Only by recognition of the clinical signs and selection of appropriate diagnostic tests will a prompt diagnosis be made before the disability becomes too severe, and thus treatment more likely to be effective. But for now, in the domestic species, LSDs carry a poor prognosis.
Acknowledgment
LA is part funded by the RCVS Trust.