Novel CLN1 mutation with atypical juvenile neuronal ceroid lipofuscinosis.

We detected a novel CLN1 gene mutation (p.Arg151X, heterogenous) in a 12-year-old boy. Low level of palmitoyl protein thioesterase and granular inclusion pattern in lymphocytes were also consistent with infantile Neuronal ceroid lipofuscinosis (INCL). However, the clinical phenotype was that of atypical juvenile neuronal ceroid lipofuscinosis (JNCL) and consisted of progressive visual loss from the age of 8 years. His visual acuity was 6/60 in both eyes at first presentation, 6/36 one month later, then 6/6 (right eye), and 6/60 (left eye) 6 months later. However, after 4 months, visual acuity dropped to 6/60 in both eyes and at last follow-up, it was 6/60 (right eye) and 3/60 (left eye). Visual hallucinations were also reported. Persistent normal fundi findings, normal electroretinogram (ERG), and delayed visual evoked potentials (VEP) were suggestive of non-retinal adolescence form/atypical JNCL. Visual loss in JNCL is secondary to retinal dystrophy. Our observations suggest that JNCL should be considered in any children presenting with bilateral progressive visual loss even with normal fundi and/or delayed VEP. Electron microscopy of buffy coat and palmitoyl protein thioesterase enzyme study are useful tools in diagnosis. Pertinent issues regarding clinical symptomatology, ophthalmologic findings, and laboratory results are discussed.

Introduction

Neuronal Ceroid Lipofuscinoses (NCLs) are a group of rare neurodegenerative disorders with a prevalence of approximately 1 in 12,500 of the population.[1] These are characterized by an accumulation of autofluorescent lipopigments in neurons and extraneuronal tissues e.g., muscle, skin, conjunctivae, and rectal mucosa. The main features are variable yet progressive symptoms including seizures, dementia, visual loss and/or cerebral atrophy with typical onset in infancy but can be delayed until adulthood. The disorders are classified into four major forms on the basis of age-at-onset, clinical presentation, histological findings, and genetic loci: The infantile (INCL), late infantile (LINCL), juvenile (JNCL), adult-onset NCL, as well as a heterogenous group of atypical subtypes. Within the last decade, mutations that cause NCL have been found in six human genes (CLN1, CLN2, CLN3, CLN5, and CLN8). Approximately, 160 NCL disease-causing mutations have been described.

We describe an 11-year-old boy with atypical juvenile NCL who presented with progressive visual loss of 3-year period. The unusual features include visual hallucinations, unexplained transient normalization of visual acuity in right eye, normal fundi, normal electroretinogram (ERG) and delayed visual evoked potentials (VEP). The biochemical, histological, and genetic analysis confirmed a novel CLN1 mutation.

Case Report

An 11-year 10-month-old Caucasian boy with normal perinatal and neuro-developmental history was referred to us with a 3 year history of progressive visual loss and visual hallucinations. His symptoms started with difficulty in reading, worsening handwriting and flashes of lights in both eyes. He gave us a description and consistently drew figures [Figure 1], which he appeared to be seeing in front of his eyes. The figures had a red border with circles and lines criss-crossing the circles.

Figure 1

Depiction of visual hallucinations perceived by the patient

Over the past 12 months, he had been investigated extensively and no cause had been identified. The visual acuity (VA) at first presentation was 6/60 in both eyes, when he was 10 years and 11 months old. VA improved to 6/36 in both eyes after 1 month. Interestingly, 6 months later, his left eye deteriorated to 6/60, but right eye had normal visual acuity (6/6). Ophthalmologist was pleased and found that he had enrolled in an archery course for the summer and was doing fairly well. However, 4 months later, VA deteriorated to 6/60 in both eyes. At the last follow-up, at 12 years and 4 months of age, VA was 6/60 in the right eye and 3/60 in the left eye. He was able to detect colors on the Ishihara chart but could not read any letters. His visual field test did not show specific defect. Pupillary reaction was normal with no relative afferent pupillary defect. Fundoscopic examination still showed healthy optic disc and healthy macula at the last ophthalmology follow-up. Neurological and other examinations were unremarkable.

VEP of both eyes were delayed while ERG was normal at 12 years. Cranial/orbit magnetic resonance imaging (MRI) and electroencephalography (EEG) were normal. Test for Leber′s hereditary optic neuropathy was normal. Skin and muscle biopsy did not demonstrate any inclusion bodies. Blood film showed no lymphocyte vacuolation. However, electron microscopy of buffy coat showed rare, membrane bound, electron dense, granular material associated with a lipid droplet in the cytoplasm of lymphocytes. Leukocyte enzyme analysis showed very low level of Palmitoyl protein transferase (PPT) activity measuring 2.2 nmol/hr/mg ptn (Normal = 17–139) and normal Tripeptidyl peptidase I (TPP-I) 214 nmol/hr/mg ptn (Normal: 42–339). Molecular genetic analysis of exon 1 to 9 of CLN1 gene showed that he was heterozygote for p.Cys96Tyr and p.Arg151X mutations.

A diagnosis of milder form of INCL (CLN1) was made. At his last neurology follow-up at 12 years and 10 months, his vision remained poor. He had developed behavior and cognitive problems. His behavior was becoming increasingly immature. There had been loss of skills and significant change in his memory. He had never had any seizures and motor dysfunction to date.

Discussion

Juvenile NCL is diagnosed on the basis of age-at-onset, initial clinical symptoms, clinical progression of the disease, and pathologic findings. Typical/classic JCNL is characterised clinically by slowly progressive visual loss at age 4–7 years, with blindness within 2–10 years. Speech disturbance, cognitive decline, epilepsy, behavioural or psychiatric symptoms, and motor dysfunction, leading to vegetative state usually in later life.[1] The consistent findings in all typical JNCL are fingerprint profile at the ultra structural (EM) level and the common 1.02 kb CLN3 mutation.[12] Atypical forms of JNCL, up to 20% of JNCL cases, have been described and further divided into the several subgroups.[2] Our case manifested atypical clinico-pathological features and probably belongs to a non-retinal adolescent form of JCNL. Further pathological, biochemical and genetic analysis confirmed that our case represented a milder form of INCL presenting at a later stage.

Visual loss in typical JNCL is secondary to retinal dystrophy. Funduscopic examination reveals a bull′s eye maculopathy, diffuse pigmentary degeneration, arteriolar attenuation, and optic atrophy, and an extinguished ERG.[3] Differences from these typical features in our case included normal ERG, delayed VEP, and persistent normal fundi even at an advanced stage where profound and prolonged visual loss was noted. Other disorders consistent with a delayed VEP include demyelinating optic neuropathy, dominant optic atrophy, Leber′s optic neuropathy, and others but abnormal appearance of the optic disc is expected at this late stage. Medline search found that normal fundi/non-retinal adolescence form,[24] and delayed VEP[5] have been reported. Retinal dystrophy is common features in JCNL while optic atrophy may be the sole finding in INCL (CLN1).[1] This may explain normal ERG, delayed VEP, and normal fundi as our case belongs to a non-retinal adolescence form of atypical JCNL secondary to CLN1 mutation, rather than CLN3. However, normal fundi may still correspond to a very early stage (4 years duration) of juvenile phenotype of NCL although ophthalmologic changes usually appear early.

Our case developed consistent visual hallucinations with the development of blindness. This rare feature has been described in the literature.[67] The most puzzling feature in our case is that vision of his right eye was reported normal (6/6) 6 months after the first presentation. Our ophthalmologist was pleased with his improvement and commented that patient had enrolled in an archery course for the summer and was doing fairly well. We are unable to explain this phenomenon but we exclude any technical or typing errors. At early stage, visual loss in juvenile phenotype of CLN1 could be due to optic neuritis in which intermittent visual loss is a known feature, rather than chronic progressive loss secondary to retinal dystrophy.

Low level of PTT, granular profiles of storage material in lymphocyte and positive CLN1 mutation confirmed that our case represented a milder form of INCL presenting at juvenile stage. Over the past few decades, new molecular findings have provided evidence of far more overlap for the different genetic variants than what was previously suggested by clinical phenotypes.[1] CLN1 mutations are probably the best described disorders with up to 40 different mutations.[1] Infantile NCL (INCL) is the most common form of CLN1 mutation, presenting at the first year of life with rapid decline in psychomotor development, seizures, and blindness. Milder forms including late infantile, juvenile, or adult phenotype are increasingly reported. The variability of presentation could be explained by different mutations of the CLN1 gene. Factors influencing age of onset and clinical severity remain obscured but the variability of disease does not seem to be related to degree of PPT deficiency. Other unknown genetic factors are supposed to modify the course of disease in such patients.[8]

Our case is an apparent compound heterozygote for the two different alleles. P.Arg151X is predicted to cause premature termination of the PTT protein and has previously been described in patients with Batten disease.[9] C.287G > A is predicted to result in the substitution of cysteine for tyrosine at residue 96 (p.Cys96Tyr). Cysteine and tyrosine are aminoacids with different physico-chemical properties. Cys96 is conserved between species (human, mouse, rat, and dog) and no other mutations have been found in the CLN gene. Assuming that p.Cys96Tyr and p.Arg151X occur in trans, the result is consistent with a diagnosis of infantile NCL/Batten disease.

To conclude, our reports add another case to the list of CLN1 variants known worldwide. Our experiences might recommend that NCL should be excluded in any unexplained progressive visual loss even with normal fundi, delayed VEP, and/or normal ERG. EM of buffy coat and PTT enzyme study are useful screening tools for NCL.