Electroclinical spectrum of the neuronal ceroid lipofuscinoses associated with CLN6 mutations

Natural History of Neuronal Ceroid Lipofuscinosis Type 6, Late Infantile Disease

BACKGROUND

Mutations in the CLN6 gene cause late infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disease of childhood onset. Clinically, individuals present with progressive motor and cognitive regression, ataxia, and early death. The aim of this study is to establish natural history data of individuals with classic, late-infantile-onset (age less than five years) CLN6 disease.

METHODS

We analyzed the natural history of 25 patients with late-infantile-onset CLN6, utilizing the Hamburg motor-language scale to measure disease progression. The key outcomes were CLN6 disease progression, assessed by rate of decline in motor and language clinical domain summary scores (0 to 6 total points); onset and type of first symptom; onset of first seizure; and time from first symptom to complete loss of function.

RESULTS

Median age of total motor and language onset of decline was 42 months (interquartile range 36 to 48). The estimated rate of decline in total score was at a slope of -1.20 (S.D. 0.30) per year, after the start of decline. Complete loss of both motor and language function was found to be, on average, 88.1 months (S.D. 13.5).

CONCLUSIONS

To our knowledge, this is the largest international study that monitors the longitudinal natural history and progression of CLN6 disease. These data may serve as a template for future interventional trials targeted to slow the progression of this devastating disease.

Progressive Myoclonus Epilepsy: A Scoping Review of Diagnostic, Phenotypic and Therapeutic Advances

The progressive myoclonus epilepsies (PME) are a diverse group of disorders that feature both myoclonus and seizures that worsen gradually over a variable timeframe. While each of the disorders is individually rare, they collectively make up a non-trivial portion of the complex epilepsy and myoclonus cases that are seen in tertiary care centers. The last decade has seen substantial progress in our understanding of the pathophysiology, diagnosis, prognosis, and, in select disorders, therapies of these diseases. In this scoping review, we examine English language publications from the past decade that address diagnostic, phenotypic, and therapeutic advances in all PMEs. We then highlight the major lessons that have been learned and point out avenues for future investigation that seem promising.

Adult-onset Kufs disease

A young man from Pakistan had his first-ever tonic-clonic seizure while playing cricket. Since age 12 years, he had reported involuntary jerks and tremulousness, sometimes with falls, particularly with bright lights. Family history included a brother who developed seizures with myoclonus in his mid-20s and parental consanguinity. Developmental history was normal. Examination identified cognitive impairment with action myoclonus. His clinical presentation raised suspicion of a progressive myoclonus epilepsy. MR scan of the brain showed white matter changes suggesting leucodystrophy with cortical atrophy. Electroencephalogram showed generalised epileptiform abnormalities with photoparoxysmal responses, including at low frequencies (1 Hz). Cortical hyperexcitability was confirmed with giant median somatosensory evoked potentials and long loop reflexes at rest. Multichannel electromyography showed action myoclonus with variable synchronous and asynchronous agonist and antagonist muscle activation with short-burst duration of 25-75 ms, and jerk-locked back-averaging showed premyoclonic potentials consistent with cortical myoclonus. Genetic sequencing identified a homozygous missense variant in the CLN6 gene (c.768C>G p.(Asp256Glu), confirming Kufs disease type A.

The involvement of Purkinje cells in progressive myoclonic epilepsy: Focus on neuronal ceroid lipofuscinosis

The progressive myoclonic epilepsies (PMEs) are a group of rare neurodegenerative diseases characterized by myoclonus, epileptic seizures, and progressive neurological deterioration with cerebellar involvement. They include storage diseases like Gaucher disease, Lafora disease, and forms of neuronal ceroid lipofuscinosis (NCL). To date, 13 NCLs have been reported (CLN1-CLN8, CLN10-CLN14), associated with mutations in different genes. These forms, which affect both children and adults, are characterized by seizures, cognitive and motor impairments, and in most cases visual loss. In NCLs, as in other PMEs, central nervous system (CNS) neurodegeneration is widespread and involves different subpopulations of neurons. One of the most affected regions is the cerebellar cortex, where motor and non-motor information is processed and transmitted to deep cerebellar nuclei through the axons of Purkinje cells (PCs). PCs, being GABAergic, have an inhibitory effect on their target neurons, and provide the only inhibitory output of the cerebellum. Degeneration of PCs has been linked to motor impairments and epileptic seizures. Seizures occur when some insult upsets the normal balance in the CNS between excitatory and inhibitory impulses, causing hyperexcitability. Here we review the role of PCs in epilepsy onset and progression following their PME-related loss. In particular, we focus on the involvement of PCs in seizure phenotype in NCLs, highlighting findings from case reports and studies of animal models in which epilepsy can be linked to PC loss.

Recent Insight into the Genetic Basis, Clinical Features, and Diagnostic Methods for Neuronal Ceroid Lipofuscinosis

Neuronal ceroid lipofuscinoses (NCLs) are a group of rare, inherited, neurodegenerative lysosomal storage disorders that affect children and adults. They are traditionally grouped together, based on shared clinical symptoms and pathological ground. To date, 13 autosomal recessive gene variants, as well as one autosomal dominant gene variant, of NCL have been described. These genes encode a variety of proteins, whose functions have not been fully defined; most are lysosomal enzymes, transmembrane proteins of the lysosome, or other organelles. Common symptoms of NCLs include the progressive loss of vision, mental and motor deterioration, epileptic seizures, premature death, and, in rare adult-onset cases, dementia. Depending on the mutation, these symptoms can vary, with respect to the severity and onset of symptoms by age. Currently, all forms of NCL are fatal, and no curative treatments are available. Herein, we provide an overview to summarize the current knowledge regarding the pathophysiology, genetics, and clinical manifestation of these conditions, as well as the approach to diagnosis.

Neurophysiological Findings in Neuronal Ceroid Lipofuscinoses

Neuronal ceroid lipofuscinoses (NCLs) are a heterogeneous group of neurodegenerative diseases, characterized by progressive cerebral atrophy due to lysosomal storage disorder. Common clinical features include epileptic seizures, progressive cognitive and motor decline, and visual failure, which occur over different time courses according to subtypes. During the latest years, many advances have been done in the field of targeted treatments, and in the next future, gene therapies and enzyme replacement treatments may be available for several NCL variants. Considering that there is rapid disease progression in NCLs, an early diagnosis is crucial, and neurophysiological features might have a key role for this purpose. Across the different subtypes of NCLs, electroencephalogram (EEG) is characterized by a progressive deterioration of cerebral activity with slowing of background activity and disappearance of spindles during sleep. Some types of heterogeneous abnormalities, diffuse or focal, prevalent over temporal and occipital regions, are described in many NCL variants. Photoparoxysmal response to low-frequency intermittent photic stimulation (IPS) is a typical EEG finding, mostly described in CLN2, CLN5, and CLN6 diseases. Visual evoked potentials (VEPs) allow to monitor the visual functions, and the lack of response at electroretinogram (ERG) reflects retinal neurodegeneration. Taken together, EEG, VEPs, and ERG may represent essential tools toward an early diagnosis of NCLs.

Higher order visual dysfunction and myoclonic-atonic seizure: an atypical presentation of CLN6 neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinosis is a rare childhood neurodegenerative disease, classified under the spectrum of progressive myoclonic epilepsy (PME). Cognitive decline, seizures including myoclonus, vision loss and ataxia are the commonly described manifestations of this illness. While visual failure in this disease is largely attributed to retinal, macular degeneration and optic atrophy, with this index case, we report an atypical presentation in the form of higher order visual dysfunction. The pattern of cognitive regression has further been explored here with higher order visual dysfunction and language regression being the predominant manifestations, stemming from an involvement of bilateral occipitoparietal/occipitotemporal networks. Yet another unique feature of this case also lies in the occurrence of myoclonic-atonic seizure, a semiology rarely reported before in PME.

Genetics in Epilepsy

The presence of unprovoked, recurrent seizures, particularly when drug resistant and associated with cognitive and behavioral deficits, warrants investigation for an underlying genetic cause. This article provides an overview of the major classes of genes associated with epilepsy phenotypes divided into functional categories along with the recommended work-up and therapeutic considerations. Gene discovery in epilepsy supports counseling and anticipatory guidance but also opens the door for precision medicine guiding therapy with a focus on those with disease-modifying effects.

Management of CLN1 Disease: International Clinical Consensus

BACKGROUND

CLN1 disease (neuronal ceroid lipofuscinosis type 1) is a rare, genetic, neurodegenerative lysosomal storage disorder caused by palmitoyl-protein thioesterase 1 (PPT1) enzyme deficiency. Clinical features include developmental delay, psychomotor regression, seizures, ataxia, movement disorders, visual impairment, and early death. In general, the later the age at symptom onset, the more protracted the disease course. We sought to evaluate current evidence and to develop expert practice consensus to support clinicians who have not previously encountered patients with this rare disease.

METHODS

We searched the literature for guidelines and evidence to support clinical practice recommendations. We surveyed CLN1 disease experts and caregivers regarding their experiences and recommendations, and a meeting of experts was conducted to ascertain points of consensus and clinical practice differences.

RESULTS

We found a limited evidence base for treatment and no clinical management guidelines specific to CLN1 disease. Fifteen CLN1 disease experts and 39 caregivers responded to the surveys, and 14 experts met to develop consensus-based recommendations. The resulting management recommendations are uniquely informed by family perspectives, due to the inclusion of caregiver and advocate perspectives. A family-centered approach is supported, and individualized, multidisciplinary care is emphasized in the recommendations. Ascertainment of the specific CLN1 disease phenotype (infantile-, late infantile-, juvenile-, or adult-onset) is of key importance in informing the anticipated clinical course, prognosis, and care needs. Goals and strategies should be periodically reevaluated and adapted to patients' current needs, with a primary aim of optimizing patient and family quality of life.

Ocular Manifestations of Neuronal Ceroid Lipofuscinoses

Neuronal ceroid lipofuscinoses (NCLs) are a group of rare neurodegenerative storage disorders associated with devastating visual prognosis, with an incidence of 1/1,000,000 in the United States and comparatively higher incidence in European countries. The pathophysiological mechanisms causing NCLs occur due to enzymatic or transmembrane defects in various sub-cellular organelles including lysosomes, endoplasmic reticulum, and cytoplasmic vesicles. NCLs are categorized into different types depending upon the underlying cause i.e., soluble lysosomal enzyme deficiencies or non-enzymatic deficiencies (functions of identified proteins), which are sub-divided based on an axial classification system. In this review, we have evaluated the current evidence in the literature and reported the incidence rates, underlying mechanisms and currently available management protocols for these rare set of neuroophthalmological disorders. Additionally, we also highlighted the potential therapies under development that can expand the treatment of these rare disorders beyond symptomatic relief.

Myoclonus-Ataxia Syndromes: A Diagnostic Approach

Background

A myriad of disorders combine myoclonus and ataxia. Most causes are genetic and an increasing number of genes are being associated with myoclonus-ataxia syndromes (MAS), due to recent advances in genetic techniques. A proper etiologic diagnosis of MAS is clinically relevant, given the consequences for genetic counseling, treatment, and prognosis.

Objectives

To review the causes of MAS and to propose a diagnostic algorithm.

Methods

A comprehensive and structured literature search following PRISMA criteria was conducted to identify those disorders that may combine myoclonus with ataxia.

Results

A total of 135 causes of combined myoclonus and ataxia were identified, of which 30 were charted as the main causes of MAS. These include four acquired entities: opsoclonus-myoclonus-ataxia syndrome, celiac disease, multiple system atrophy, and sporadic prion diseases. The distinction between progressive myoclonus epilepsy and progressive myoclonus ataxia poses one of the main diagnostic dilemmas.

Conclusions

Diagnostic algorithms for pediatric and adult patients, based on clinical manifestations including epilepsy, are proposed to guide the differential diagnosis and corresponding work-up of the most important and frequent causes of MAS. A list of genes associated with MAS to guide genetic testing strategies is provided. Priority should be given to diagnose or exclude acquired or treatable disorders.

Moyamoya and progressive myoclonic epilepsy secondary to CLN6 bi-allelic mutations - A previously unreported association

The neuronal ceroid lipofuscinoses (NCL) are a collection of lysosomal storage diseases characterised by the accumulation of characteristic inclusions containing lipofuscin in various tissues of the body and are one of the causes of progressive myoclonic epilepsy. Mutations in at least thirteen genes have been identified as causes of NCL, which can present as infantile, late-infantile, juvenile or adult forms. CLN6 codes for an endoplasmic reticulum transmembrane protein of unknown function. Homozygous and compound heterozygous mutations of the gene are associated with both late-infantile (LINCL) and adult onset (ANCL) forms of NCL, including Kufs disease, comprising ANCL without associated visual loss. Moyamoya, a rare vasculopathy of the circle of Willis, has been reported in conjunction with a number of inflammatory and other diseases, as well as a handful of lysosomal storage diseases. To our knowledge, this is the first reported case of Moyamoya in the context of the neuronal ceroid lipofuscinoses or a CLN6-related disease.

Experimental gene therapies for the NCLs

The neuronal ceroid lipofuscinoses (NCLs), also known as Batten disease, are a group of rare monogenic neurodegenerative diseases predominantly affecting children. All NCLs are lethal and incurable and only one has an approved treatment available. To date, 13 NCL subtypes (CLN1-8, CLN10-14) have been identified, based on the particular disease-causing defective gene. The exact functions of NCL proteins and the pathological mechanisms underlying the diseases are still unclear. However, gene therapy has emerged as an attractive therapeutic strategy for this group of conditions. Here we provide a short review discussing updates on the current gene therapy studies for the NCLs.

Inherited RORB pathogenic variants: Overlap of photosensitive genetic generalized and occipital lobe epilepsy

Variants in RORB have been reported in eight individuals with epilepsy, with phenotypes ranging from eyelid myoclonia with absence epilepsy to developmental and epileptic encephalopathies. We identified novel RORB variants in 11 affected individuals from four families. One was from whole genome sequencing and three were from RORB screening of three epilepsy cohorts: developmental and epileptic encephalopathies (n = 1021), overlap of generalized and occipital epilepsy (n = 84), and photosensitivity (n = 123). Following interviews and review of medical records, individuals' seizure and epilepsy syndromes were classified. Three novel missense variants and one exon 3 deletion were predicted to be pathogenic by in silico tools, not found in population databases, and located in key evolutionary conserved domains. Median age at seizure onset was 3.5 years (0.5-10 years). Generalized, predominantly absence and myoclonic, and occipital seizures were seen in all families, often within the same individual (6/11). All individuals with epilepsy were photosensitive, and seven of 11 had cognitive abnormalities. Electroencephalograms showed generalized spike and wave and/or polyspike and wave. Here we show a striking RORB phenotype of overlap of photosensitive generalized and occipital epilepsy in both individuals and families. This is the first report of a gene associated with this overlap of epilepsy syndromes.

Gene Therapy Corrects Brain and Behavioral Pathologies in CLN6-Batten Disease

CLN6-Batten disease, a form of neuronal ceroid lipofuscinosis is a rare lysosomal storage disorder presenting with gradual declines in motor, visual, and cognitive abilities and early death by 12–15 years of age. We developed a self-complementary adeno-associated virus serotype 9 (scAAV9) vector expressing the human CLN6 gene under the control of a chicken β-actin (CB) hybrid promoter. Intrathecal delivery of scAAV9.CB.hCLN6 into the cerebrospinal fluid (CSF) of the lumbar spinal cord of 4-year-old non-human primates was safe, well tolerated, and led to efficient targeting throughout the brain and spinal cord. A single intracerebroventricular (i.c.v.) injection at post-natal day 1 in Cln6 mutant mice delivered scAAV9.CB.CLN6 directly into the CSF, and it prevented or drastically reduced all of the pathological hallmarks of Batten disease. Moreover, there were significant improvements in motor performance, learning and memory deficits, and survival in treated Cln6 mutant mice, extending survival from 15 months of age (untreated) to beyond 21 months of age (treated). Additionally, many parameters were similar to wild-type counterparts throughout the lifespan of the treated mice.

Next-Generation Sequencing Analysis Reveals Novel Pathogenic Variants in Four Chinese Siblings With Late-Infantile Neuronal Ceroid Lipofuscinosis

Neuronal Ceroid Lipofuscinoses (NCLs) are progressive degenerative diseases mainly affect brain and retina. They are characterized by accumulation of autofluorescent storage material, mitochondrial ATPase subunit C, or sphingolipid activator proteins A and D in lysosomes of most cells. Heterogenous storage material in NCLs is not completely disease-specific. Most of CLN proteins and their natural substrates are not well-characterized. Studies have suggested variants of Late-Infantile NCLs (LINCLs) include the major type CLN2 and minor types CLN5, CLN6, CLN7, and CLN8. Therefore, combination of clinical and molecular analysis has become a more effective diagnosis method. We studied 4 late-infantile NCL siblings characterized by seizures, ataxia as early symptoms, followed by progressive regression in intelligence and behavior, but mutations are located in different genes. Symptoms and progression of 4 types of LINCLs are compared. Pathology of LINCLs is also discussed. We performed Nest-Generation Sequencing on these phenotypically similar families. Three novel variants c.1551+1insTGAT in TPP1, c.244G>T in CLN6, c.554-5A>G in MFSD8 were identified. Potential outcome of the mutations in structure and function of proteins are studied. In addition, we observed some common and unique clinical features of Chinese LINCL patient as compared with those of Western patients, which greatly improved our understanding of the LINCLs.

Therapeutic landscape for Batten disease: current treatments and future prospects

Batten disease (also known as neuronal ceroid lipofuscinoses) constitutes a family of devastating lysosomal storage disorders that collectively represent the most common inherited paediatric neurodegenerative disorders worldwide. Batten disease can result from mutations in 1 of 13 genes. These mutations lead to a group of diseases with loosely overlapping symptoms and pathology. Phenotypically, patients with Batten disease have visual impairment and blindness, cognitive and motor decline, seizures and premature death. Pathologically, Batten disease is characterized by lysosomal accumulation of autofluorescent storage material, glial reactivity and neuronal loss. Substantial progress has been made towards the development of effective therapies and treatments for the multiple forms of Batten disease. In 2017, cerliponase alfa (Brineura), a tripeptidyl peptidase enzyme replacement therapy, became the first globally approved treatment for CLN2 Batten disease. Here, we provide an overview of the promising therapeutic avenues for Batten disease, highlighting current FDA-approved clinical trials and prospective future treatments.

Novel mutations in CLN6 cause late-infantile neuronal ceroid lipofuscinosis without visual impairment in two unrelated patients

CLN6 is a transmembrane protein located in the endoplasmic reticulum that is involved in lysosomal acidification. Mutations in CLN6 cause late-infantile neuronal ceroid lipofuscinosis (LINCL), and teenage and adult onset NCL without visual impairment. Here we describe two pediatric patients with LINCL from unrelated families who were evaluated at the National Institutes of Health. Both children exhibited typical phenotypes associated with LINCL except that they lacked the expected visual impairment. Whole exome sequencing identified novel biallelic mutations in CLN6, i.e., c.218-220dupGGT (p.Trp73dup) and c.296A > G (p.Lys99Arg) in Proband 1 and homozygous c.723G > T (p.Met241Ile) in Proband 2. Expression analysis in dermal fibroblasts showed a small increase in CLN6 protein levels. Electron micrographs of these fibroblasts demonstrated large numbers of small membrane-bound vesicles, in addition to lipofuscin deposits. LysoTracker™ Red intensity was increased in fibroblasts from both patients. This study supports a role for CLN6 in lysosomal homeostasis, and highlights the importance of considering CLN6 mutations in the diagnosis of Batten Disease even in patients with normal vision.

Tracking sex-dependent differences in a mouse model of CLN6-Batten disease

Background

CLN6-Batten disease is a rare neurodevelopmental disorder characterized pathologically by the accumulation of lysosomal storage material, glial activation and neurodegeneration, and phenotypically by loss of vision, motor coordination, and cognitive ability, with premature death occurring in the second decade of life. In this study, we investigate whether sex differences in a mouse model of CLN6-Batten disease impact disease onset and progression.

Results

A number of noteworthy differences were observed including elevated accumulation of mitochondrial ATP synthase subunit C in the thalamus and cortex of female Cln6 mutant mice at 2 months of age. Moreover, female mutant mice showed more severe behavioral deficits. Beginning at 9 months of age, female mice demonstrated learning and memory deficits and suffered a more severe decline in motor coordination. Further, compared to their male counterparts, female animals succumbed to the disease at a slightly younger age, indicating an accelerated disease progression. Conversely, males showed a marked increase in microglial activation at 6 months of age in the cortex relative to females.

Conclusions

Thus, as female Cln6 mutant mice exhibit cellular and behavioral deficits that precede similar pathologies in male mutant mice, our findings suggest the need for consideration of sex-based differences in CLN6 disease progression during development of preclinical and clinical studies.

Electronic supplementary material

The online version of this article (10.1186/s13023-019-0994-8) contains supplementary material, which is available to authorized users.

Clinical challenges and future therapeutic approaches for neuronal ceroid lipofuscinosis

Treatment of the neuronal ceroid lipofuscinoses, also known as Batten disease, is at the start of a new era because of diagnostic and therapeutic advances relevant to this group of inherited neurodegenerative and life-limiting disorders that affect children. Diagnosis has improved with the use of comprehensive DNA-based tests that simultaneously screen for many genes. The identification of disease-causing mutations in 13 genes provides a basis for understanding the molecular mechanisms underlying neuronal ceroid lipofuscinoses, and for the development of targeted therapies. These targeted therapies include enzyme replacement therapies, gene therapies targeting the brain and the eye, cell therapies, and pharmacological drugs that could modulate defective molecular pathways. Such therapeutic developments have the potential to enable earlier diagnosis and better targeted therapeutic management. The first approved treatment is an intracerebroventricularly administered enzyme for neuronal ceroid lipofuscinosis type 2 disease that delays symptom progression. Efforts are underway to make similar progress for other forms of the disorder.

The Spectrum of Movement Disorders in Childhood-Onset Lysosomal Storage Diseases

Background

Movement disorders are a significant clinical problem in lysosomal storage diseases (LSD) and account for substantial morbidity. The spectrum of movement disorders in childhood-onset LSD, however, remains poorly defined.

Objectives

To define the spectrum of movement disorders in a well-characterized cohort of children with LSD.

Methods

A retrospective chart review at a single tertiary care center (Boston Children's Hospital, Boston, MA, USA). Patients up to the age of 18 years with a clinical, genetic and/or biochemical diagnosis of an LSD and at least one predefined movement disorder (parkinsonism, dystonia, ataxia, tremor, chorea, myoclonus, ballism, restless leg syndrome) were included.

Results

96 patients were identified and 76 patients had a sufficiently document biochemical and/or genetic diagnosis. Of these, 18 patients met inclusion criteria (mean age: 10.3±5.8 (SD) years, range: 3-18 years; 72% male). The most common LSD associated with a movement disorder was Niemann-Pick disease type C (NPC), followed by several types of neuronal ceroid lipofuscinosis (NCL) and different mucopolysaccharidoses. The most common movement disorder was ataxia followed by rest tremor, dystonia and myoclonus. The other predefined movement disorders were rare. The majority of patients presented with more than one movement disorder. The movement disorder was slowly progressive in all patients. Brain MRI changes included diffuse cerebral volume loss, white matter abnormalities with thinning of the corpus callosum, and cerebellar atrophy.

Conclusions

Movement disorders develop in a significant number of LSD patients. Ataxia, often in patients with NPC and NCL, is the most common phenotype but significant heterogeneity exists within and between different LSD.

Diagnosis and misdiagnosis of adult neuronal ceroid lipofuscinosis (Kufs disease)

OBJECTIVE

To critically re-evaluate cases diagnosed as adult neuronal ceroid lipofuscinosis (ANCL) in order to aid clinicopathologic diagnosis as a route to further gene discovery.

METHODS

Through establishment of an international consortium we pooled 47 unsolved cases regarded by referring centers as ANCL. Clinical and neuropathologic experts within the Consortium established diagnostic criteria for ANCL based on the literature to assess each case. A panel of 3 neuropathologists independently reviewed source pathologic data. Cases were given a final clinicopathologic classification of definite ANCL, probable ANCL, possible ANCL, or not ANCL.

RESULTS

Of the 47 cases, only 16 fulfilled the Consortium's criteria of ANCL (5 definite, 2 probable, 9 possible). Definitive alternate diagnoses were made in 10, including Huntington disease, early-onset Alzheimer disease, Niemann-Pick disease, neuroserpinopathy, prion disease, and neurodegeneration with brain iron accumulation. Six cases had features suggesting an alternate diagnosis, but no specific condition was identified; in 15, the data were inadequate for classification. Misinterpretation of normal lipofuscin as abnormal storage material was the commonest cause of misdiagnosis.

CONCLUSIONS

Diagnosis of ANCL remains challenging; expert pathologic analysis and recent molecular genetic advances revealed misdiagnoses in >1/3 of cases. We now have a refined group of cases that will facilitate identification of new causative genes.