Most types of NCL manifest in infancy or early childhood and are characterized by developmental regression recognizable in the progressive loss of motor and cognitive skills, the onset of seizures and behavioral problems, as well as retinal degeneration resulting in visual impairment progressing to blindness. These symptoms are triggered by profound neuronal degeneration, cortical thinning, and overall brain atrophy, and they may be accompanied by progressive microcephaly [1] [2] [3].

Following normal development during the first few months of life, children suffering from classical infantile NCL begin to lose previously acquired skills and are eventually diagnosed with progressive cognitive and motor decline. By contrast, epilepsy is the leading symptom in most patients with NCL type 2 [4]. While vision loss is the presenting symptom in about 80% of patients with NCL type 3 [3], it is not usually observed in adult-onset NCL. The latter typically manifests as early dementia, Parkinson-like movement disorders, and myoclonic epilepsy. Patients suffering from congenital NCL may present with microcephaly at birth [5]. In general, the sequence of onset of symptoms may vary from patient to patient [3].

The presence of two or more of the main symptoms (motor deterioration, dementia, epilepsy, and vision loss) should raise suspicion as to NCL. Except from NCL type 10, these symptoms are developed by initially healthy patients of different ages.

Initial laboratory studies comprise enzymatic tests (when NCL types 1, 2, or 10 are considered), the analyses of blood smears in search of vacuolated lymphocytes (to be expected in patients with NCL types 3 or 12), as well as light and electron microscopic investigations of intracellular storage [6]. NCL is related to the accumulation of autofluorescent material in lysosomes in and outside of the central nervous system, and thus, biopsy samples are most commonly obtained from the skin [7]. The respective lipopigments are recognizable by light microscopy, while further hints on the underlying type of the disease can be gathered by means of electron microscopy. Here, distinctive ultrastructural patterns may be identified: NCL type 1, for instance, is characterized by granular osmiophilic deposits, as are NCL types 4, 6, 8, 10, and 14. By contrast, curvilinear patterns are observed in classical late-infantile NCL and NCL types 3, 5, 6, and 8. Besides the curvilinear ultrastructure, NCL type 3 may also be associated with rectilinear or fingerprint profiles. Rectilinear and fingerprint patterns may similarly be noticed in samples from patients with NCL types 5 and 7, whereas fingerprints may also be related to NCL types 6, 11, 12, 13, and 14. TDP-43 inclusions have been reported for NCL type 11, while brain iron accumulation is typical of NCL type 12 [5].

As can be inferred from the previous descriptions, the combination of certain ultrastructural patterns may further support the tentative diagnosis of a determined type of NCL. This tentative diagnosis should be confirmed with molecular biological analyses aiming at identifying the underlying mutation. This will at the same time lay the foundation for a thorough familial workup.

To date, treatment options are limited to symptomatic and palliative care [6], but new approaches to the management of NCL are intensively studied by researchers all over the world. In this context, they follow distinct strategies in agreement with the function of the deficient protein [7]:

• NCL types 1 and 2 are caused by the deficiency of lysosomal enzymes and may be amenable to exogenous sources of functional proteins. Furthermore, gene therapy and stem-cell therapy have been discussed as treatment options for these enzymatic variants of NCL [1] [2].
• Similarly, other types of NCL related to the deficiency of a secretable protein may possibly be managed with gene therapy and stem-cell therapy. Besides NCL types 1 and 2, this applies to NCL types 5, 10, 11, and 13. The underlying phenomenon is cross-correction, which is not to be expected in cases of mutations affecting transmembrane proteins.
• Gliosis has been shown to precede NCL-related neurodegeneration, which indicates a role of neuroinflammation in the pathogenesis of the disease. Accordingly, anti-inflammatory agents have been considered as potential aids in the retardation of disease progression. Mycophenolate mofetil, for instance, has been studied in a short-term clinical trial including patients with NCL type 3 [8]. Long-term results have yet to be provided.

Employing experimental therapies, remarkable improvements in neurological function and life expectancy have been achieved in animal models. While this gives hope to many patients who wish for improvements in their quality of life, the fact that cure has not been reached for any type NCL does point out the necessity to further intensify research efforts [7].

It should be considered that the age at symptom onset as well as the presentation and progression of NCL may vary even when occurring within the same family. Nevertheless, all cases of NCL are associated with significant morbidity. There is no cure for this disease, which is associated with a limited expectancy of life. Patients diagnosed with classical infantile NCL don't usually survive beyond the age of 6, while those suffering from classical late-infantile NCL may reach the early teenage years. Patients with NCL type 3, the classical juvenile variant, often succumb to the disease during the third or fourth decade of life [3]. As a rule of thumb, the later the onset of symptoms, the higher the life expectancy. In line with this rule, congenital NCL is related to the shortest lifespan, with affected individuals often dying within their first year of life, and adult-onset disease may allow for long-term survival [5].

To date, NCL has been related to several hundred mutations in more than a dozen genes [4]. Most types of NCL are inherited in an autosomal recessive manner, with autosomal dominant inheritance having been described for NCL types 4 and 11 [5].

NCL has been described in patients of all ages, both genders, and most ethnicities worldwide. The overall incidence has been estimated at 1-2.5 per 100,000 live births, with classical late-infantile NCL being the most common type of the disease [1] [5].

The original, phenotypic classification of NCL has been based on the age of onset, which was defined as follows:

• Congenital or neonatal NCL is present at birth.
• Infantile NCL manifests within the first two years of life.
• Late infantile NCL becomes symptomatic in children aged 2-4 years.
• Juvenile NCL manifests in preschoolers.
• Adult NCL is associated with symptom onset beyond the age of 16.

The age at symptom onset remains an important aspect in the clinical evaluation and diagnosis of NCL, but more modern classification schemes are based on the underlying gene defects as well as the properties and functions of the affected proteins [6].

Distinct types of NCL are caused by mutations in different genes. Although the function of the respective gene products remains incompletely understood, many of these proteins have been shown to participate in lysosomal catabolism and the recycling of proteins and lipids [4] [9]. NCL may be related to enzyme deficiencies, defects in transmembrane proteins, mutations in ATPase or potassium channel genes, or the dysfunction of proteins putatively implicated in synapse functions [6].

Genotype-phenotype correlations have been described, and the degree of protein dysfunction may indeed correspond to the severity of the disease. Such has initially been described for those types of NCL related to deficiencies of lysosomal enzymes, but it may also apply to other variants of the disease [4]. In this context, it has been speculated that the complete loss of function of the CLN6 gene and its product, a protein likely to be involved in the degradation of post-translationally modified proteins in lysosomes, provokes a more severe disease than the partial reduction of CLN6 protein activity [10]. On the other hand, gain-of-function mutations resulting in protein aggregation is assumed to be the cause of NCL type 4 [7]. In sum, a better understanding of the individual functions of NCL-related genes is urgently required to better target experimental treatment regimens.

Affected families may benefit from genetic counseling. Precise knowledge regarding the underlying mutation allows for the prenatal diagnosis of NCL, which may weigh on the parent's decision regarding the maintenance of pregnancy. Similarly, prenatal testing is feasible in the case of enzyme deficiencies.

NCL refers to a heterogeneous group of lysosomal storage disorders. According to the current classification scheme, there are 13 types of NCL. For reasons of clarity, they are briefly described in this section and related to the underlying mutation [5] [6]:

• NCL type 1 in its classical infantile, late infantile, or juvenile phenotype due to mutations in the PPT1 gene
• NCL type 2 as a variant of the late infantile NCL type 1 is also referred to as classical late-infantile NCL and is caused by mutations in the TPP1 gene
• NCL type 3 or classical juvenile NCL is also referred to as Batten disease and has been linked to mutations in the CLN3 gene
• NCL type 4 or Parry type NCL, which may manifest in adolescence or adulthood, is provoked by DNAJC5 mutations
• NCL type 5 is another variant of the late infantile NCL type 1 and is related to mutations in the CLN5 gene
• NCL type 6 may similarly manifest as late infantile NCL, but adult-onset disease has also been described and may be referred to as Kufs disease type A; both variants are due to mutations in the CLN6 gene
• NCL type 7 corresponds to yet another variant of the late infantile NCL type 1 but is provoked by MFSD8 mutations
• NCL type 8 may present as late infantile NCL or late-onset Northern epilepsy, both of which are associated with mutations in the CLN8 gene
• NCL type 10 is also known as congenital NCL and is caused by CTSD mutations
• NCL type 11 is not usually diagnosed before the third decade of life and may be related to mutations in the GRN gene
• NCL type 12 is also referred to as Kufor-Rabek syndrome and constitutes a juvenile variant of NCL, which is caused by ATP13A2 mutations
• NCL type 13 or Kufs disease type B is an adult-onset variant of NCL that is related to mutations in the CTSF gene
• NCL type 14 as an infantile variant of NCL due to KCTD7 mutations

As described in the list, there are certain types of NCL that are referred to as classical variants of the disease according to the average age at symptom onset. These are NCL type 1 (classical infantile NCL), NCL type 2 (classical late-infantile NCL), and NCL type 3 (classical juvenile NCL). It is beyond the scope of this article to provide a detailed description of the individual types (and subtypes) of NCL, so it is focused on these classical forms of the disease.

Neuronal ceroid lipofuscinosis (NCL) is a general term referring to more than a dozen diseases caused by mutations of different genes. They do, however, share important clinical features. Most types of NCL manifest in infancy or early childhood and are associated with a progressive decline in motor function and cognitive skills, seizures, behavioral disorders, and vision loss. Very rarely, NCL is present at birth, or symptom onset is delayed until adulthood.

To date, there is no cure for this disease, which is invariably fatal. Patients are provided symptomatic and palliative care but eventually succumb to the consequences of neurodegeneration and brain atrophy. Their life expectancy depends on the age of symptom onset: the later the onset of symptoms, the higher the life expectancy. In any case, NCL is associated with significant morbidity.

In families known to harbor NCL-related mutations, the identification of the respective gene defects makes prenatal diagnosis feasible and gives parents the opportunity to decide with regards to the maintenance of pregnancy.

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