Dear Sirs,

The neuronal ceroid lipofuscinoses (NCLs) are the largest group of neurodegenerative diseases of childhood [1]. In the majority of cases, the clinical presentation occurs at preschool age, and such cases account for around 70 % of the NCL cohort recruited by the Italian network CLNet [2]. Clinical onset of NCL within the first year of life (infantile NCL; INCL) is less frequent; this form is mostly related to mutations in the PPT1/CLN1 gene [3]. Congenital NCLs are extremely rare. To date, five cases, all microcephalic, have been reported; three of these babies carried homozygous mutations in cathepsin D (CTSD), a gene coding for a lysosomal aspartic protease [4–7], and none of them lived longer than 10 days. Biallelic mutations in CTSD have been reported in five patients, who developed either late infantile or juvenile forms of the disease associated with granular osmiophilic deposits (GRODs) in both skin and skeletal muscle [8, 9]. Cases carrying CTSD mutations have been classified, together, as affected by CLN10 disease [10]. We report the case of a female child with INCL, who developed early and dramatic decreased rate of head growth with acquired microcephaly and marked cerebral atrophy (Fig. 1), a relatively late appearance of severely progressive epilepsy (displaying a “vanishing” EEG pattern), GRODs in a skin biopsy, and hypertrophic cardiomyopathy (see Supplementary Material). Defective enzymatic activity of CTSD was detected by a mass spectrometry (MS)-based assay in dried blood spots (DBSs). Sanger sequencing showed that the child harbored a novel homozygous c.205G>A, p.Glu69Lys located in exon 2 of CTSD (Fig. 2). The mutation, absent in a large series of exome polymorphic databases and in a set of ethnically matched healthy controls, was heterozygous in her healthy parents and sister, and it was associated with nearly absent enzymatic activity of CTSD in DBSs and a significant reduction of protein expression in cultured skin fibroblasts. The storage material present in the skin biopsy of this patient has seldom been described in CLN10 disease [5, 8, 9], whereas it is commonly detected in both CNS and peripheral tissues in the CLN1 disease and adult-onset CLN4 [11]. On the basis of the Cln1 −/− mouse model, it is hypothesized that the lack of synergic action of CTSD and PPT1 engulfed lysosomes with undegraded proteins and organelles, which favors the formation of GRODs [12].

Fig. 1
figure 1

Head MRI of the patient. Coronal sections T2 inversion recovery sequence. Upper row 8-month old. The subarachnoid spaces of the frontal lobes are enlarged bilaterally; the cerebellum is not affected; and the lateral ventricles and the third ventricle are mildly dilated. Lower row 18-month old. Dramatic cerebral atrophy is observed affecting the cortex diffusely; the centrum semovale is shrunken bilaterally; the corpus callosum is severely thinned; the ventricular system is markedly enlarged; and the cerebellum is modestly affected

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Fig. 2
figure 2

a Biochemical enzyme analysis of cathepsin D in dried blood spots obtained from the patient (II-02), her carrier parents (I-01 and I-02), a healthy sister (II-01), and six healthy, age/sex-matched controls. The enzymatic activities were expressed as μM/L blood/h. b Family pedigree of a novel family harboring a mutation in CTSD. M presence of the mutation and hyphen wild-type sequence. Circles are women, and squares are men; filled symbol indicates the patient. The figure also shows the electropherogram flanking the novel c.205G>A, p.Glu69Lys in CTSD detected in II-02 (P) as compared with a control (WT), and the level of conservation among different species of the glutamic acid at residue 69. c Western blotting analysis of cathepsin D in cultured skin fibroblasts from a patient harboring the novel homozygous mutation in CTSD. The band at 34 kDa represents the mature cathepsin D. The pro (52 kDa) and the single chain (43 kDa) forms of the protein (indicated by asterisks) are slightly visible in human fibroblasts. The LAMP2 protein has been used as a lysosomal marker, whereas β-tubulin was used as control for loading. The panel on the right illustrates the quantification of the levels of mature protein in the patient (P) as compared with a control (C). Data are mean ± SEM of three independent experiments

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The present case presented the atypical association of encephalopathy with hypertrophic cardiomyopathy, already observed in juvenile CLN10 disease (case 2 in [9]), and previously reported in CLN3 and, occasionally, in CLN2 [13, 14]. By extending the life expectancy of patients, through attenuation of neurological symptoms with novel therapeutic opportunities [15], further pathologies may arise in the peripheral organs, including the heart. An evaluation of heart function and volume should always be considered in the follow-up of young NCL patients regardless of their clinical form, because cardiac symptoms can be as life threatening as the CNS features.

To summarize, in this patient, who broadens the phenotypic spectrum of CLN10 disease, a conclusive diagnosis of NCL was reached using the MS-based multiplex enzyme testing in DBSs. The benefit and efficiency of this method should be borne in mind when planning the early diagnostic screening of toddlers with symptoms suggesting early onset encephalopathies, given that such onsets can occur in enzymatic forms of NCLs.