In the cohort under study, a total of eight patients were enrolled, with ages spanning from 4 to 6 years. The studies group participants consisted of 5 females and 3 males. All the patients included in this study were from distinct families and were unrelated to each other. The collected dataset is described below and encompasses the demographic information, genotypic profile, key clinical, neuroimaging and electrophysiologic characteristics. Among the clinical phenotypic spectrum, our group focused on studying the onset and progression of neurologic symptomatology and comprehensive neuropsychological evaluation.

Demographics/genotype

All 8 patients were clinically diagnosed with CLN7 deficiency, and the diagnosis was molecularly confirmed with homozygous or bi-allelic heterozygous pathogenic mutations in MFSD8 gene. Analysis of the MFSD8 gene mutations in our patient population revealed 12 distinct pathogenic variants, out of which 6 variants (Table 1) are novel to this study and have not been described in literature before.

Table 1 Demographics- participants gender, country of origin and genotype

The genetic diagnosis was made through epilepsy gene panels, whole exome sequencing and MFSD8 sequence analysis. Descriptive analysis was used to analyze the data which revealed heterogeneity of geographic, cultural, and linguistic and demographics backgrounds as detailed in Table 1.

Clinical characteristicsBirth history, early development, and age of onset of symptoms

All patients (n = 8) were reported to have an uncomplicated birth and an unremarkable early post-natal period. Early development was normal until 2 years of age.

The earliest symptom onset was at two years of age. In all the patients (n = 8) the onset of symptoms was noted before 4 years of age (Table 2) and the age of diagnosis was between 4 and 5 years. In our study cohort, median age of onset of symptoms is 3.4 years and median age of diagnosis is 4.8 years.

Table 2 Developmental regression

In the study cohort, the initial presenting symptoms prompting the families to seek medical attention varied among the patients. Per parent report, 2 out of 8 patients presented with gait disturbance as the initial symptom, while another 2 exhibited signs of tremors and recurrent falls. Additionally, one patient presented with vision changes, and difficulties in language were reported in 2 patients, either in terms of articulation or fluency and 1 patient was brought to medical attention because family was concerned about developmental delays, which was described as an overall concern about the lack of progress in neurologic development specifically in language and fine motor domains.

It is noteworthy, that the first clinical symptoms, prompting the family to seek medical attention is parent reported and is described here as it was noted in medical records. It is important to mention that detailed clinical and neuropsychological assessments were not conducted at the time of onset to provide detailed characterization of specific nature and extent of language difficulties or developmental delays.

Another noteworthy observation emerged during retrospective analysis, that the majority of patients, 6 out of 8 to be specific, had a paucity in language development, occurring at or right after 2 years of age. This plateau in language development, however, did not attain a level of significance which would either prompt families to seek medical attention or alert the primary care physician to pursue further diagnostic testing.

Neurological symptomsDevelopmental regression

Gait Problems

Gait disturbance was reported by families as early as 2 and a half years of age and as late as 4 and half years of age. Gait progressively deteriorated requiring an aid to walk within 1–2 years from onset of gait difficulty (Table 2). Once walking difficulties were noticed, the earlier level of function was never obtained. All the patients who were seen at advanced disease stage were non-ambulatory within 1 year of age when they needed help with walking (Fig. 1).

Fig. 1

Box plots of the age of onset for stages of ambulatory regression

Language difficulties

Language difficulties or delays were noted as early as 2 years of age in 3/8 patients, 2.5 years in 3/8 patients, 3.5 years in 1/8 patients and by 4 years in 1/8 patients. All the patients at the time of visit had moderate to severe language issues.

Seizures

In this patient cohort the onset of seizures ranged from 3 years and 5 months to 4 years and 10 months. Various seizure types were reported including myoclonic, atonic, and bilateral tonic–clonic seizures. In addition to seizures, myoclonic jerks were reported in all but one study participant. One unprovoked seizure was captured during EEG, identified by the caregiver, and characterized clinically by a brief head drop (atonic seizure).

In this study cohort, the majority (6 out of 8) of patients required more than one anti-seizure medication to achieve adequate seizure control. The most common anti-seizure medication used in this study cohort was Valproic acid, and 6 out of 8 patients were taking Valproic acid. In one patient, Valproic acid was stopped later in the disease course due to ineffectiveness and replaced with an alternative medicine. Clobazam and Levetiracetam were the second most common medications used and 5 out of 8 patients were taking these anti-seizure medications. Lamotrigine was also used for seizure control in 2 out of 8 patients in this cohort. Cannabidiol was used in 2 out of 8 patients. One study patient was on Rufinamide and Clonazepam. There was no clear benefit of one anti-seizure medication over the other, however, this data are not sufficient to establish such a correlation. One patient had excessive sedation on combination of Clobazam and Cannabidiol (FDA approved prescribed formulation used), however there were no other harmful effects or side effects reported for any other anti-seizure medication at the time of the study. It should be noted that one of the limitations of the retrospective study is paucity of data and a complete profile of longitudinal use of anti-seizure medications along with the side effects and interactions of various medications is not available to report in some cases.

Figure 1 shows box plots of the age of onset for stages of ambulatory regression. Stages range from gait problems (light blue) down to non-ambulatory status (dark blue). Ages range from roughly 3 years of age to around 6 years of age. Each line corresponds to one of the 8 individual patients.

Neuro-imaging findings

Abnormal MRI results were observed in all (8/8) cases reviewed. All patients had generalized findings with diffuse cortical and cerebellar atrophy with white matter involvement (see Table 3). Diffuse white matter involvement with T2 hyperintensities were noted in 7/8 cases and periventricular gliosis was reported in 2/8 cases. Thalami were involved in all (8/8) cases with a variable degree of volume loss and gliosis. The neuroimaging data was compared to the previous scans for cases where the previous reports or images were avaialble.

Table 3 Neuro-imaging findingsElectrophysiological findings

Electroencephalographic interictal background activity for all eight patients was abnormal. Interictal background activity showed continuous generalized delta and theta slowing in 37.5% and 62.5% of patients, respectively. In 5 out of 8 patients generalized rhythmic delta activity was seen which often was maximal in the bioccipital or bifrontal regions (Fig. 2). Independent multifocal spikes (100% of patients) along with generalized (75% of patients) with posterior maximum epileptiform discharges were seen. During photic stimulation no electroretinogram artifact, photomyoclonus or photoparoxysmal responses were seen in 6 of 8 patients. In one patient photic stimulation triggered a focal motor (myoclonic) seizure with concomitant spike and wave activity seen in the bioccipital regions. One unprovoked seizure was captured, identified by the caregiver, and characterized clinically by a brief head drop (atonic seizure). Ictal EEG showed a generalized spike and wave correlate. One event of non-epileptic staring was seen with no electrographic correlation.

Fig. 2

Interictal background showing generalized rhythmic delta activity

Figure 2 This figure illustrates the EEG recording of a patient during the interictal period. The data depict a consistent pattern of generalized rhythmic delta activity, characterized by a regular, rhythmic waveform. The presence of rhythmic delta waves in the interictal phase is indicative of potential underlying neurological conditions

Neuropsychological assessment results

Assessment of developmental/cognitive abilities was limited for this population given advanced disease progression. Of the 8 children seen, 6 had complete Mullen administrations (participants 3–8). The other two were both evaluated on receptive and expressive language and only one (participant 1) on fine motor abilities. Except for participant 8, all others obtained a standard score at the floor (i.e., lowest score available) of the measure (Standard Score = 49) and participant 8 performed in the exceptionally low range (Early Learning Composite Standard Score = 63). Additionally, participants 1, 2, and 7 were outside of the normative age-range. As such, Developmental Quotients are thought to best capture the participants’ functioning and are presented in Fig. 3. Participants 5 and 6 underwent 6-month follow-up evaluations with the Mullen demonstrating regression in all domains for both participants (Fig. 1 in supplementary Materials. The slope of the regression was most significant in the area of receptive language (− 0.13 participant 5 and − 0.45 participant 6) compared to fine motor (− 0.02 participant 5 and − 0.18 participant 6), visual reception (− 0.08 participant 5 and − 0.15 participant 6), and expressive language (− 0.16 participant 5 and − 0.21 participant 6) slopes.

Fig. 3

Boxplots of developmental quotients from the mullen for subjects 1–8

Figure 3 Boxplots of Developmental Quotients from the Mullen for subjects 1–8. The scores represented are the Gross Motor (GM), Fine Motor (FM), Visual Reception (VR), Receptive Language (RL), and Expressive Language (EL) subtests. Subjects from the natural history study (NH) are colored red, and subjects from the gene therapy clinical trials (Clinical) are colored blue

With respect to adaptive functioning, there was noted to be significant variability although all but participant 8 was noted to be in the exceptionally low range on the Vineland-3 Adaptive Behavior Composite (Standard Score mean = 51.5 ± 31.5). Composite standard scores for the adaptive measure are presented in Fig. 4. Participants in the clinical trial were rated overall at a higher level of functioning than the participants in the natural history portion. Similar to results from the Mullen, follow-up evaluations on the Vineland-3 for participants 1, 2, 5 and 6 also demonstrated parent-reported regression overall (Adaptive Behavior Composite; see supplementary Fig. 2). Individually, participant 5 had sharp regressions in Communication (slope of − 31) while the others remained at the floor of the subdomain (participants 1 and 2 had slopes of 0 and participant 6 a slope of − 2). Daily Living remained stable for participants 1 (slope of 0) and 5 (slope of 1) but was noted to regress for participants 2 (slope of − 16) and 6 (slope of − 17). Social skills were also relatively stable for participants 1 (slope of 2) and 2 (slope of − 4) but showed a sharper decline in 5 and 6 (both with slopes of − 18). Further, Motor Skills were already at the floor for participants 1 and 2 (both with slopes of 0) but demonstrated rapid regression for participants 5 (slope of − 26) and 6 (slope of − 32). A look at the Developmental Quotients for the motor and communication subscales of the Vineland-3 revealed smaller differences between natural history and clinical trial participants. Further, the language scales were rated more highly than motor scales (Fig. 5).

Fig. 4

Boxplots of standard scores from the vineland-3 for subjects 1–8

Fig. 5

Boxplots of developmental quotients scores from the vineland-3 for subjects 1–8

Figure 4 Boxplots of Standard Scores from the Vineland-3 for subjects 1–8. The scores collected are the Adaptive Behavior Composite made up of the other subdomains including Socialization, Daily Living Skills, Communication, and Motor Skills composites. Subjects from the natural history study (NH) are colored red, and subjects from the gene therapy clinical trials (Clinical) are colored blue

For the clinical trial portion, assessment of health-related quality of life was assessed (participants 5–8). Three participants received the ITQOL (participants 5, 6, and 8). There was significant variability in responses (Supplementary Fig. 3), but parents rated Overall Health as good to excellent. While parents for participants 6 and 8 rated their child’s Change in Health much worse than one year ago, parents of participant 5 rated it to be about the same. Parental Emotional Impact and Family Cohesion also revealed significant variability. The QI-Disability was administered to participants 5 and 7 (Supplementary Fig. 4). The greatest variability was seen in Negative Emotions (e.g., behavioral outbursts, withdrawn behavior), but both families rated generally positive Physical Health, Positive Emotions (e.g., smiling, laughing), Social Interaction, and Leisure Skills.

Figure 5 Boxplots of Developmental Quotients Scores from the Vineland-3 for subjects 1–8. The scores represented are the Gross Motor (GM), Fine Motor (FM), Receptive Language (RL) and Expressive Language (EL) subscales. Subjects from the natural history study (NH) are colored red, and subjects from the gene therapy clinical trials (Clinical) are colored blue