Alpha-mannosidosis in Tunisian consanguineous families: Potential involvement of variants in GHR and SLC19A3 genes in the variable expressivity of cognitive impairment

Exome sequence analysis identifies a homozygous, pathogenic, frameshift variant in the MAN2B1 gene underlying clinical variant of α-mannosidosis

Background

α-mannosidosis (MAN) is a rare genetic condition that segregates in an autosomal recessive manner. Lack of lysosomal alpha-mannosidase is the underlying cause of the disease. Symptoms of the disease gradually worsen with the age. Newborns are usually asymptomatic, however, some cases are reported with either congenital ankle equinus or hydrocephalus during the first year. Primary symptoms are characterized by immune deficiency, hearing loss, skeletal abnormalities, progressive mental, motor and speech functions' impairment followed by facial asymmetry.

Methods

We studied two Saudi families (A and B) with bilateral moderate hearing loss (family A) and clubfoot with glaucoma (family B). Clinical diagnosis was not reached based on phenotype of patients. Therefore, hypothesis-free whole exome sequencing (WES) was performed on DNA samples from affected individuals of both the families, followed by Sanger sequencing and segregation analysis to validate the segregation of the identified variant. Furthermore, 3D protein modelling was performed to determine the in silico effects of the identified variant on the protein structure and function.

Results

Re-examination of clinical features revealed that the patients in family A have speech delay and hearing impairment along with craniostenosis, whereas the patients from family B have only clubfoot and glaucoma. WES identified a well known pathogenic homozygous frameshift variant (NM_000528.4: c.2402dupG; p.S802fs*129) in MAN2B1 in both the families. Sanger sequencing confirmed the segregation of the variant with the disease phenotype in both the families. 3D structural modeling of the MAN2B1 protein revealed significant changes in the tertiary structure of the mutant protein, which would affect enzyme function. This report presents a new case where partial and novel α-mannosidosis phenotypes are associated with a MAN2B1 gene pathogenic variant.

Conclusion

Patients in both the families have manifested peculiar set of clinical symptoms associated with α-mannosidosis. Family A manifested partial clinical symptoms missing several characteristic features like intellectual disability, dysmorphic features, neurological and abdominal manifestations, whereas family B has no reported clinical symptoms related to α-mannosidosis except the novel symptoms including club foot and glaucoma which has never been reported earlier The current findings support the evidence that biallelic variants of MAN2B1 are associated with new clinical variants of α-mannosidosis.

Current phenotypic and genetic spectrum of syndromic deafness in Tunisia: paving the way for precision auditory health

Hearing impairment (HI) is a prevalent neurosensory condition globally, impacting 5% of the population, with over 50% of congenital cases attributed to genetic etiologies. In Tunisia, HI underdiagnosis prevails, primarily due to limited access to comprehensive clinical tools, particularly for syndromic deafness (SD), characterized by clinical and genetic heterogeneity. This study aimed to uncover the SD spectrum through a 14-year investigation of a Tunisian cohort encompassing over 700 patients from four referral centers (2007-2021). Employing Sanger sequencing, Targeted Panel Gene Sequencing, and Whole Exome Sequencing, genetic analysis in 30 SD patients identified diagnoses such as Usher syndrome, Waardenburg syndrome, cranio-facial-hand-deafness syndrome, and H syndrome. This latter is a rare genodermatosis characterized by HI, hyperpigmentation, hypertrichosis, and systemic manifestations. A meta-analysis integrating our findings with existing data revealed that nearly 50% of Tunisian SD cases corresponded to rare inherited metabolic disorders. Distinguishing between non-syndromic and syndromic HI poses a challenge, where the age of onset and progression of features significantly impact accurate diagnoses. Despite advancements in local genetic characterization capabilities, certain ultra-rare forms of SD remain underdiagnosed. This research contributes critical insights to inform molecular diagnosis approaches for SD in Tunisia and the broader North-African region, thereby facilitating informed decision-making in clinical practice.

Long-term clinical evaluation of patients with alpha-mannosidosis - A multicenter study

BACKGROUND

Alpha mannosidosis is an autosomal recessive lysosomal storage disorder caused by biallelic pathogenic variants in the MAN2B1 gene. It manifests with clinical features, including intellectual disability, hearing impairment, coarse facial appearance, skeletal anomalies, immunodeficiency, central nervous system involvement, psychiatric comorbidities, corneal opacity, and hepatosplenomegaly. This multicenter study assesses the long-term outcomes of individuals diagnosed with alpha-mannosidosis, examining demographic, clinical, laboratory, and molecular characteristics.

METHOD

Sixteen patients diagnosed with alpha-mannosidosis who presented to four pediatric metabolic units were included in the study. The patients' medical records were analyzed and data on demographics, clinical presentation and laboratory findings were recorded.

RESULTS

Of the 16 patients (6 females, 10 males) with alpha mannosidosis included in the study, the mean age at the time of diagnosis was 79.4 ± 56.1 (16-208) months, and the mean diagnosis delay time was 57.9 ± 51.9 (4-181) months. Hearing loss was the primary manifestation found in seven out of 16 patients (43.8%), followed by speech delay in 37.8%. On clinical follow-up, 87.5% of patients experienced recurrent infections, mainly in the upper respiratory tract, with 12 requiring the use of a hearing aid. Hepatomegaly was found in six out of 13 patients who received abdominal ultrasonography; two out of 12 patients who underwent echocardiography were found to have mitral valve prolapse (16.6%). Upon neurological evaluation, five patients displayed no neurological manifestation. Delayed language development was observed in nine (56.3%) patients, intellectual disability in eight (50%) patients, and hypertonicity was identified in one (6.3%) patient with the severe form of the disease. Homozygous c.2477C>A (p.Ser826Ter) and homozygous c.967G>A (p.Glu323Lys) novel variants were detected in four patients and one patient, respectively. The most common variant observed in the study was c.2477C>A (p.Ser826Ter).

CONCLUSION

The present study identified two novel MAN2B1 variants. An evaluation of the long-term outcome of alpha-mannosidosis, in which the early initiation of enzyme replacement therapy (ERT) may lead to a better clinical outcome, can permit a better analysis of the effect of ERT on the natural progression of the disease.

Audiological and radiological study of eight polish patients with alpha-mannosidosis

Alpha-mannosidase catalyze lysosomal cleaving of mannose residues from glycoproteins. The enzyme is encoded by the MAN2B1 gene. Biallelic pathogenic variants cause enzymatic deficiency, which clinically results in alpha-mannosidosis (AM), an autosomal recessively inherited condition. Typical features observed in AM patients include intellectual disability, loss of speech, dysmorphic features, progressive motor problems, ataxia, hearing impairment and recurrent otitis. The cause of the latter is mainly attributed to immunodeficiency. The aim of our study was to demonstrate the otolaryngologic and hearing outcomes in patients with AM. The study group consisted of 8 AM patients: 6 males and 2 females, aged 2.5-37 yrs. The clinical course, dysmorphic ENT features, hearing status and the HRCT scans of the temporal bones were analyzed. MS Excel for Windows and Statistica software package were used for the comparison of interaural audiometric loss, mean hearing loss and mean hearing threshold for each patient's audiometric frequency tested. We identified ENT dysmorphic features in all of our AM patients, while the hearing loss was detected in 6 out of our 8 patients. For those cases, the onset of deafness was noted in the first decade of life, this impairment was sensorineural, of cochlear origin, bilateral, of a moderate degree (mean loss 62.76 dB; median 60 dB, standard deviation 12.5 dB), symmetrical and stable. The shape of the audiometric curves of our patients can be described as slightly sloping towards the higher tested frequencies, with a marked improvement at 4 kHz. The radiological examination revealed normal structures of the ears, with the exception of one case where a persistent otitis generated a cochlear gap. We therefore concluded that the hearing loss in our AM patients derived from cochlear impairment unrelated with recurrent otitis.