Clinical utility gene card for: Biotinidase deficiency-update 2015

Sequence variants in the BTD underlying biotinidase deficiency in families of Pakistani origin

BACKGROUND

Biotinidase deficiency (BTD) is a rare autosomal recessive metabolic disease, which develops neurological symptoms because of the impaired biotin recycling. Pathogenic mutations on BTD gene cause BTD deficiency. The clinical features and mutation analysis of Pakistani children with BTD deficiency have rarely been described. Herein, for the first time, we report the clinical features, BTD gene mutations and biochemical analysis of seven symptomatic children with BTD deficiency from Pakistan.

METHODS

Seven suspected BTD-deficient patients who presented abnormal organic acid profiles and clinical features were subjected to Sanger sequencing to identify pathogenic mutations in the BTD gene. The results were analyzed by Mutation Surveyor Software.

RESULTS

All seven patients exhibited common biotinidase deficiency symptoms including hypotonia, developmental delay and seizures. Biochemical analysis shows marked excretion of 3-hydroxy isovalerate in all cases, followed by 3-hydroxy propionate and methyl citrate. Sanger sequencing revealed one frame-shift mutation, c.98_104delinsTCC (p.Cys33Phefs), and two missense mutations, c.1612C>A (p.Arg538Ser) and c.1330G>C (p.Asp444His). All mutations were in the homozygous state and classified as pathogenic in published studies and mutation databases.

CONCLUSIONS

This study has validated the BTD variants as the underlying cause of biotinidase deficiency in which molecular testing of BTD is supported by urinary organic acid analysis and clinical diagnosis. Secondly, the strength of the local availability of this test in Pakistan will paved the way for the neonatal screening of biotinidase deficiency.

Atopic Dermatitis-like Genodermatosis: Disease Diagnosis and Management

Eczema is a classical characteristic not only in atopic dermatitis but also in various genodermatosis. Patients suffering from primary immunodeficiency diseases such as hyper-immunoglobulin E syndromes, Wiskott-Aldrich syndrome, immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome, STAT5B deficiency, Omenn syndrome, atypical complete DiGeorge syndrome; metabolic disorders such as acrodermatitis enteropathy, multiple carboxylase deficiency, prolidase deficiency; and other rare syndromes like severe dermatitis, multiple allergies and metabolic wasting syndrome, Netherton syndrome, and peeling skin syndrome frequently perform with eczema-like lesions. These genodermatosis may be misguided in the context of eczematous phenotype. Misdiagnosis of severe disorders unavoidably affects appropriate treatment and leads to irreversible outcomes for patients, which underlines the importance of molecular diagnosis and genetic analysis. Here we conclude clinical manifestations, molecular mechanism, diagnosis and management of several eczema-related genodermatosis and provide accessible advice to physicians.

Immunophenotypic analysis of lymphocyte subsets in newborns with biotinidase deficiency

BACKGROUND/AIM

Biotin is a vital micronutrient that plays a role in metabolic homeostasis and the regulation of innate and adaptive immune system functions. Biotinidase deficiency (BTD) leads to impairment in biotin-dependent immune functions. This study focused on immunophenotypic analysis of lymphocyte subsets in newborns with BTD.

PATIENTS AND METHODS

A total of 181 (95 female and 86 male; 114 had BTD and 67 were healthy) newborns underwent biotinidase enzyme activity, molecular and lymphocyte immunophenotyping analyses. BTD is classified into four biochemical phenotypes: profound, partial, heterozygous and normal. The following lymphocyte subsets were studied in all participants: total B lymphocyte (CD19), total T lymphocyte (CD3), helper T lymphocyte (CD3/CD4), cytotoxic T lymphocyte (CTL) (CD3/CD8), natural killer T lymphocyte (CD16/56) and a T-lymphocyte activation marker (HLA-DR).

RESULTS

The percentages of lymphocyte subsets were similar in newborns with and without BTD. In all newborns with BTD, the mean CD3/CD4 levels were higher in females, while the CD3/CD8 levels were higher in males (P < .001 for each). In female and male newborns, the CD3/CD4 levels were 53.83 ± 9.46 and 16.82 ± 5.19, respectively, and the CD3/CD8 levels were 48.80 ± 8.65 and 21.48 ± 6.02, respectively. A moderate negative correlation was found between CD3/CD4 and CD3/CD8 in female and male newborns (r_female  = -0.488, r_male  = -0.574, P < .001).

CONCLUSION

This study showed that although there were no differences in the lymphocyte subsets in newborns with BTD, the CD3/CD4 levels were higher in females, and the CD3/CD8 levels were higher in males. In addition, there was a negative correlation between the CD3/CD4 and CD3/CD8 levels in both genders. Although these results indicate sexual dimorphism between CD3/CD4 and CD3/CD8 levels, whether this dissociation is unique to BTD in newborns is not fully clear.

High frequency of biotinidase deficiency in Italian population identified by newborn screening

The biotinidase (BTD) enzyme is essential for recycling biotin, a water-soluble B-complex vitamin that is the coenzyme of four carboxylases involved in fatty acid synthesis, amino acid catabolism and gluconeogenesis. If untreated, total or partial BTD deficiencies lead to an autosomal recessive inherited organic aciduria whose clinical features, mainly presenting in the first years of life, include, seizures, skin rash, and alopecia. Based on residual BTD enzyme activity it is possible to identify partial or total biotinidase deficiency. The incidence of profound and partial biotinidase deficiency worldwide is estimated to be about 1 in 60.000.

We report twelve years of experience in the newborn screening of biotinidase deficiency on 466.182 neonates. When a positive screening result occurred, a clinical evaluation was made of the patient and genetic counselling was offered to the family. Molecular analysis the BTD gene was carried out in all recalled neonates.

Newborn screening lead to the identification of 75 BTD deficiencies with an incidence of about 1:6.300 births, ten times higher than the reported worldwide incidence. BTD deficiency was confirmed at a genomic level in all patients, demonstrating a high frequency of the p.(Asp444His) amino acid substitution and the complex allele p.(Ala171Thr)/p.(Asp444His) in the analyzed Italian newborns. Four new mutations (two small deletions, one stop mutation and one missense mutation) and a new combined allelic alteration were identified.

Our data suggests that there is a high incidence of the biotinidase defect in the Italian population, most likely due to the high frequency of certain mutations.

Severe Distal Motor Involvement in a Non-compliant Adult With Biotinidase Deficiency: The Necessity of Life-Long Biotin Therapy

Biotinidase deficiency is an autosomal recessive disorder in which affected individuals are unable to recycle biotin. Untreated, children usually exhibit hypotonia, seizures, ataxia, developmental delay, and/or hearing loss. Individuals diagnosed by newborn screening have an excellent prognosis with life-long biotin supplementation. We report a young adult diagnosed with profound biotinidase deficiency by newborn screening who was asymptomatic while on therapy. At 18 years of age, 6 months after voluntarily discontinuation of biotin, he developed a progressive distal muscle weakness. Molecular analysis of the BTD gene showed a pathogenic homozygous duplication c.1372_1373dupT p.(Cys458LeufsTer26) (1). Despite 16 months since reintroduction of biotin, muscle strength only partially recovered. Transition to adulthood in chronic metabolic diseases is known to be associated with an increased risk for non-compliance. Neurological findings in this adult are similar to those described in others with adult-onset biotinidase deficiency. Long-term prognosis in non-compliant symptomatic adult with biotinidase deficiency likely depends on the delay and/or severity of intervening symptoms until reintroduction of biotin.

Novel familial IQSEC2 pathogenic sequence variant associated with neurodevelopmental disorders and epilepsy

Pathogenic sequence variants in the IQ motif- and Sec7 domain-containing protein 2 (IQSEC2) gene have been confirmed as causative in the aetiopathogenesis of neurodevelopmental disorders (intellectual disability, autism) and epilepsy. We report on a case of a family with three sons; two of them manifest delayed psychomotor development and epilepsy. Initially proband A was examined using a multistep molecular diagnostics algorithm, including karyotype and array-comparative genomic hybridization analysis, both with negative results. Therefore, probands A and B and their unaffected parents were enrolled for an analysis using targeted "next-generation" sequencing (NGS) with a gene panel ClearSeq Inherited Disease^XT (Agilent Technologies) and verification analysis by Sanger sequencing. A novel frameshift variant in the X-linked IQSEC2 gene NM_001111125.2:c.1813_1814del, p.(Asp605Profs*3) on protein level, was identified in both affected probands and their asymptomatic mother, having skewed X chromosome inactivation (XCI) (100:0). As the IQSEC2 gene is a known gene escaping from XCI in humans, we expect the existence of mechanisms maintaining the normal or enough level of the IQSEC2 protein in the asymptomatic mother. Further analyses may help to the characterization of the presented novel frameshift variant in the IQSEC2 gene as well as to elucidate the mechanisms leading to the rare asymptomatic phenotypes in females.

Biotinidase Deficiency: Prevalence, Impact And Management Strategies

Biotinidase deficiency is an autosomal recessive inherited neurocutaneous disorder. Clinically untreated patients with BD can present with variable neurological and dermatological signs, such as seizures, hypotonia, feeding problems, developmental delay, hearing loss, optic atrophy ataxia, alopecia, and skin rash. Clinical findings of patients with partial BD reported in the literature show that it can occur from infancy to adulthood. Outcomes of newborn screening programs support the fact that biotin treatment started after birth prevents patients with biotinidase deficiency from developing symptoms. Presence of late-onset cases with different clinical findings indicates that there is still much to learn about BD.

Laboratory diagnosis of biotinidase deficiency, 2017 update: a technical standard and guideline of the American College of Medical Genetics and Genomics

Disclaimer: These ACMG Standards and Guidelines are intended as an educational resource for clinical laboratory geneticists to help them provide quality clinical laboratory genetic services. Adherence to these Standards and Guidelines is voluntary and does not necessarily assure a successful medical outcome. These Standards and Guidelines should not be considered inclusive of all proper procedures and tests or exclusive of others that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, clinical laboratory geneticists should apply their professional judgment to the specific circumstances presented by the patient or specimen. Clinical laboratory scientists and geneticists are encouraged to document in the patient's record the rationale for the use of a particular procedure or test, whether or not it is in conformance with these Standards and Guidelines. They also are advised to take notice of the date any particular guideline was adopted, and to consider other relevant medical and scientific information that becomes available after that date. It also would be prudent to consider whether intellectual property interests may restrict the performance of certain tests and other procedures.Biotinidase deficiency is an autosomal recessively inherited disorder of biotin recycling that is associated with neurologic and cutaneous consequences if untreated. Fortunately, the clinical features of the disorder can be ameliorated or prevented by administering pharmacological doses of the vitamin biotin. Newborn screening and confirmatory diagnosis of biotinidase deficiency encompasses both enzymatic and molecular testing approaches. These guidelines were developed to define and standardize laboratory procedures for enzymatic biotinidase testing, to delineate situations for which follow-up molecular testing is warranted, and to characterize variables that can influence test performance and interpretation of results.