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A homozygous double mutation in SMN1: a complicated genetic diagnosis of SMA.

ABSTRACT: Spinal muscular atrophy (SMA), the most common autosomal recessive cause of infant death, is typically diagnosed by determination of SMN1 copy number. Approximately 3-5% of patients with SMA retain at least one copy of the SMN1 gene carrying pathogenic insertions, deletions, or point mutations. We report a patient with SMA who is homozygous for two mutations carried in cis: an 8 bp duplication (c.48_55dupGGATTCCG; p.Val19fs*24) and a point mutation (c.662C>T; p.Pro221Leu). The consanguineous parents carry the same two mutations within one SMN1 gene copy. We demonstrate that a more accurate diagnosis of the disease is obtained through a novel diagnostic assay and development of a capillary electrophoresis method to determine the copy number of their mutant alleles. This illustrates the complexity of SMN mutations and suggests additional testing (gene sequencing) may be appropriate when based on family lines.

SUBMITTER: Kirwin SM

PROVIDER: S-EPMC3865576 | biostudies-literature | 2013 Jul

REPOSITORIES: biostudies-literature

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A homozygous double mutation in SMN1: a complicated genetic diagnosis of SMA.

Kirwin Susan M SM Vinette Kathy M B KM Gonzalez Iris L IL Abdulwahed Hind Al HA Al-Sannaa Nouriya N Funanage Vicky L VL

Molecular genetics & genomic medicine 20130530 2

Spinal muscular atrophy (SMA), the most common autosomal recessive cause of infant death, is typically diagnosed by determination of SMN1 copy number. Approximately 3-5% of patients with SMA retain at least one copy of the SMN1 gene carrying pathogenic insertions, deletions, or point mutations. We report a patient with SMA who is homozygous for two mutations carried in cis: an 8 bp duplication (c.48_55dupGGATTCCG; p.Val19fs*24) and a point mutation (c.662C>T; p.Pro221Leu). The consanguineous par ...[more]

PMID: 24498607

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Project description:ObjectivesPreprotein convertase 1/3 deficiency is a rare autosomal recessive disorder in which patients present with malabsorptive diarrhea and a series of symptoms of endocrine disorders such as polydipsia, reactive hypoglycemia, growth hormone deficiency, hypothyroidism, adrenal insufficiency, and early onset obesity. In its essence, pituitary hormone deficiency is caused by insufficient cleavage of pituitary prohormones. Here, we describe a female child with a rare double-site homozygous mutation in PCSK1 (Proprotein convertase subtilisin/kexin-type 1) gene, and thereby intend to investigate the relationship between these novel mutation sites and changes in protein synthesis and function.MethodsWe tested this patient's blood and urine fecal indicators of infection, blood electrolytes, and relevant endocrine hormone levels in the laboratory. Next Generation Sequencing was applied to screen the patient's DNA. Western Blot was performed to evaluate the mutant protein's expression. The enzymatic activity was measured as the rate of cleavage of a synthetic fluorogenic substrate in a specific solution.ResultsWe found that this patient presented shortly after birth with uncorrectable diarrhea and symptoms of metabolic acidosis with hypothyroidism. Next Generation Sequencing revealed that a rare double-site homozygous missense mutation, c.763G > A (p.G255R) and c.758C > T (p.S253L), were detected in exon 7 of PCSK1 (Proprotein convertase subtilisin/kexin-type 1) gene on chromosome 5 of the patient. Western blotting revealed that there was no significant decrease in protein synthesis levels in the mutant phenotype compared to the wild type. Compared with WT type, the proteins expressed by the mutations showed a significant decrease in the enzyme activity towards the fluorescent substrates. However, neither the single site mutation p.S253L or p.G255R, nor the double-site mutation of both, all showed no significant differences from each other.ConclusionsThese two missense mutations have not been reported before, and it is even rarer to find homozygous variation of two sites in one patient. This study identifies two novel mutations for the first time and further investigates the changes in protein synthesis and enzyme activity, providing a new pathway to continue to explore the pathogenesis of diseases associated with the function of PC1/3.

Project description:Abstract Background: Werner syndrome (WS; MIM# 277700) is a rare genetic disorder characterized by accelerated aging and predisposition to cancers. The causal gene, WRN, encodes a multifunctional nuclear protein that belongs to the RecQ family of DNA helicases. The earliest sign of WS is short stature due to the lack of pubertal growth spurt. In addition to the aged-appearance, WS patients typically develop functional declines of multiple organs including the endocrine system in their middle ages. Frequent Endocrine disorders include Type 2 Diabetes (T2D), dyslipidemia (DL), hypothyroidism, hypogonadism and osteoporosis. Most common causes of deaths are myocardial infarction and cancers in their 50s. Clinical Case: A 54-year old man was admitted for right wrist septic arthritis. His medical history was pertinent for premature cataract diagnosed in his 2nd decade. He had uncontrolled T2D (peak HbA1C level 10%) since the age of 32 years, treated with oral hypoglycemics including glimepiride, sitagliptin and metformin then shifted to insulin over the past 2 years. He also had DL since the age of 41 years, characterized by hypertriglyceridemia (766 mg/dL) with very low HDL (7 mg/dL). At age 50, he was diagnosed with hypothyroidism and negative anti-TPO antibodies. Moreover, he had refractory bilateral leg ulcers that manifested at the age of 35 years requiring multiple debridments, premature coronary artery disease at the age of 38 years and ischemic cardiomyopathy. On examination, his height was 154 cm and weight 42 kgs. He had prominent eyes, a pinched nose, grey and sparse scalp hair with absence of eyebrows and eyelashes. Hoarse voice was easily noticeable. He had very thin limbs, flat feet, central fat distribution and muscle wasting. Skin was shiny, tight, and atrophic with overlying hypermelanosis. Old bilateral hyperkeratotic deep feet ulcers were noticed with variable necrosis. Blood chemistry testing showed: HbA1C 8.2%, disturbed lipid profile, TG 341 mg/dL, HDL 17 mg/dL, LDL 155 mg/dL, and TSH 21 µU/mL (0.2 - 4.2 µU/mL). MRI of the foot revealed Achilles’ tendon calcification. His parents were 2nd degree cousins and had normal features. Three of his siblings had similar features and premature death. His diabetes was atypical due to his low BMI, high insulin requirements and the relatively early onset. WS was suspected based on the overall features of accelerated aging with characteristic deep leg ulcers and the family history. Genetic sequence analysis revealed a novel pathogenic homozygous nonsense variant mutation c.1111G>T, p.Glu371* of the WRN gene. Conclusion: Werner syndrome is a rare genetic disease, but it carries high morbidity and mortality burden, in addition to impaired quality of life. It should be highly suspected in patients with atypical T2D, DL, premature coronary disease, refractory leg ulcers with features of accelerated aging and short stature.

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A homozygous double mutation in SMN1: a complicated genetic diagnosis of SMA.

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A homozygous double mutation in SMN1: a complicated genetic diagnosis of SMA.

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