Genotype–Phenotype Associations in 72 Adults with Suspected ALPL-Associated Hypophosphatasia

Abstract

Hypophosphatasia (HPP) is a rare inborn error of metabolism due to a decreased activity of tissue nonspecific alkaline phosphatase (TNSALP). As the onset and severity of HPP are heterogenous, it can be challenging to determine the pathogenicity of detected rare ALPL variants in symptomatic patients. We aimed to characterize patients with rare ALPL variants to propose which patients can be diagnosed with adult HPP. We included 72 patients with (1) clinical symptoms of adult HPP or positive family history and (2) low TNSALP activity and/or high pyridoxal 5′-phosphate (PLP) levels, who underwent ALPL gene sequencing. The patients were analyzed and divided into three groups depending on ALPL variant pathogenicity according to the classification of the American College of Medical Genetics and Genomics (ACMG). Reported pathogenic (n = 34 patients), rare (n = 17) and common (n = 21) ALPL variants only were found. Muscular complaints were the most frequent symptoms (> 80%), followed by bone affection (> 50%). Tooth involvement was significantly more common in patients with pathogenic or rare ALPL variants. Seven rare variants could be classified as likely pathogenic (ACMG class 4) of which five have not yet been described. Inconclusive genetic findings and less specific symptoms make diagnosis difficult in cases where adult HPP is not obvious. As not every pathogenic or rare ALPL variant leads to a manifestation of HPP, only patients with bone complications and at least one additional complication concerning teeth, muscle, central nervous and mental system, repeated low TNSALP activity and high PLP levels should be diagnosed as adult HPP if rare ALPL gene variants of ACMG class 4 or higher support the diagnosis.

Biochemical, clinical and genetic characteristics in adults with persistent hypophosphatasaemia; Data from an endocrinological outpatient clinic in Denmark

“…2 ). In addition, a recently published study, presenting a large cohort of patients suspected for HPP, reported that low normal ALP activity can occur in mild symptomatic HPP patients ( Jandl et al, 2020 ). However, PLP is assumed to be a more specific and sensitive marker to detect HPP ( Riancho-Zarrabeitia et al, 2016 ).…”

Clinical, biochemical and genetic findings in adult patients with chronic hypophosphatasemia

“…This is the first study reporting about Italian hypophosphatasemic osteoporotic patients. Previous series performing genetic analysis for the detection of ALPL aberrations in adult patients with persistent hypophosphatasemia at high risk for diagnosis of genetic HPP, defined as presence of clinical symptoms of adult HPP or positive family history, detected a prevalence of pathogenic variants of 43% [20] and 47% [21] or of rare/uncertain variants in 24% of patients [21]. Other studies evaluated the prevalence of HPP in hypophosphatasemic patients detected in different clinical settings.…”

Periodontal Disease Associated with Genetic Disorders

“…At the same time, extracellular pyrophosphate induces osteopontin production and the latter inhibits formation of hydroxyapatite. The aforementioned are the main mechanisms that causes early tooth loss and abnormal bone mineralization in HPP patients [73,74].…”