Utility of genetic testing for prenatal presentations of hypophosphatasia

Disease burden by ALPL variant number in patients with non-life-threatening hypophosphatasia in the Global HPP Registry

BACKGROUND

Hypophosphatasia (HPP) is a rare metabolic disease caused by autosomal dominant or recessive inheritance of ALPL variants resulting in low alkaline phosphatase activity. The objective of this analysis was to compare HPP disease burden between patients with non-life-threatening disease in the Global HPP Registry who have one ALPL variant versus two or more ALPL variants.

METHODS

Patients were included if they had one or more ALPL variants identified through genetic testing and first HPP manifestations after 6 months of age. Assessments included history of HPP manifestations, Brief Pain Inventory-Short Form (BPI-SF), Health Assessment Questionnaire-Disability Index (HAQ-DI), 6-Min Walk Test (6MWT), Paediatric Quality of Life Inventory (PedsQL) and 36-Item Short-Form Survey V.2 (SF-36v2).

RESULTS

Of 685 included patients, 568 (82.9%) had one ALPL variant, 116 (16.9%) had two variants, and one (0.1%) had three variants. Patients with two or more ALPL variants had higher proportions of skeletal (52.1% vs 32.6%), dental (73.5% vs 56.0%), muscular (36.8% vs 23.6%) and neurological (22.2% vs 8.8%) manifestations at last assessment. BPI-SF, HAQ-DI, PedsQL and SF-36v2 scores were similar between groups. Distances walked on the 6MWT were similar between groups for children. Distance walked was lower among adults with two or more variants (293 m (n=8)) than adults with one variant (466 m (n=103)), although the former group was very small.

CONCLUSION

HPP disease burden is high in patients with HPP, regardless of ALPL variant number. While prevalence of HPP-specific manifestations was higher in patients with two or more variants than those with one variant, patient-reported outcomes were similar between groups.

TRIAL REGISTRATION NUMBER

NCT02306720; EUPAS13514.

The diagnosis of hypophosphatasia in children as a multidisciplinary effort: an expert opinion

Hypophosphatasia (HPP) is a rare genetic disorder in which pathogenic variants of the ALPL gene lead to a marked decrease of tissue non-specific alkaline phosphatase (TNSALP) activity. Although HPP is a systemic disorder, its clinical manifestations are more evident on bones, teeth, muscle and central nervous system. The clinical spectrum ranges from severe forms with extreme skeletal deformities, respiratory impairment, seizures, to very mild forms with onset in late adulthood and few clinical signs. The diagnosis can be suspected by measurement of TNSALP activity, but the insufficient awareness among health professionals and the lack of official guidelines are responsible for delayed diagnosis in children with HPP. The purpose of the current document is to provide an expert opinion directed at optimizing the diagnostic pathway of pediatric HPP. From April to December 2022, a multidisciplinary working group of 6 experts including two pediatric endocrinologists, a pediatric neurologist, a pediatric odontologist, a clinical geneticist, and a molecular biologist gathered in a series of periodic meetings to discuss the main issues related to the diagnosis of HPP in children and formalize an Expert Opinion statement. The experts agreed on a diagnostic trail that begins with the recognition of specific clinical signs, leading to biochemical analyses of TNSALP activity and vitamin B6 serum concentration. Very important are the neurological and dental manifestation of the disease that should be thoroughly investigated. The evaluation of TNSALP activity must consider sex and age variability and low activity must be persistent. Repeated blood measurements are thus necessary. The molecular analysis is then mandatory to confirm the diagnosis and for genetic counseling.

Hypophosphatasia

Hypophosphatasia (HPP) is an inherited metabolic disease caused by loss-of-function mutations in the tissue non-specific alkaline phosphatase (TNAP) gene. Reduced activity of TNAP leads to the accumulation of its substrates, mainly inorganic pyrophosphate and pyridoxal-5′-phosphate, metabolic aberrations that largely explain the musculoskeletal and systemic features of the disease. More than 400 ALPL mutations, mostly missense, are reported to date, transmitted by either autosomal dominant or recessive mode. Severe disease is rare, with incidence ranging from 1:100,000 to 1:300,000 live births, while the estimated prevalence of the less severe adult form is estimated to be between 1:3100 to 1:508, in different countries in Europe. Presentation largely varies, ranging from death in utero to asymptomatic adults. In infants and children, clinical features include skeletal, respiratory and neurologic complications, while recurrent, poorly healing fractures, muscle weakness and arthropathy are common in adults. Persistently low serum alkaline phosphatase is the cardinal biochemical feature of the disease. Management requires a dedicated multidisciplinary team. In mild cases, treatment is usually symptomatic. Severe cases, with life-threating or debilitating complications, can be successfully treated with enzyme replacement therapy with asfotase alfa.