Mild hypophosphatasia may be twice as prevalent as previously estimated: an effective clinical algorithm to detect undiagnosed cases.

OBJECTIVES: Since the prevalence of hypophosphatasia (HPP), a rare genetic disease, seems to be underestimated in clinical practice, in this study, a new diagnostic algorithm to identify missed cases of HPP was developed and implemented. METHODS: Analytical determinations recorded in the Clinical Analysis Unit of the Hospital Universitario Clínico San Cecilio in the period June 2018 - December 2020 were reviewed. A new clinical algorithm to detect HPP-misdiagnosed cases was used including the following steps: confirmation of persistent hypophosphatasemia, exclusion of secondary causes of hypophosphatasemia, determination of serum pyridoxal-5'-phosphate (PLP) and genetic study of ALPL gene. RESULTS: Twenty-four subjects were selected to participate in the study and genetic testing was carried out in 20 of them following clinical algorithm criteria. Eighty percent of patients was misdiagnosed with HPP following the current standard clinical practice. Extrapolating these results to the current Spanish population means that there could be up to 27,177 cases of undiagnosed HPP in Spain. In addition, we found a substantial proportion of HPP patients affected by other comorbidities, such as autoimmune diseases (∼40 %). CONCLUSIONS: This new algorithm was effective in detecting previously undiagnosed cases of HPP, which appears to be twice as prevalent as previously estimated for the European population. In the near future, our algorithm could be globally applied routinely in clinical practice to minimize the underdiagnosis of HPP. Additionally, some relevant findings, such as the high prevalence of autoimmune diseases in HPP-affected patients, should be investigated to better characterize this disorder.

Characterization of Genetic Variants of Uncertain Significance for the ALPL Gene in Patients With Adult Hypophosphatasia.

Hypophosphatasia (HPP) a rare disease caused by mutations in the ALPL gene encoding for the tissue-nonspecific alkaline phosphatase protein (TNSALP), has been identified as a potentially under-diagnosed condition worldwide which may have higher prevalence than currently established. This is largely due to the overlapping of its symptomatology with that of other more frequent pathologies. Although HPP is usually associated with deficient bone mineralization, the high genetic variability of ALPL results in high clinical heterogeneity, which makes it difficult to establish a specific HPP symptomatology. In the present study, three variants of ALPL gene with uncertain significance and no previously described (p.Del Glu23_Lys24, p.Pro292Leu and p.His379Asn) were identified in heterozygosis in patients diagnosed with HPP. These variants were characterized at phenotypic, functional and structural levels. All genetic variants showed significantly lower in vitro ALP activity than the wild-type (WT) genotype (p-value <0.001). Structurally, p.His379Asn variant resulted in the loss of two Zn2+ binding sites in the protein dimer which may greatly affect ALP activity. In summary, we identified three novel ALPL gene mutations associated with adult HPP. The correct identification and characterization of new variants and the subsequent study of their phenotype will allow the establishment of genotype-phenotype relationships that facilitate the management of the disease as well as making it possible to individualize treatment for each specific patient. This would allow the therapeutic approach to HPP to be personalized according to the unique genetic characteristics and clinical manifestations of each patient.

Hypophosphatasia: A Unique Disorder of Bone Mineralization.

Hypophosphatasia (HPP) is a rare genetic disease characterized by a decrease in the activity of tissue non-specific alkaline phosphatase (TNSALP). TNSALP is encoded by the ALPL gene, which is abundantly expressed in the skeleton, liver, kidney, and developing teeth. HPP exhibits high clinical variability largely due to the high allelic heterogeneity of the ALPL gene. HPP is characterized by multisystemic complications, although the most common clinical manifestations are those that occur in the skeleton, muscles, and teeth. These complications are mainly due to the accumulation of inorganic pyrophosphate (PPi) and pyridoxal-5'-phosphate (PLP). It has been observed that the prevalence of mild forms of the disease is more than 40 times the prevalence of severe forms. Patients with HPP present at least one mutation in the ALPL gene. However, it is known that there are other causes that lead to decreased alkaline phosphatase (ALP) levels without mutations in the ALPL gene. Although the phenotype can be correlated with the genotype in HPP, the prediction of the phenotype from the genotype cannot be made with complete certainty. The availability of a specific enzyme replacement therapy for HPP undoubtedly represents an advance in therapeutic strategy, especially in severe forms of the disease in pediatric patients.

Serum free light chains in the evaluation of the response to treatment and biological progression in primary amyloidosis / Cadenas ligeras libres séricas en la evaluación de la respuesta al tratamiento y progresión biológica en Amiloidosis Primaria

Primary amyloidosis is a rare condition characterised by the deposition of free light chains in different tissues and organs (e.g. kidney, heart, liver, gastrointestinal system). The aim of the therapy in patients with primary amyloidosis is to suppress the monoclonal plasma cells that produce the amyloidogenic free light chains and to preserve the organ function. Thus, the new criteria for the haematological disease response include the measurement of serum free light chains concentrations. The case is presented on a patient diagnosed with primary amyloidosis, where the difference between bound and free serum free light chains (dFLC) was used to evaluate the haematological response to the treatment, as well as any biological progression. In contrast to dFLC, Bence Jones Protein in urine was positive but ineffective to evaluate the response to the treatment (AU)

La amiloidosis primaria es una entidad rara caracterizada por el depósito de cadenas ligeras libres en diferentes tejidos y órganos (riñón, corazón, hígado, aparato gastrointestinal). El objetivo en la terapia de los pacientes con amiloidosis primaria consiste en suprimir las células plasmáticas monoclonales que producen las cadenas ligeras libres amiloidogénicas y preservar la función de los órganos afectados. Así, los nuevos criterios de respuesta hematológica de la enfermedad incorporan la medida de las concentraciones séricas de cadenas ligeras libres. Presentamos el caso de una paciente a quien se diagnosticó amiloidosis primaria y en la cual la diferencia de concentración en suero entre la cadena ligera libre monoclonal implicada y la no implicada (dFLC) nos permitió evaluar la respuesta hematológica al tratamiento y la presencia de progresión biológica. En contraste a la dFLC, la proteinuria de Bence Jones fue positiva pero ineficaz en la evaluación de la respuesta al tratamiento (AU)