Molecular-genetic causes for the high frequency of phenylketonuria in the population from the North Caucasus

Mutation Analysis of PAH Gene in Phenylketonuria Patients from the North of Iran: Identification of Three Novel Pathogenic Variants

Background

There are more than 1100 different pathogenic variants in the phenylalanine hydroxylase (PAH) gene that are responsible for phenylketonuria (PKU) diseases, and the spectrum of these mutations varies in different ethnic groups. The aim of the present study was to identify the frequency of pathogenic variants in all 13 exons of the PAH gene among patients with PKU in Mazandaran and Golestan provinces in the north of Iran.

Methods

Forty unrelated PKU patients from Mazandaran and Golestan provinces were enrolled in the study. Genomic DNA was extracted from leukocytes using a Qiagen DNA extraction kit and polymerase chain reaction - restriction fragment length polymorphism (PCR-RFLP), and Sanger sequencing methods were applied to detect the variants. In the case of new variants, the InterVar online tool (PMID: 28132688) was used to classify the variants.

Results

Twenty-one different pathogenic variants were observed among the 40 investigated patients. The c.106611G>A variant had the highest frequency (27.5%) in the region, and the c.168+5G>C, c.473G>A, and c.782 G>A variants were the other most frequent mutations with allelic frequencies of 7.5, 5, and 5%, respectively. Three novel pathogenic variants including c.773T>G, c.878 T>C, and c. 1245del variants were observed among the investigated patients.

Conclusions

The introduction of pathogenic variants in the PAH gene in each ethnic group provides valuable data regarding the understanding of the pathogenesis of the disease and can be helpful for prenatal diagnosis programs.

Increasing Diversity, Equity, Inclusion, and Accessibility in Rare Disease Clinical Trials

Diversity, equity, inclusion, and accessibility (DEIA) are foundational principles for clinical trials and medical research. In rare diseases clinical research, where numbers of participants are already challenged by rarity itself, maximizing inclusion is of particular importance to clinical trial success, as well as ensuring the generalizability and relevance of the trial results to the people affected by these diseases. In this article, we review the medical and gray literature and cite case examples to provide insights into how DEIA can be proactively integrated into rare diseases clinical research. Here, we particularly focus on genetic diversity. While the rare diseases DEIA literature is nascent, it is accelerating as many patient advocacy groups, professional societies, training and educational organizations, researcher groups, and funders are setting intentional strategies to attain DEIA goals moving forward, and to establish metrics to ensure continued improvement. Successful examples in underserved and underrepresented populations are available that can serve as case studies upon which rare diseases clinical research programs can be built. Rare diseases have historically been innovation drivers in basic, translational, and clinical research, and ultimately, all populations benefit from data diversity in rare diseases populations that deliver novel insights and approaches to how clinical research can be performed.

Genetic Landscape and Clinical Features of Hyperphenylalaninemia in North Ossetia-Alania: High Frequency of P281L and P211T Genetic Variants in the PAH Gene

This study, conducted in the Republic of North Ossetia-Alania (RNOA), aimed to explore the genetic landscape of hyperphenylalaninemia (HPA) and phenylketonuria (PKU) in the Ossetian population using data from newborn screening (NBS). Through comprehensive molecular genetic analysis of 29 patients with HPA from diverse ethnic backgrounds, two major genetic variants in the PAH gene, P281L and P211T, were identified, constituting 50% of all detected pathogenic alleles in Ossetian patients. Remarkably, these variants exhibited an exceptionally high frequency in the Ossetian population, surpassing global prevalence rates. This study unveiled a notable prevalence of mild forms of HPA (78%), underscoring the importance of genetic counseling for carriers of pathogenic variants in the PAH gene. Moreover, the findings emphasized the necessity for ongoing monitoring of patients with mild forms, as they may lack significant symptoms for diagnosis, potentially impacting offspring. Overall, this research offers valuable insights into the genetic landscape of HPA and PKU in the Ossetian population.

Step-by-Step Double-Trouble OBAIRH and DMD Diagnosis in a One-Year-Old Boy

We present a case of a combination of two rare hereditary disorders: obesity, adrenal insufficiency and red hair syndrome (OBAIRH) and Duchenne muscular dystrophy (DMD) in a boy. Both diseases were diagnosed during the first year of life. OBAIRH was suggested based on the ethnicity and family history of the patient, while DMD was based on an extreme increase in transaminase and CK (creatine kinase) levels during a biochemical analysis of his blood. The OBAIRH syndrome was caused by a pathogenic homozygous variant in the regulatory region of the POMC gene (NM_001035256.3): c.-71+1G>A, while DMD was caused by the de novo deletion of exons 38-45 of the DMD (NM_004006.3) gene (NC_000023.10:g.(?_32380941)(31950285_?)del).

Identification of Variants Underlying Phenylalanine Hydroxylase Deficiency in Saudi Arabia

Background: Deleterious mutations in the human gene phenylalanine hydroxylase (PAH) encoding the phenylalanine hydroxylase enzyme give rise to classic phenylketonuria and hyperphenylalaninemia. Our study was designed to characterize the spectrum of variants in the PAH gene in Saudi patients. Materials and Methods: We screened a cohort of 72 Saudi patients with clinical and biochemical diagnoses of hyperphenylalaninemia at the largest tertiary care center in Saudi Arabia; the King Faisal Specialist Hospital and Research Center (KFSH&RC), Riyadh. All patient's charts were reviewed under an approved study by Institutional Review Board. Results: Twenty-one different PAH variants were identified among the 144 PAH alleles assessed by targeted gene sequencing. Within the studied cohort, 60 of 72 patients had homozygous mutations with the the remaining 12 being compound heterozygotes. The most prevalent of the disease alleles identified in this study was the p.(Arg252Trp) mutation, which accounted for 38 of 144 alleles (26.4%). With the high incidence of genetic disorders in the population, religiously permissible preventive reproductive measures are a priority in our practice. Prenatal diagnoses carried out on four fetuses revealed two that were homozygous for PAH pathogenic variants. In addition, pre-implantation genetic diagnoses were initiated for 19 families. Eight of these families completed more than one full cycle of treatment, from which one healthy newborn was delivered. Conclusions: This study describes the spectrum of PAH variants in the Saudi population and highlights the molecular heterogeneity underlying phenylketonuria and hyperphenylalaninemia. These results add to the existing knowledge about PAH variants in Middle Eastern Countries. These results can be further translated to provide: informed counseling; cascade carrier testing in extended family members; and pre-marital screening.

The spectrum of phenylalanine hydroxylase variants and genotype-phenotype correlation in phenylketonuria patients in Gansu, China

BACKGROUND

Phenylketonuria (PKU) is a common, congenital, autosomal recessive, metabolic disorder caused by Phenylalanine hydroxylase (PAH) variants.

METHODS

967 PKU patients from Gansu, China were genotyped by Sanger sequencing, multiplex ligation-dependent probe amplification, and whole exome sequencing. We analyzed the variants of PAH exons, their flanking sequences, and introns.

RESULTS

The detection of deep intronic variants in PAH gene can significantly improve the genetic diagnostic rate of PKU. The distribution of PAH variants among PKU subtypes may be related to the unique genetic background in Gansu, China.

CONCLUSION

The identification of PAH hotspot variants will aid the development of large-scale neonatal genetic screening for PKU. The five new PAH variants found in this study further expand the spectrum of PAH variants. Genotype-phenotype correlation analysis may help predict the prognosis of PKU patients and enable precise treatment regimens to be developed.

Epidemiology of Rare Hereditary Diseases in the European Part of Russia: Point and Cumulative Prevalence

The issue of point prevalence, cumulative prevalence (CP), and burden of rare hereditary diseases (RHD), comprising 72–80% of the group of rare diseases, is discussed in many reports and is an urgent problem, which is associated with the rapid progress of genetic technology, the identification of thousands of genes, and the resulting problems in society. This work provides an epidemiological analysis of the groups of the most common RHDs (autosomal dominant, autosomal recessive, and X-linked) and their point prevalence (PP) and describes the structure of RHD diversity by medical areas in 14 spatially remote populations of the European part of Russia. The total size of the examined population is about 4 million. A total of 554 clinical forms of RHDs in 10,265 patients were diagnosed. The CP for all RHDs per sample examined was 277.21/100,000 (1:361 people). It is worth noting that now is the time for characterizing the accumulated data on the point prevalence of RHDs, which will help to systematize our knowledge and allow us to develop a strategy of care for patients with RHDs. However, it is necessary to address the issues of changing current medical classifications and coding systems for nosological forms of RHDs, which have not kept pace with genetic advances.

Genetic etiology of hearing loss in Russia

Prevalence and locus/allelic heterogeneity of the hereditary hearing loss (HL) vary significantly in different human populations. Investigation of the hereditary HL diversity and the evaluation of the factors determining the region-specific landscapes of genetic HL are important for local healthcare and medical genetic services. This review presents the summarized data from the published studies concerning the genetic etiology of HL in different populations of Russia. Multiethnic population of Russia (in total, about 146 million on 2021) includes over 180 different ethnic groups, the number of which varies from millions to just several thousand people. Among them, Russians are the largest group (about 111 million). The contribution of GJB2 gene in the HL etiology in patients of different ethnicities and ethnic-specific prevalence of the GJB2 pathogenic variants were studied in many local populations of Russia. However, the investigation of other "deafness" genes is still limited to a relatively small number of studies on patients with HL of unsolved etiology.

Phenylketonuria

Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.

Comparison of phenylalanine tolerance in singleton and twin pregnancies in patients with phenylketonuria

Objectives

Empirical determination of phenylalanine (Phe) tolerance in patients with phenylketonuria (PKU) relies on frequent assessment of blood Phe concentrations in relation to Phe intake from detailed meal records. This study aimed to determine Phe tolerance in twin pregnancies.

Methods

The reviewed cases included three women with PKU who each had a singleton and twin pregnancy (i.e., they were pregnant twice). All patients were under regular supervision to maintain Phe concentrations in a steady state and determine safe Phe intake. Restriction of Phe in the patient’s diet was determined depending on the amount of Phe intake, which allowed for stable blood Phe concentrations within the target range.

Results

In all three patients with PKU, the ratio of Phe tolerance during the course of the twin and singleton pregnancies was <1 for most of the pregnancy. The ratio of the increase in Phe tolerance between 29 and 34 weeks of gestation and that between 15 and 28 weeks of gestation was 0.66 and 1.17, 0.51 and 0.14, and 0.76 and 1.42 in the twin and singleton pairs of pregnancies, respectively.

Conclusions

Our study shows that Phe tolerance in a twin pregnancy is not greater than that in a singleton pregnancy.

A new approach to estimating the prevalence of hereditary hearing loss: An analysis of the distribution of sign language users based on census data in Russia

The absence of comparable epidemiological data challenges the correct estimation of the prevalence of congenital hearing loss (HL) around the world. Sign language (SL) is known as the main type of communication of deaf people. We suggest that the distribution of SL can be interpreted as an indirect indicator of the prevalence of congenital HL. Since a significant part of congenital HL is due to genetic causes, an assessment of the distribution of SL users can reveal regions with an extensive accumulation of hereditary HL. For the first time, we analyzed the data on the distribution of SL users that became available for the total population of Russia by the 2010 census. Seventy-three out of 85 federal regions of Russia were ranked into three groups by the 25th and 75th percentiles of the proportion of SL users: 14 regions-"low proportion"; 48 regions-"average proportion"; and 11 regions-"high proportion". We consider that the observed uneven prevalence of SL users can reflect underlying hereditary forms of congenital HL accumulated in certain populations by specific genetic background and population structure. At least, the data from this study indicate that the highest proportions of SL users detected in some Siberian regions are consistent with the reported accumulation of specific hereditary HL forms in indigenous Yakut, Tuvinian and Altaian populations.

Variants of the phenylalanine hydroxylase gene in neonates with phenylketonuria in Hainan, China

OBJECTIVE

To investigate the incidence of phenylalanine hydroxylase (PAH) deficiency and PAH genotypes in neonates in Hainan, China. Methods: We performed heal stick to collect blood and obtain dry blood spot specimens from newborns in Hainan from January 2007 to December 2016. Phenylalanine (Phe) concentration in these dry blood spots was measured by the fluorescence method to screen phenylketonuria (PKU). For suspicious samples, the genotypes of the PAH gene were amplified by biotin labeled oligonucleotide primers. Polymerase chain reaction (PCR) products were then analyzed by flow-through hybridization to detect genotypes. At the same time, peripheral blood samples of children suspicious of PKU and their parents were used to perform gene sequencing. Results: Of the 914,520 newborns screened, 29 of them had PAH deficiency. The incidence of PAH deficiency in Hainan was 3.17/100,000. A total of 58 mutant alleles belonging to 15 different types were identified in the 29 patients. In terms of genotypes frequency, the top 4 were: c.611A > G 20.7% (12/58) , c.728G > A 17.2%, c.158G > A 15.2% (9/58) and c.721C > T 13.8% (8/58). The frequencies of other genotypes were all below 10%. Conclusion: The incidence of PAH deficiency in Hainan is relatively high among all provinces in southern China. With a total frequency of 67.2%, c.611A > G, c.728G > A, c.158G > A and c.721C > T, and are the most common PAH gene genotypes.

Molecular genetics of phenylketonuria and tetrahydrobiopterin deficiency in Jordan

BACKGROUND

Information regarding the prevalence of PKU in the Middle East in comparison to other world regions is scarce, which might be explained by difficulties in the implementation of national newborn screening programs.

OBJECTIVE

This study seeks for the first time to genotype and biochemically characterize patients diagnosed with hyperphenylalaninemia (HPA) at the Pediatric Metabolic Genetics Clinic at the King Hussein Medical Center, Amman, Jordan.

METHODS

A total of 33 patients with HPA and 55 family members were investigated for pterins (neopterin and biopterin) and dihydropteridine reductase (DHPR) activity in dried blood spots. Patients with HPA were genotyped for phenylketonuria (PKU) and the genes involved in tetrahydrobiopterin (BH4) metabolism.

RESULTS

In total 20 patients were diagnosed with PKU due to phenylalanine hydroxylase (PAH) deficiency, 2 with GTP cyclohydrolase I (GTPCH) deficiency, 6 with DHPR deficiency, and 3 with the 6-pyruvoyl-tetrahydropterin synthase (PTPS) deficiency. Diagnosis was not possible in 2 patients. This study documents a high percentage of BH4 deficiencies within HPA patients. With one exception, all patients were homozygous for particular gene variants.

CONCLUSIONS

This approach enables differentiation between PKU and BH4 deficiencies and, thus, allows for critical selection of a specific treatment strategies.

The Genetic Landscape and Epidemiology of Phenylketonuria

Phenylketonuria (PKU), caused by variants in the phenylalanine hydroxylase (PAH) gene, is the most common autosomal-recessive Mendelian phenotype of amino acid metabolism. We estimated that globally 0.45 million individuals have PKU, with global prevalence 1:23,930 live births (range 1:4,500 [Italy]-1:125,000 [Japan]). Comparing genotypes and metabolic phenotypes from 16,092 affected subjects revealed differences in disease severity in 51 countries from 17 world regions, with the global phenotype distribution of 62% classic PKU, 22% mild PKU, and 16% mild hyperphenylalaninemia. A gradient in genotype and phenotype distribution exists across Europe, from classic PKU in the east to mild PKU in the southwest and mild hyperphenylalaninemia in the south. The c.1241A>G (p.Tyr414Cys)-associated genotype can be traced from Northern to Western Europe, from Sweden via Norway, to Denmark, to the Netherlands. The frequency of classic PKU increases from Europe (56%) via Middle East (71%) to Australia (80%). Of 758 PAH variants, c.1222C>T (p.Arg408Trp) (22.2%), c.1066-11G>A (IVS10-11G>A) (6.4%), and c.782G>A (p.Arg261Gln) (5.5%) were most common and responsible for two prevalent genotypes: p.[Arg408Trp];[Arg408Trp] (11.4%) and c.[1066-11G>A];[1066-11G>A] (2.6%). Most genotypes (73%) were compound heterozygous, 27% were homozygous, and 55% of 3,659 different genotypes occurred in only a single individual. PAH variants were scored using an allelic phenotype value and correlated with pre-treatment blood phenylalanine concentrations (n = 6,115) and tetrahydrobiopterin loading test results (n = 4,381), enabling prediction of both a genotype-based phenotype (88%) and tetrahydrobiopterin responsiveness (83%). This study shows that large genotype databases enable accurate phenotype prediction, allowing appropriate targeting of therapies to optimize clinical outcome.

Epidemiology of Hereditary Diseases in the Karachay-Cherkess Republic

Prevalence and allelic heterogeneity of hereditary diseases (HDs) could vary significantly in different human populations. Current knowledge of HDs distribution in populations is generally limited to either European data or analyses of isolated populations which were performed several decades ago. Thus, an acknowledgement of the HDs prevalence in different modern open populations is important. The study presents the results of a genetic epidemiological study of hereditary diseases (HDs) in the population of the Karachay-Cherkess Republic (KChR). Clinical screening of a population of 410,367 people for the identification of HDs was conducted. The population surveyed is represented by five major ethnic groups-Karachays, Russians, Circassians, Abazins, Nogais. The study of the populations was carried out in accordance with the proprietary protocol of genetic epidemiological examination designed to identify >3500 HDs easily diagnosed during clinical examination by qualified specialists specializing in the HDs. The protocol consists of the population genetic and medical genetic sections and is intended for comprehensive population analysis based on the data on different genetic systems, including the genes of HDs, DNA polymorphisms, demographic data collected during hospital-based survey. 8950 families (with 10,125 patients) with presumably the HDs were initially identified as a result of the survey and data collection through various sources of registration (from 1156 medical workers from 163 medical institutions). A diagnosis of hereditary pathology was established in 1849 patients (from 1295 families). Two hundred and thirty nosological forms were revealed (in 1857 patients from 1295 families). The total prevalence of HDs was 1:221. Differences between populations and ethnic groups were identified: 1:350 in Russians, 1:195 in Karachays, 1:199 in Circassians, 1:218 in Abazins, 1:135 in Nogais. Frequent diseases were determined, the presence of marked genetic heterogeneity was identified during the confirmatory DNA diagnosis. To explain the reasons for the differentiation of populations by load of HD, a correlation analysis was carried out between the FST (random inbreeding) in populations and HDs load values. This analysis showed genetic drift is probably one of the leading factors determining the differentiation of KChR populations by HDs load. For the first time, the size of the load and spectrum of HDs in the populations of the KChR are determined. We have demonstrated genetic drift to be one of the main factors of the population dynamics in studied population. A significant genetic heterogeneity of HDs, both allelic and locus, was revealed in KChR.

Expanded Newborn Screening for Inborn Errors of Metabolism by Tandem Mass Spectrometry in Suzhou, China: Disease Spectrum, Prevalence, Genetic Characteristics in a Chinese Population

Expanded newborn screening for inborn errors of metabolism (IEMs) by tandem mass spectrometry (MS/MS) could simultaneously analyze more than 40 metabolites and identify about 50 kinds of IEMs. Next generation sequencing (NGS) targeting hundreds of IMEs-associated genes as a follow-up test in expanded newborn screening has been used for genetic analysis of patients. The spectrum, prevalence, and genetic characteristic of IEMs vary dramatically in different populations. To determine the spectrum, prevalence, and gene mutations of IEMs in newborns in Suzhou, China, 401,660 newborns were screened by MS/MS and 138 patients were referred to genetic analysis by NGS. The spectrum of 22 IEMs were observed in Suzhou population of newborns, and the overall incidence (excluding short chain acyl-CoA dehydrogenase deficiency (SCADD) and 3-Methylcrotonyl-CoA carboxylase deficiency (3-MCCD)) was 1/3,163. The prevalence of each IEM ranged from 1/401,660 to 1/19,128, while phenylketonuria (PKU) (1/19,128) and Mild hyperphenylalaninemia (M-HPA) (1/19,128) were the most common IEMs, followed by primary carnitine uptake defect (PCUD) (1/26,777), SCADD (1/28,690), hypermethioninemia (H-MET) (1/30,893), 3-MCCD (1/33,412) and methylmalonic acidemia (MMA) (1/40,166). Moreover, 89 reported mutations and 51 novel mutations in 25 IMEs-associated genes were detected in 138 patients with one of 22 IEMs. Some hotspot mutations were observed for ten IEMs, including PAH gene c.728G > A, c.611A > G, and c.721C > T for Phenylketonuria, PAH gene c.158G > A, c.1238G > C, c.728G > A, and c.1315+6T > A for M-HPA, SLC22A5 gene c.1400C > G, c.51C > G, and c.760C > T for PCUD, ACADS gene c.1031A > G, c.164C > T, and c.1130C > T for SCAD deficiency, MAT1A gene c.791G > A for H-MET, MCCC1 gene c.639+2T > A and c.863A > G for 3-MCCD, MMUT gene c.1663G > A for MMA, SLC25A13 gene c.IVS16ins3Kb and c.852_855delTATG for cittrullinemia II, PTS gene c.259C > T and c.166G > A for Tetrahydrobiopterin deficiency, and ACAD8 gene c.1000C > T and c.286C > A for Isobutyryl coa dehydrogenase deficiency. All these hotspot mutations were reported to be pathogenic or likely pathogenic, except a novel mutation of ACAD8 gene c.286C > A. These mutational hotspots could be potential candidates for gene screening and these novel mutations expanded the mutational spectrum of IEMs. Therefore, our findings could be of value for genetic counseling and genetic diagnosis of IEMs.