Metabolic Diseases Glossary

AAXO

Type of chromosomal aberration involving the loss of an X chromosome.

Adenine

Purine base of DNA and RNA. Pairs with thymine in double-stranded DNA sequences, with adenine (in DNA) and with uracil (in RNA) in DNA-RNA duplexes.

AMNIOCENTESIS

Procedure used to diagnose fetal chromosome abnormalities. Involves taking a sample of amniotic fluid to collect fetal cells.

ANEUPLOIDY

Alteration in the number of one or more chromosomes, for example, a trisomy.

Array CGH

Comparative genomic hybridisation. Cutting-edge genetic testing technique for diagnosing chromosomal anomalies with a higher resolution than conventional techniques (conventional cytogenetic analysis that studies the structure, function and behaviour of chromosomes). A fast and accurate tool for studying the human genome, this test can detect DNA alterations, including microdeletions and duplications.

AUTOSOMAL DOMINANT

Classical or Mendelian pattern of inheritance characterised by genetic dominance in an allele of a gene located on an autosome. Through this mechanism, a certain heritable characteristic is transmitted, which can be predicted regardless of the sex of the offspring. Furthermore, the offspring only has to receive the gene from one of its parents in order for this heritable characteristic to be expressed. In an autosomal dominant disease, the individual only has to receive the abnormal allele from one parent in order to inherit the disease.

AUTOSOMAL RECESSIVE

Classical or Mendelian inheritance pattern characterised by the absence of genetic dominance. The phenotype that characterises the recessive allele is encoded by a gene located on one of the autosomes. Through this mechanism, a certain heritable characteristic is transmitted, which can be predicted regardless of the sex of the offspring. In order for this heritable characteristic to be expressed, the offspring has to receive the mutated allele or gene from both parents.

ANAPHASE

Stage of cell division where newly-copied chromosomes move to opposite ends of the cell.

Autosome

Somatic chromosome; any chromosome that is not a sex chromosome. In humans, chromosome pairs 1 to 22 are autosomes, and pair 23 corresponds to the X and Y sex chromosomes.

ALLELE

Alternative forms of a gene at the same locus.

Amniotic fluid

Liquid fluid that surrounds and cushions the embryo and later the fetus inside the amniotic sac (the bag where the fetus develops until birth). Protects and hydraulically supports the fetus during the gestation process.

BIOMARKER

Fraction of DNA that indicates a differential characteristic between two individuals for screening, or a DNA sequence that causes a particular disease or is associated with susceptibility to a disease.

Beta HCG

Free beta-subunit of human chorionic gonadotropin; glycoprotein hormone produced in pregnancy by the developing embryo shortly after conception and later by the syncytiotrophoblast (part of the placenta). Its function is to prevent the disintegration of the corpus luteum in the ovary and thus maintain the production of progesterone, which is essential for pregnancy in humans. Levels can be measured initially through a blood test about 10-11 days after conception, and approximately 12-14 days later through a urine test.

BENIGN VARIANT

Variant that has been shown to have no functional effect.

CONGENITAL DISEASE

Disease that manifests before birth, either caused by a disorder during embryonic development or resulting from an inherited defect.

CHROMOSOMAL DISEASE

Refers to alterations in the number of chromosomes or even chromosomal translocations.

CYTOGENETICS

Study of inheritance by applying cytological and genetic techniques.

CYTOSINE

Pyrimidine base found in DNA and RNA. It is linked to guanine by three hydrogen bridge bonds in double-stranded sequences.

COVERAGE

Depth in DNA sequencing; the number of unique reads that include a given nucleotide in the reconstructed sequence. Deep sequencing generally refers to aiming for a high number of unique reads for each region of a sequence.

Codon

Trinucleotide sequence of DNA or mRNA that represents a specific amino acid in the genetic code and is translated into its corresponding amino acid in the translation process. Some codons do not represent amino acids and function as translation termination signals.

CERVICAL CONIZATION

Surgical procedure to obtain a sample of abnormal cervical tissue for further analysis in order to diagnose or treat precancerous changes in the cervix.

CHROMOSOMAL ABERRATION

Type of mutation associated with large chromosomal changes.

CHORIONIC VILLUS SAMPLING

Invasive prenatal diagnostic procedure that involves the removal of a small amount of tissue from the chorionic villi from the chorionic sac, in order to determine if the baby has a genetic condition or a chromosomal abnormality.

CONGENITAL HEART DISEASES

Structural and/or functional heart defects present from birth.

CFTS

Combined First Trimester Screening (Triple Screening); technique used to detect chromosome abnormalities, involving an ultrasound scan and blood tests between the ten and twelfth weeks of pregnancy.

CNV

Copy number variation; genomic DNA alteration that results in the loss or gain of genetic material, compared to a "normal" or "healthy" individual.

CHROMOSOME

Structure found in the nucleus of cells, formed of DNA and proteins. This is where the genes containing all the genetic information of the cell and its hereditary characteristics are located.

CLINICAL VALIDITY

Extent to which proposals about the investigated hypotheses or theories are justified by the results of the study.

CHROMOSOME ABNORMALITY

Type of genetic alteration that affects part or all of a chromosome and can alter the expression of hundreds or thousands of genes simultaneously. Down's syndrome is an example.

CARRIER

Individual that has inherited a mutated genetic trait of a disease, but does not show any symptoms. For example, haemophilia: a recessive X-linked genetic disorder that does not show any symptoms in the carrier.

CHROMOSOMAL POLYMORPHISM

DNA sequence with two or more variants in the population.

CLINICAL GENETICS

Application of genetic knowledge in the clinical world. Its objectives include genotype-phenotype association (search for genetic associations), studying disorder heritability and genetic counselling.

CLINICAL EXOME

Information on more than 6,400 genes, all of which are included in the OMIM. Therefore, if any changes are found, we know that we will be able to retrieve the gene description.

CHROMATIN

Substance used to create chromosomes, made up of DNA, RNA and various protein molecules, located in the nucleus of each human cell.

DNA

Deoxyribonucleic acid. Molecule containing genetic instructions for all known living organisms and some viruses to be able to develop and function. Has a double helix structure where genetic information is encoded by a nucleotide sequence (guanine, adenine, thymine and cytosine), represented by the letters G, A, T and C, respectively.

DELETION

Type of mutation that occurs in point mutations (loss of a base) and also due to chromosomal aberration (loss of chromosome material).

Down’s Syndrome

Clinical condition in humans characterized by various phenotypic alterations. Genotypically affected individuals carry a trisomy for chromosome 21 (2n + 1). A low percentage of these individuals carry a translocation between one of the group 13 chromosomes with chromosome 21 and, therefore, there is extra information for chromosome 21.

DE NOVO MUTATION

Genetic alteration that appears for the first time in the family.

DNA METHYLATION

Addition of methyl groups to the DNA, usually associated with gene inactivation in chromosomes.

Edwards syndrome

Aneuploid abnormality that affects the human population at a frequency of 1 / 10,000 at birth and is related to a trisomy of chromosome 18.

EXPRESSIVITY

The degree of variability in a gene's phenotypic manifestation.

Exome

Coding segments of genes that are part of messenger RNA, which in turn results in protein synthesis. It is the most important functional part of the human genome and contributes the most to the final phenotype of an organism.

Exon

DNA sequence that is transcribed and part of the functional molecule.

EPIGENETICS

Study of heritable changes in gene function that occur without alterations to the DNA sequence and cannot be expressed, i.e. phenomena that do not affect the DNA sequence but do change gene expression.

FALSE POSITIVE RATE

Proportion of negative cases that the test detects as positive.

FOETAL DNA RATIO

The amount of fetal (placental) DNA present in the mother's blood.

FN

False negative; obtaining negative results with the technique being validated, which are positive with another technique, diagnosis or positive reference material.

FP

False positive; obtaining positive results with the technique being validated, which are negative with another technique, diagnosis or negative reference material.

FOETAL FRACTION

Amount of free (circulating-free) fetal DNA found in a given maternal blood sample, divided by the total free DNA present (free fetal DNA plus free maternal DNA). It is of particular clinical interest, as it affects how well the fetal genetic material can be detected.

GENETIC DIAGNOSIS

Lets us know the genetic basis of a hereditary disease, essential information for making decisions regarding possible treatment options or for monitoring its development and prognosis. This genetic diagnosis can provide us with the necessary tools to help prevent the transmission of the disease and to consider extending it to other family members who may be affected or may be carriers of the condition.

GENETIC COUNSELLING

Communication process of informing, educating and supporting individuals and families with or at risk of a genetic disease. It is part of the clinical genetics consultation and can be provided by a clinical geneticist or genetic counsellor.

Genomic library

Total genome cloned in a set of vectors.

Genome

Complete set of genetic material in a cell or organism, including both nuclear and cytoplasmic genetic material.

Genotype

Genetic constitution of an organism. Generally not observable in the living organism.

GENETIC MUTATION

Refers to any alteration that changes the genetic code. Usually refers to the change of one or a few nucleotides within a gene.

GENETIC MOSAIC

Also known as mosaicism; genetic alteration where two or more cell populations with different genotypes, originating from the same zygote, coexist in the same individual. For example, tumour cells.

Genetics

Scientific discipline that studies the meaning, properties and function of genetic material.

Genomics

Area of knowledge concerned with deciphering the structure and function of genomes as a whole. It requires high-throughput methods and makes intensive use of bioinformatics.

Guanine

Purine base found in DNA and RNA. It is linked to cytosine by three hydrogen bridge bonds in double-stranded sequences.

Gene

Sequence of nucleotides coding for a polypeptide or RNA strand that performs a function in the organism.

GENE EXPRESSION

Refers to the processes of DNA replication, transcription and translation.

GENETIC TEST

Test to identify the mutation (error) in the gene responsible for the hereditary disease.

HISTONE ACETYLATION

Addition of an acetyl group (three-carbon molecule) to a lysine residue at one end of a histone molecule.

Histone

Proteins found in complexes with nuclear RNA, rich in basic amino acids (lysine and arginine) and involved in DNA coiling to form nucleosomes.

HETEROZYGOUS

Individual with two different alleles for a given characteristic.

HOMOZYGOTIC

Individual with two identical alleles of a gene.

Intron

DNA segment within a eukaryotic gene, which is transcribed but removed from the primary transcript by post-transcriptional modification (splicing). However, as a result of alternative splicing, a particular intron may be part of the informational sequence of functional mRNA. This has also been described for some prokaryotic genes.

INHERITED MUTATION

Genetic alteration that comes from one or both parents (the parent may or may not be affected).

INCIDENTAL FINDING

Finding of a pathogenic (or likely pathogenic) and clinically relevant variant in a gene unrelated to the syndrome or condition for which the patient was referred.

KARYOTYPE

Chromosome complement of a cell made up of metaphase chromosomes arranged in homologous pairs according to length and centromere position.

Klinefelter syndrome

Chromosomal aberration affecting the male X heterochromosome in humans. Male carriers of this aberration have an extra X chromosome. Phenotypically they are characterized by being sterile and occasionally have mental retardation.

Kilobase

Unit designating the length of nucleic acids, corresponding to 1000 nucleotides. It is abbreviated as kb for single-stranded nucleic acids and kbp (kilobase pair) for double-stranded nucleic acids.

LIKELY BENIGN VARIANT

When the nucleotide alteration has a greater than 90% likelihood of not being pathogenic.

LIKELY PATHOGENIC VARIANT

When the nucleotide alteration has a greater than 90% likelihood of being pathogenic.

Locus

Chromosomal location of a given gene (the locus is the same for two identical chromosomes). For example, chromosomal locus 22q11.

MOLECULAR DIAGNOSTICS

Set of biological techniques used to identify and analyse biomarkers in the genome and proteome (the genetic code and how these genes are expressed as proteins), making it possible to diagnose and monitor diseases, detect risk and apply a personalised treatment.

MITOCHONDRIAL DISEASE

Caused by genes in the mitochondrial DNA. An exclusively maternal inheritance occurs.

MATERNAL BLOOD FREE FOETAL DNA TEST

Fast, safe, effective and accurate prenatal screening test based on analysing free fetal DNA in maternal blood. Enables early detection of trisomy 22, 21, 18, 16, 13 and 9. Also provides information on sex chromosomes (indicating fetal sex), the most common sex chromosome anomalies and allows certain microdeletion syndromes to be ruled out.

Microarray

Genetic technique to test for the loss and gain of genetic material along the whole genome. It is a diagnostic test (not a screening test), and in the prenatal context it detects a high number of syndromes in a very short period of time, unlike conventional karyotyping.

MONOGENIC DISEASE

Genetic disorder caused by only one gene in each patient.

NON-INVASIVE PRENATAL SCREENING

Maternal blood fetal DNA test performed in the laboratory that helps to calculate the likelihood of trisomy 9, 13, 16, 18, 21 and 22, the most common sex chromosome aneuploidies and microdeletion syndromes. These tests can be performed from the tenth week of pregnancy.

NUCHAL TRANSLUCENCY

Parameter obtained by abdominal ultrasound between 11 and 13 weeks of pregnancy, measuring the thickness of the fetal nuchal fold (back of the neck). This thickening is caused by increased fluid in the area and is associated with Down's syndrome and other genetic alterations.

NEGATIVE PREDICTIVE VALUE

Likelihood of not having the disease if the test is negative.

NGS

Next-Generation Sequencing. High-throughput methodology for rapid sequencing of base pairs in DNA or RNA samples.

POSITIVE PREDICTIVE VALUE

Likelihood of having the disease if the test is positive.

PATHOGENIC VARIANT

Variant that has a functional effect and may cause a disease as a result.

Promoter

Region essential for gene transcription that has a regulatory function and is recognised by RNA polymerase. Prokaryotic RNA polymerase binds directly to the promoter while eukaryotic RNA polymerase recognises the promoter through transcriptional factors.

PSEUDOGENE

Nucleotide sequence similar to a gene that does not result in a functional product (it is not expressed).

Patau syndrome

Chromosomal aberration that affects humans and occurs with a frequency of 1 / 5,000 at birth and is related to a trisomy of chromosome 13.

Purine

Nitrogenous bases. In DNA and RNA they are adenine (A) and guanine (G). In double-stranded nucleic acid sequences a purine base recognises a pyrimidine, so that purine A recognises pyrimidine T (in DNA) and U (in RNA). Purine G recognises pyrimidine C.

PAPP-A

Pregnancy-associated plasma protein-A; glycoprotein synthesised by the syncytiotrophoblast and in the endometrium from the seventh week of pregnancy. Inhibits elastase, preventing the trophoblast implantation zone from intruding into the endometrium. It is also a biochemical marker.

PENETRANCE

Ability of a gene or set of genes to express an associated trait.

PYRIMIDINE

Type of nitrogenous base. Cytosine is found in both DNA and RNA, thymine only in DNA and uracil only in RNA.

PHENOTYPE

Observable physical characteristics of an organism, including physiology, morphology and behaviour at all levels of description; in short, the observable properties of an organism.

POLYGENIC DISEASE

Also known as multifactorial or complex diseases; genetic disorder caused by more than one gene, in addition to environmental factors.

QF-PCR

Genetic technique that, in the prenatal context, identifies certain short tandem repeats (STR) in chromosomes 21, 18, 13, X and Y. This identification and quantification enables us to find out the number of these chromosomes in just 24 hours.

RNA

Ribonucleic acid; macromolecules that perform various vital roles in the coding, decoding, regulation, and expression of genes. Like DNA, it is a nucleic acid and, together with proteins, they constitute the three great types of macromolecules essential for all forms of life.

RNAm

RNA molecule that is translated into one or more proteins on ribosomes.

REPLICATION

Process of DNA duplication, whereby two identical copies are generated from a single copy. This process requires both strands of the original molecule to be separated.

RARE DISEASES

Also known as minority, orphan or uncommon diseases, including genetic disorders. Life-threatening or chronic disabling diseases with a prevalence of less than 5 cases per 10,000 people.

SEX CHROMOSOMES

Type of chromosome involved in determining the sex of an organism. Humans have two sex chromosomes: the X and Y chromosomes. Females have two X chromosomes in their somatic cells, while males have one X and one Y chromosome. However, as eggs contain only one X chromosome, and sperm can contain either an X or a Y chromosome, it is the male that determines the sex of the offspring after fertilisation.

Splicing

A splicing process that takes place on the early RNA transcript, removing introns and subsequently assembling exons to form mature DNA.

SNPs

Single-nucleotide polimorphisms (polimorfismos de un único nuleótido). Tipo de polimorfismo más común en el genoma humano, que se diferencia en un solo par de bases de un gen, con respecto al mismo gen en otro individuo. Son estables y en promedio se encuentran cada 1000 pb en el genoma. Aquellos incluídos en secuencias codificadoras se llaman cSNPs y pueden ser utilizados como marcadores de enfermedades.

Sensitivity

Ability of the test to detect disease (correctly classify a sick individual).

SPECIFICITY

Likelihood of a negative sample being identified as such. It is calculated as TN/(TN+FP) i.e. the fraction of assigned negative results out of the total known negatives. The value sought corresponds to 99% specificity.

SCREENING TEST

Test carried out to identify individuals in the general population who are affected by a disease or abnormality that was previously undetected by diagnostic tests, examinations or other rapid techniques.

TN

Number of true negatives; negative results obtained with the technique being validated, and the same result from another comparative technique, diagnosis or negative reference material.

TP

Number of true positives; positive results obtained with the technique being validated, and the same result from another comparative technique, diagnosis or positive reference material.

Transposon

DNA sequences that can change their position within a genome. This allows mutations to be created or reversed, altering the size and genetic identity of the cell genome. They are made up of approximately half of the non-coding DNA.

NUCHAL TRANSLUCENCY

Parameter obtained by abdominal ultrasound between 11 and 13 weeks of pregnancy, measuring the thickness of the fetal nuchal fold (back of the neck). This thickening is caused by increased fluid in the area and is associated with Down's syndrome and other genetic alterations.

Translation

Protein biosynthesis process that occurs due to the interaction of mature transcriptional products (mRNA, aa-tRNA and ribosomes).

TRANSCRIPTION

Síntesis de una copia de ARN complementaria a partir de una de las cadenas sencillas de ADN.

TRISOMY

Presence of an extra chromosome (three, instead of two) in human cells.

Tuner syndrome

Chromosome aberration affecting the X heterochromosome in humans. Female carriers of this aberration have one less X chromosome (monosomy). Thus the normal AAXX genotype changes to AAXO. Phenotypically they are characterized by being sterile. They tend to be short, have a wide neck, and less mental development.

TARGETED EXOME

Possibility of analysing select gene panels according to pathology. This is the best option when faced with a Mendelian disorder with defined clinical features and several associated genes.

TRIO WHOLE EXOME

Information on more than 19,000 coding genes in the exome. Making it a trio reduces the bioinformatic complexity and it is possible to compare the changes of the index case against the parents, thus increasing its prognostic value.

VUS

Variant of uncertain significance; i.e. it is not possible to define whether it is pathogenic or not.

Variant

Gene that is essentially the same as another, but has differences due to mutations.

VANISHING TWIN

Syndrome that occurs when one of the embryos of a multiple pregnancy disappears in the uterus as a result of miscarriage and is partially or completely reabsorbed by the mother or placenta.

WES

Whole Exome Sequencing. Genomic technique for sequencing all protein-coding regions of the genes in a genome (known as the exome).

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